Green Roof and Rooftop Agrivoltaic Benefit-Cost Analysis — Washington State Department of Commerce (June 22, 2023)

Issuer: Congressional materials
Document type: Report

Summary

A final report, Green Roof and Rooftop Agrivoltaic Benefit-Cost Analysis, prepared by EES Consulting for the Washington State Department of Commerce and dated June 22, 2023. It states that the legislature appropriated $200,000 in the 2021-2023 Operating Budget for a cost-benefit analysis of green roof and agrivoltaic systems on projected new buildings of 10,000 square feet or larger. The report analyzes over 250 rooftop agrivoltaic measures for Seattle, Yakima and Spokane using the Power Council's ProCost model. It reports that solar PV was not cost-effective in any case under a 15-year lifecycle, while green roofs alone were cost-effective in some instances when stormwater benefits were quantified. Table 1.1 gives benefit-cost ratios by building type and city, and the report closes with Appendix B interview responses from installers and design firms.

Summary drafted by a model from the document's text below and checked by script against that text before publication. It is a navigation aid, not a reading of what the document proves. Where AI is used

Cite this document

“Green Roof and Rooftop Agrivoltaic Benefit-Cost Analysis — Washington State Department of Commerce (June 22, 2023).” Original at app.leg.wa.gov. Archived at Pandemic Darlings, https://pandemicdarlings.com/source-documents/2023-commercereports-agrivoltaics-benefit-cost-analysis-final-report-final/, SHA-256 e7399dbdd52ed656e5c654505128ceca6beab6d20bd351189d41130644836595.

Full text

PREPARED BY EES CONSULTING

WASHINGTON
STATE
DEPARTMENT OF COMMERCE

Final Report
Green Roof and Rooftop
Agrivoltaic Benefit-Cost
Analysis


June 22, 2023
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE OF CONTENTS
1 EXECUTIVE SUMMARY ........................................................................................................................................................ 1
1.1 Purpose................................................................................................................................................................................1
1.1.1 Green Roof Types........................................................................................................................................................1
1.2 Overview of Existing Programs ............................................................................................................................................2
1.3 Summary of Benefits and Costs...........................................................................................................................................2
1.3.1 Quantitative ...............................................................................................................................................................2
1.3.2 Qualitative..................................................................................................................................................................2
1.4 Methodology .......................................................................................................................................................................2
1.5 Benefit-Cost Analysis Results ..............................................................................................................................................3
1.6 Study Findings......................................................................................................................................................................4
2 STUDY OVERVIEW AND METHODOLOGY ...................................................................................................................... 6
2.1 Green Roof Background ......................................................................................................................................................6
2.1.1 Climate Considerations..............................................................................................................................................7
2.2 Work Plan ............................................................................................................................................................................7
2.2.1 Measure Specification ................................................................................................................................................7
2.2.2 Research Objectives................................................................................................................................................. 11
2.2.3 Limitations on Proposed Research ......................................................................................................................... 11
2.3 Data Collection Plan ..........................................................................................................................................................11
2.4 Analysis Plan ......................................................................................................................................................................14
2.5 Collaboration .....................................................................................................................................................................15
3 EXISTING PROGRAMS ....................................................................................................................................................... 17
3.1 Green Roof and combined Solar Programs .......................................................................................................................17
3.1.1 San Francisco Better Roofs Ordinance ................................................................................................................... 17
3.1.2 New York City Administrative Code and Building Code ...................................................................................... 17
3.1.3 City of Chicago Green Roofs; Solar Express Permit Program............................................................................... 18
3.1.4 Summary Green Roof and Combined Solar Programs ......................................................................................... 18
3.2 Green Roof Only Programs................................................................................................................................................18
3.3 Rooftop Solar Programs ....................................................................................................................................................20
3.4 International Programs......................................................................................................................................................21
3.5 Summary............................................................................................................................................................................21
4 REGIONAL STUDY COORDINATION .............................................................................................................................. 22
4.1 Green Roof Installers .........................................................................................................................................................23
5 COSTS AND BENEFITS ....................................................................................................................................................... 24
5.1 Stormwater Quality ...........................................................................................................................................................24
5.2 Improved Air Quality (AQ) and Reduced Greenhouse Gas (GHG) Emissions....................................................................25
5.3 Food Production ................................................................................................................................................................28
5.4 Biodiversity, Ecosystem Health and Human Quality of Life Considerations.....................................................................29
5.5 Stormwater Quantity and Peak Flow Benefits ..................................................................................................................30

prepared byE E S C O N S U L T I N G i 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

5.5.1 Stormwater Benefit Modeling ................................................................................................................................ 31
5.6 Building Performance Energy Impacts ..............................................................................................................................38
5.7 Leadership in Energy and Environmental Design (LEED) Credits ......................................................................................40
5.7.1 Green Roof LEED Credits ......................................................................................................................................... 41
5.7.2 Solar LEED Credits ................................................................................................................................................... 42
5.8 Green Roofs installation costs and operation & maintenance (O&M) Costs ...................................................................43
5.9 Electric System Benefits ....................................................................................................................................................45
5.9.1 Avoided Cost Overview ........................................................................................................................................... 46
5.9.2 Social Cost of Carbon .............................................................................................................................................. 47
5.9.3 Avoided Renewable Energy Purchases .................................................................................................................. 48
5.9.4 Electric Wholesale Market Price ............................................................................................................................. 48
5.9.5 Deferred Transmission and Distribution System Investment .............................................................................. 48
5.9.6 Deferred Investment in Generation Capacity ........................................................................................................ 48
5.9.7 Northwest Power Act Credit ................................................................................................................................... 49
5.10 Rooftop Solar ...................................................................................................................................................................49
5.10.1 Solar O&M ............................................................................................................................................................. 50
5.10.2 Solar Capital Costs ................................................................................................................................................ 50
5.11 Battery Storage ................................................................................................................................................................51
5.11.1 Battery O&M.......................................................................................................................................................... 52
5.11.2 Battery Capital Costs ............................................................................................................................................ 53
5.12 CoLocation Benefits of Green Roofs with Photovoltaic Arrays .......................................................................................54
5.13 Economic Development...................................................................................................................................................55
5.13.1 Solar PV Economic Impacts .................................................................................................................................. 55
5.14 Summary of Quantified Benefits and Costs.....................................................................................................................56
6 BENEFIT-COST ANALYSIS RESULTS ............................................................................................................................... 57
6.1 Financial Assumptions .......................................................................................................................................................57
6.1.1 Measure Life ............................................................................................................................................................ 57
6.1.2 Discount Rate .......................................................................................................................................................... 58
6.2 Results: Green Roof and Solar PV No Colocation ..............................................................................................................58
6.3 Results: Green Roof and Solar PV with Colocation ...........................................................................................................62
6.4 Results: Green Roof, Solar PV, and short-Term Battery STorage......................................................................................63
7 KEY FINDINGS ..................................................................................................................................................................... 65
7.1 Study Findings....................................................................................................................................................................65
7.2 Barriers Assessment ..........................................................................................................................................................65
7.3 Recommendations.............................................................................................................................................................66
8 REFERENCES ......................................................................................................................................................................... 67
9 APPENDIX A GREEN ROOF INSTALLATION PROVIDER CONTACTS AND INTERVIEW QUESTIONS .............. 70
10 APPENDIX B INTERVIEW RESPONSES......................................................................................................................... 74


prepared byE E S C O N S U L T I N G i i 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

1 Executive Summary
1.1 P URP O SE
The Washington state legislature appropriated $200,000 to the Washington Department of Commerce
(Commerce) in its 2021-2023 Operating Budget to:

“…Conduct a cost-benefit analysis on the use of agrivoltaic and green roof systems on projected new buildings with
a floor area of 10,000 square feet or larger to be developed over the next 20 years in communities of 50,000 or
greater. The department shall consult with the department of ecology, private sector representatives, and an
organization that has experience conducting cost-benefit analyses on green roofing. The cost-benefit analysis must
include:
(i) The impact of widespread green and agrivoltaic roof installation on stormwater runoff and water treatment
facilities in communities with a population of greater than 50,000;
(ii) Potential water quality and peak flow benefits of widespread green and agrivoltaic roof installation; (iii)
Public health impacts;
(iv) Air quality impacts;
(v) Reductions in fossil fuel use for buildings with agrivoltaic systems;
(vi) Energy efficiency of buildings with agrivoltaic systems;
(vii) Job creation; and
(viii) Agrivoltaic installation and maintenance costs.”1

A green roof is a layer of contained vegetation which sits over a waterproofing system, installed on top of a flat or
slightly sloped roof. Agrivoltaics, for the purposes of this study, are the colocation of solar panels (photovoltaics)
and green roof technology on commercial rooftops. All references to agrivoltaics, and colocation, throughout this
report refer to rooftop applications only.

The objective of this work is to better understand the opportunity for green roof and rooftop agrivoltaic systems
on new commercial buildings in densely-populated areas of Washington. The results of this analysis will support
the development of policies and legislation to achieve increased green roof and rooftop agrivoltaic adoption in
Washington cities.

1. The BCA must examine the costs and benefits associated with green roof and agrivoltaic systems on
projected new buildings with a floor area of 10,000 square feet or greater.
2. The study will cover 3 urban areas in Washington with a population greater than 50,000 people.

1 .1 .1 G r een Roof Typ es
Green (or eco-) roofs are a green infrastructure alternative to conventional roofs that reduce stormwater
discharge and provide a wide range of additional environmental and aesthetic benefits. A green roof consists of a
living layer of vegetation placed over a growing medium on top of a synthetic, waterproof membrane, the entire
multi-layer system being supported by a building structural roof system. Living green roofs can significantly
decrease stormwater runoff, save energy, improve water quality, absorb carbon dioxide, cool urban heat islands,
and filter air pollutants. A vegetated roof can increase habitat for birds and insects and provide much needed
greenspace for urban dwellers. Vegetated roof technologies can provide building owners with a proven return on
investment, and create opportunities for significant social, economic and environmental benefits, particularly in
urban areas. Green roofs are broadly divided into two types as follows:

1
2021-2023 Washington State Operating Budget, pg. 833

prepared byE E S C O N S U L T I N G 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Extensive – Consists of low growing plants with 2-6 inches of soil. Irrigation after plant establishment may not be
necessary in Western WA. These types of roofs have the best stormwater retention and require the least amount
of maintenance, depending on the types of plants chosen. They can be placed on individual tr ays which spread
out the load with easy access to the roof structure or membrane for easy replacement. Dormant species can
return after a “brown out” in dry months.

Intensive – These roofs can be elaborate gardens with great plant diversity can include trees with the right quantity
of soil and can be a “destination” for tenants or visitors to enjoy. While more visually attractive than extensive
roofs, they need much more maintenance, irrigation and structural support.

1.2 O V E RV IE W O F E XISTING P RO G RAMS
A number of existing green roof infrastructure programs of varying levels of scope and maturity have been or are
being implemented at the local and state level in the United States as well as abroad, particularly in Europe.
Programs vary in their emphasis on building types, where the focus may be on residential, commercial, mixed-use
and/or public office buildings. In general, western European countries have developed more advanced and robust
programs for encouraging green roof alternatives for both new and existing building infrastructure. Many of these
programs offer various incentive mechanisms that can make green infrastructure alternatives more attractive
financially or more desirable from an end-user perspective.

1.3 SUMMAR Y O F BE NE FITS AND C O STS
The project included an extensive review of benefits and associated costs of green roofs alone and in combination
with solar cell energy production (photovoltaic). The combination of the two systems has been termed Agrivoltaic
for the purposes of this study. The benefits of these green roof and solar applications can be quantitative, where
positive influences are expressed in terms of discreet units of measure, or can be qualitative, when benefits are
tangible but not amenable to evaluation using discrete units of measurement.

1 .3 .1 Q uant it at ive
The following potential quantitative benefits were evaluated in this study:
 Reduction in stormwater runoff volumes and associated management costs
 Impacts on building energy performance (HVAC)
 Leadership In Energy and Environmental Design (LEED) Credits
 Roof Installation and Operation & Maintenance (O&M) Costs
 Greenhouse Gas reduction or sequestration
 Avoided Costs (deferred capital expenses, social cost of carbon emissions, energy infrastructure)

1 .3 .2 Q ualit at ive
The following potential qualitative benefits were evaluated in this study:
 Improvements in water and air quality
 Reduced surface temperatures and urban heat island effects
 Increased food production
 Biodiversity, ecosystem health and human quality of life considerations
 Highly impacted community effects

1.4 ME THO DO LO G Y

prepared byE E S C O N S U L T I N G 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

The costs and benefits are quantified for several measure permutations that vary by climate zone, roof coverages,
and building type. In total over 250 rooftop agrivoltaic measures were analyzed. The benefits and costs were used
as input in the Northwest Power and Conservation Council (Power Council)’s ProCost model for evaluating energy
efficiency savings and demand side resources such as behind-the-meter solar. The value assumptions such as the
price of energy are updated with current market conditions, and each measure is analyzed based on the time-of-
day energy savings or production occurs. The three cities selected for the measure analysis include Seattle,
Yakima, and Spokane. Each of these cities has unique attributes in terms of population density, rainfall, climate,
solar availability, and development. The benefit-cost analysis was prepared from a total resource cost or regional
perspective.2

1.5 BE NE FIT -C O ST ANAL YSI S RE SULTS
The benefit-cost analysis resulted in mixed cost-effectiveness levels. Because the analyses are prepared
separately, their results can also be viewed separately. Mainly, solar PV was not cost-effective in any of the cases
when a 15-year lifecycle is assumed. This result is not surprising from a wholesale energy perspective. Most
rooftop solar installations are deemed cost effective when incentives are included and the energy produced is
compared to retail utility rates which include costs that are considered transfers in this benefit cost analysis.

In some instances, green roofs alone were cost-effective. This is the case when stormwater benefits are quantified
based on the analysis contained in this study. In this study, stormwater benefits are quantified only in Seattle and
Spokane. Stormwater benefits in Yakima were not quantified but are believed to be minimal.3 Table 1.1
summarizes the BCA results by combining all roof coverages for solar and green roof. Result details by roof
coverage is provided in the main body of the report in Section 6.

TABLE 1.1: BENEFIT-COST RATIOS- GREEN ROOF PLUS SOLAR PV ALL COVERAGES
Building Type Seattle Spokane Yakima
Assembly 0.82 0.50 0.41
Grocery 0.81 0.46 0.37
Hospital 0.91 0.72 0.65
Lodging 0.88 0.56 0.50
Mixed Commercial* 0.88 0.52 0.46
Multifamily High Rise (7+) 0.96 1.59 NA
Multifamily Low Rise (1-3) 0.92 0.56 0.51
Multifamily Mid Rise (4-6) 0.92 0.81 0.75
Office 0.88 0.51 0.45
Other 0.87 0.46 0.40
Residential Care 0.87 0.48 0.41
Retail / Service 0.88 0.52 0.46
School 0.86 0.46 0.40
Warehouse 0.86 0.47 0.40

2
Consistent with the approach taken by the Northwest Power and Conservation Council in its evaluation of resources.
3 The City of Yakima’s 2007 Annual Inflow and Infiltration Evaluation noted that leakage from irrigation is the primary

driver of excess inflow/infiltration on the City’s wastewater treatment plant.
www.yakimawa.gov/services/wastewater-treatment-plant/files/2012/06/IandI2007.pdf

prepared byE E S C O N S U L T I N G 3 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

1.6 STUDY FI NDING S
The research and modeling revealed several key findings, some of which are specific to Washington State.

1. The primary benefit of green roofs is stormwater retention. Energy savings from the added R-value from
green roof minimally impacted new building energy efficiency when the most recent Washington building
codes are factored into the analysis.
2. Stormwater benefits in this study are dependent on the variable cost of stormwater treatment. Not all
areas in Washington State face the same challenges in stormwater treatment. Yakima is an example from
this study where the City’s wastewater treatment facilities are impacted significantly through irrigation
system leaks and less impacted by stormwater inflow and infiltration.
3. Increasing soil depths on green roofs in Spokane results in not needing irrigation systems due to consistent
average monthly rainfall.
4. The primary quantified benefit of green roofs is the stormwater discharge reduction, and this benefit
varies widely depending on the local stormwater infrastructure and precipitation. Studies that have
shown positive cost/benefit results for green roofs have relied on quantifying softer benefits such as
increased occupant productivity, comfort, or energy savings based on buildings that are not as efficient
as required in Washington State.
5. In practice, rooftop agrivoltaic projects are not common. Most often when solar and green roofs are
combined on the same roof, they exist separately leaving any, likely small, colocation benefits unrealized.
Colocating closely together raises concerns from roofing contractors regarding how the photovoltaics will
be anchored and the potential for puncturing of the green roof liner. The panels could also interfere with
access to the green roofs for required regular maintenance.
6. Rooftop solar programs are well-defined in Washington State. The cost of the infrastructure is also well-
known. Washington has already implemented several programs to promote increased adoption of rooftop
solar including solar-ready buildings, sales tax rate exemptions, and grant monies for qualifying projects.
Additionally, significant incentives are available at the Federal level from both the American Recovery and
Reinvestment Act and the Inflation Reduction Act.
7. The colocation benefits of solar PV and green roofs related to energy production is not significant. The
study identified qualitative colocation benefits for green roofs from solar PV shading; however, the data
available was insufficient for quantifying those benefits.

From this study it can be concluded that rooftop agrivoltaics are not wholly cost-effective in the State of
Washington. Regardless, the State may wish to promote rooftop agrivoltaics for economic or other reasons that
are difficult to quantify based on the current literature. If the State were to move forward with a rooftop
agrivoltaic program, the Project Team makes the following general recommendations:

prepared byE E S C O N S U L T I N G 4 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

5. If green roof mandates are considered, it is recommended to create multiple solutions for developers to
meet the requirements. An example is the San Francisco model which lets builders choose between solar,
green roof, or both.
6. Evaluate program spending or agrivoltaic potential based on specific parameters (mandate, incentive
structure, grant funding level) for economic development impacts.
7. Carve out additional incentives for development incorporating rooftop agrivoltaics near highly impacted
communities.

prepared byE E S C O N S U L T I N G 5 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

2 Study Overview and Methodology
Washington Department of Commerce (Commerce) retained the Project Team (EES Consulting, a GDS Associates
Company and Peak Sustainability) to evaluate the costs and benefits of green roofs paired with solar photovoltaics.

Green rooftops paired with rooftop agrivoltaics have the potential to solve multiple challenges in areas with high
population growth such as Washington State. Western Washington in particular has seen a significant growth in
multifamily building investments. Urban planners must balance the trade-off between buildings and greenspace.
At the same time, climate change is likely to increase winter rainfall in Western Washington and also lead to
hotter, drier summers. Rooftop agrivoltaics could provide relief for some of these impacts.

This study was developed using a variety of methods to develop assumptions and understand implementation
barriers. Specifically, costs and benefits are estimated based on direct modeling, survey of industry experts and
practitioners, and secondary research. Our team leveraged regional contacts and existing benefit-cost analysis
construct to develop assumptions and results. This study also focused the literature review first on sources in the
regions selected for this project. If no local studies were available, the study relies on the next best alternative
including state level, adjacent or nearby states, United States, then international. Throughout the process,
modeling decisions were vetted with Commerce staff and uncertainties discussed. The results of the study provide
one of the first detailed analyses of the costs and benefits of rooftop agrivoltaic measures. The results are specific
to Washington State for the purposes of providing recommendations for potential legislation or building codes
amendments addressing green roof and rooftop agrivoltaic measures.

2.1 G RE E N RO O F BAC KG RO UN D
Green (or eco-) roofs are a green infrastructure alternative to conventional roofs that reduce stormwater
discharge and provide a wide range of additional environmental and aesthetic benefits. A green roof consists of a
living layer of vegetation placed over a growing medium on top of a synthetic, waterproof membrane, the entire
multi-layer system being supported by a building structural roof system. Living green roofs can significantly
decrease stormwater runoff, save energy, improve water quality, absorb carbon dioxide, cool urban heat islands,
and filter air pollutants. A vegetated roof can increase habitat for birds and insects and provide much needed
greenspace for urban dwellers. Vegetated roof technologies can provide building owners with a proven return on
investment,4 and create opportunities for significant social, economic and environmental benefits, particularly in
urban areas. Green roofs are broadly divided into two types as follows:

FIGURE 2.1 GREEN ROOF CROSS SECTION

4 Cushman & Wakefield. Green is Good: Sustainable Office Outperforms in Class A Urban Markets. August 2021.

prepared byE E S C O N S U L T I N G 6 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Extensive – Consists of low growing plants with 2-6 inches of soil. Irrigation after plant establishment may not be
necessary in Western WA. These types of roofs have the best stormwater retention and require the least amount
of maintenance, depending on the types of plants chosen. They can be placed on individual trays which spread
out the load with easy access to the roof structure or membrane for easy replacement. Dormant species can
return after a “brown out” in dry months.5

Intensive – These roofs can be elaborate gardens with great plant diversity and can include trees with the right
quantity of soil and can be a “destination” for tenants or visitors to enjoy. While more visually attractive than
extensive roofs, they need much more maintenance, irrigation and structural support.

2 .1 .1 C limat e C ons id er at ions
The different climate zones have variations pertaining to the design, efficiency and maintenance of green roofs.
Some of the literature recommend a deeper soil medium and roof pitch of 5-20 degrees for drier climates, where
the roof pitch recommendation for Western Washington is flat. A more level roof maximizes stormwater
retention. Depths of 2-3 inches support succulents, grasses and herbaceous plants when deeper soils substrate
can allow for drought-tolerant perennials and grasses. A diverse assortment of plants is the key to surviving the
arid summer and frigid temperatures of the winter months. The soil medium is the key factor in water retention.

2.2 WO RK P LAN
The Project Team developed a work plan to specify the benefit-cost analysis framework. This work plan is provided
below in this section.

2 .2 .1 Meas ur e Sp ec ific at ion
Rooftop agrivoltaic costs and benefits will vary by climate and solar zones across Washington State. The Project
Team selected 3 metropolitan areas that represent the climate and solar resources across the state. Seattle,
Spokane, and Yakima were selected based on representing Washington’s populous climate zones, precipitation,
and solar zones. The table below compares the three selected areas. Heating zone one is a mild heating climate
(winters are, on average, not cold). Heating Zone 2 experiences colder winter temperatures. Similarly, cooling
zone 1 represents mild summer weather while cooling zones 2 and 3 experience hotter summer

5
Paladino & Company, Inc., Green Roof Feasibility Review. February 2006. Available at:
https://kingcounty.gov/~/media/depts/dnrp/solid-waste/green-building/documents/KC_Green_Roof_case-
study.ashx?la=en

prepared byE E S C O N S U L T I N G 7 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

temperatures. Washington state is generally characterized by 2 solar zones with solar zone 1 having lower solar
generation potential.

TABLE 2-1: STUDY AREAS

Annual Rainfall
Heating Zone Cooling Zone Inches Solar Zone Other Attributes
Wenatchee 1 3 8.4 3
Yakima 1 3 8.0 3 Indian Reservation
Represented
Spokane 2 2 16.5 3 Metropolitan Area Size
Seattle 1 1 37.1 1 Previous Green Roof Programs
Bellingham 1 1 36.1 1
Note: Wenatchee and Bellingham were not initially selected for study based on lower construction rates for new commercial
buildings

The table above shows that Yakima and Wenatchee have similar climate, rainfall, and solar potential. Yakima was
selected based on its ability to represent an area where Native American populations exist. Measures developed
for Yakima could also be applied to buildings in the Wenatchee area. Similarly, Seattle and Bellingham are similarly
situated across the examined characteristics. Measures developed for Seattle could reasonably be applied to the
Bellingham area.

Based on our initial research and subject matter expertise, the Project Team has specified measures according to
the parameters in Table 2. The coverage percentage applies to 100% of roof space.

TAB LE 2-2: M E AS U R E S P E C IFIC AT IO N S

Commercial Roof Area Commercial Roof Area
Measure Solar Multifamily 2,000 to 50,000 SF above 50,000 SF
Option Panel Roof Area <25,000 SF 65% Existing Buildings 35% Existing Buildings
Green
Green Roof Roof
Extensive Intensive Green Roof Extensive Green Roof Extensive
1 10% 50% 50%
2 25% 70% 70%
3 10% 30% 30%
4 25% 50% 50%
5 50% 70%

Error! Reference source not found..2 illustrates how measure permutations might apply to different approaches t
o green roof or rooftop agrivoltaics. In the example below, the coverage varies as well as the type of green roof:
extensive or intensive. The measure permutations that are most likely to be feasible and cost-effective are
prioritized in the modeling. Any exclusions are noted and explained in this report.6

6An exclusion may be where the measure is a “small saver,” or a relatively costly measure with little expected benefit. An
example might be green roof and agrivoltaic installations in unheated buildings.

prepared byE E S C O N S U L T I N G 8 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

FIGURE 2.2: MEASURE PERMUTATION EXAMPLES

Each measure permutation will be compared to a baseline option. The baseline is the type of roof installation that
would be used in absence of green roof or rooftop agrivoltaic options. Costs and benefits will be quantified for
each measure permutation incrementally to the baseline.

FIGURE 2.3: BENEFIT COST ANALYSIS PROCESS

In the case where solar and green roofs are collocated, the solar panels considered in the modeling are specified
as bifacial technology to allow for light to pass through to plantings below. The maximum coverage of solar panels
is assumed at 50% based on initial research. There may be cases where solar and green roof vegetation do not
overlap in design. The base analysis assesses designs where there is no overlap. Additional benefits are included
in a separate analysis where varying degrees of overlap are modeled.

Green roof coverage varies by roof size and building use. While the scope of the study evaluates buildings over
10,000 square feet, most commercial building roof sizes will be above 25,000 square feet based on the 2019
Commercial Building Stock assessment supporting data (see Tables 3 and 4). Intensive coverage is reserved for
special cases such as multifamily buildings in high-density areas. The Project Team discussed the following
considerations for measure analysis:

prepared byE E S C O N S U L T I N G 9 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

1. A survey of current programs revealed the following:
a. Coverage for solar and green roof compliance can be achieved by an either/or approach
b. Minimum solar size was required by the program in some instances i.e. 4 MW
c. Programs recognized that there is a minimum roof size to allow for sufficient economies
of scale
2. Intensive green roofs are primarily installed in premium multifamily buildings to enhance real
estate value.
3. For green roof applications, the program required green roof in cases where the roof pitch is flat
or mostly flat
4. Consistent with our research, it is assumed that the maximum green roof coverage is 70% to allow
for HVAC and other necessary rooftop equipment.
5. Solar can be installed on roof pitches up to 30%
6. Very large roof areas for certain building types may not produce significant cooling benefit if those
buildings are located in lower density zones with vegetation surrounding the building.
7. Energy Use Intensity must be sufficient to produce energy savings and utilize on-site solar
production. Tables 3 and 4 summarize the relevant energy use intensity (EUI) values by building
type.

prepared by E E S C O N S U L T I N G 1 0 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

8. In practice green roofs might include a mix of intensive and extensive plantings on one roof. This
“mixed roof” scenario is highly specific to building type or use. The Project Team will define a
minimum share of extensive or intensive plantings for each case. For example:
a. Extensive – No more than 20% Intensive plantings
b. Intensive – No greater than 80% extensive

Measure specification was further verified through conversations with professional installers and additional
research.

2 .2 .2 Res ear c h O b jec t ives
The objective of the research is to quantify costs and benefits of rooftop agrivoltaics and provide qualitative
analysis where quantification is challenging, or data is not available. Data collection records will clearly define the
sources as well as any quantitative limitations and objective assumptions used in the benefit-cost analysis.

2 .2 .3 Limit at ions on P r op os ed Res ear c h
Prior to starting the research, several potential limitations were identified.

1. Data for specific areas in WA state may not be available such as energy savings attributed directly to
rooftop agrivoltaics or best practices for green roof installations in certain cities.
2. Data or analysis may not be available for certain rooftop agrivoltaic costs and benefits, including
impacts on vulnerable communities.
3. Measure specification is based on best available data and distilled to a reasonable number of
permutations. Due to the diversity in building construction and design, there may be some
permutations where the costs and benefits are not well-represented by using average building
attributes from the Commercial Building Stock Assessment.
4. The costs and benefits quantified will be specific to current market conditions, service, and values
available at the time of the study.

These limitations and others were defined throughout the research and are presented in this report. Where data
was limited, the Project Team relied on best available alternatives including studies from other regions and/or
qualitative analysis.

2.3 DATA C O LLE C TIO N P LAN
The study will produce quantitative data in terms of $/SF of roof area. These data will include the costs and
benefits detailed in Tables 2.7 and 2.8. Many of these parameters in Tables 2.7 and 2.8 will easily translate to
$/sq-ft roof area. However, some items will need additional adjustments to produce the desired units. For
example, HVAC savings for buildings with multiple stories will need to take into account the average number of
stories.

Tables 2.3 and 2.4 summarize commercial building data for Washington buildings. Table 2.3 summarizes buildings
data for 10,000 square feet or larger and Table 2.4 shows the same data for buildings with 50,000 square feet or
larger. The energy use intensity (EUI) data applies to all buildings by type regardless of size.

prepared by E E S C O N S U L T I N G 1 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

T AB LE 2 .3 : WASHIN GT ON B UILD I N GS WIT H ARE A > 10 ,0 00 SF 1
Average of Floors above
Site Floor Building Grade Inferred Roof Electric EUI Natural Gas EUI
Building Type Area Count Average Size kWh/SF Therms/SF
Assembly 36,333 24 2 19,378 7.93 0.44
Grocery 23,625 8 1 18,900 40.23 0.57
Hospital 207,115 26 3 64,107 32.34 1.35
Lodging 51,923 26 3 15,169 10.89 0.43
Mixed 44,515 33 2 19,078 15.19 0.24
Commercial*
Office 58,308 13 3 22,970 11.84 0.16
Other 13,000 2 2 8,667 6.39 0.36
Residential 61,360 25 2 30,078 14.74 0.41
Care
Retail / 78,842 19 1 59,920 9.59 0.29
Service
School 73,000 23 1 49,382 7.39 0.22
Warehouse 71,838 37 2 47,464 3.72 0.14
Grand Total 73,805 236 2 34,355 11.89 0.36
*Mixed commercial applies to buildings with multiple commercial uses. The CBSA data did not include residential within
Mixed Commercial definitions.

prepared by E E S C O N S U L T I N G 1 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 2.4: WASHINGTON BUILDINGS WITH AREA >50,000 SF2
Floors
above
Average of Building Grade Inferred Electric EUI Natural Gas EUI
Building Type Site Floor Area Count Average Roof Size kWh/SF Therms/SF
Assembly 130,333 3 4 32,508 7.93 0.44
Grocery 53,000 1 1 53,000 40.23 0.57
Hospital 248,200 20 4 62,050 32.34 1.35
Lodging 94,222 9 4 23,556 10.89 0.43
Mixed Commercial* 107,167 6 6 17,861 15.19 0.24
Office 119,400 5 3 39,800 11.84 0.16
Other NA NA NA NA 6.39 0.36
Residential Care 79,231 13 3 26,410 14.74 0.41
Retail / Service 549,500 2 2 274,750 9.59 0.29
School 107,273 11 2 53,637 7.39 0.22
Warehouse 143,364 11 2 71,500 3.72 0.14
Grand Total 152,864 81 3 50,955 11.89 0.36
*Mixed commercial applies to buildings with multiple commercial uses. The CBSA data did not include residential within
Mixed Commercial definitions.

Multifamily building data from the most recent Residential Building Stock Assessment is provided in Tables 2.5
and 2.6. Due to the sample size, buildings from Oregon, Washington, Idaho, and Montana are included these
tables.

TABLE 2.5: PACIFIC NORTHWEST MULTIFAMILY BUILDINGS3
Average Electric Natural
Average Floor EUI Gas EUI
Multifamily (stories) Stories Building Count Area SF kWh/SF Therms/SF
Low Rise (1-3) 2.2 411 5,462 9.7 0.31
Mid-Rise (4-6) 4.4 12 9,481 8.1 0.29
High Rise (7+) 13 1 17,250 5.9 0.29
All 2.3 424 9.5 0.3

Of the 424 buildings in Table 2.5, 43 are greater than 10,000 square feet in floor area (see Table 2.6). The largest
building surveyed is 3 stories and 46,720 SF. The smallest building in this subset is 2 stories and 10,075 SF.

prepared by E E S C O N S U L T I N G 1 3 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 2-6: PACIFIC NORTHWEST MULTIFAMILY BUILDINGS WITH FLOOR
AREA GREATER THAN 10,000 SF4
Average
Average Average Floor Inferred Roof
Multifamily (stories) Stories Building Count Area SF Area SF
Low Rise (1-3) 2.4 37 15,897 6,609
Mid-Rise (4-6) 4.8 5 14,501 3,021
High Rise (7+) 13 1 17,250 1,327
All 2.9 43 15,766 5,380

The above building types will be reviewed to develop measure data the options in Table 2.6 above. Some
permutations by building type may be eliminated in this process to analyze technically feasible options that will
likely be economic.

2.4 ANA LYSI S P LAN
Tables 2.7 and 2.8 summarize the parameters and evaluation methodology proposed.

TABLE 2.7: BENEFIT PARAMETERS AND PROPOSED METHODOLOGY
Benefit Methodology and Data Sources
Reduced Stormwater Reduced Stormwater quantities
Mitigation Roof slope impacts
Measured through gallons/year/SF roof space
Quantified using stormwater variable costs, deferred investments
Water Quality Qualitative evaluation of increased stormwater purity
Reduced Surface Quantified according to LEED credit value (proxy)
Temperatures Qualitative components
Impact on vulnerable communities
Reduced GHG Energy savings and market greenhouse gas (GHG) content
Consider CETA requirements
Valued at WAC 194-40-100
GHG Sequestration Carbon sequestration of green roofs
Measure in lbs CO2e per year per SF
Intensive and extensive green roof differences
Reduced Energy Use Electricity savings, kWh/SF/yr by building type per DesignBuilder 3d model
Natural Gas savings, MMBtu/SF/year
Electric distribution and transmission benefits ($/kW-yr)
Hourly HVAC savings shapes by building
Sources: NWPCC, RTF, NEEA, other
Building types (>50,000 sf): multifamily, commercial retail large, commercial office large,
warehouse, hospital, school/university, grocery, Assembly, General Industrial
Solar PV + Battery Production curves solar and solar + battery options by solar zone
Generation Battery optimized for regional electric system peak
Electric Grid Resilience and Peak reduction with Battery storage $/kW
Stability
Colocation Benefits Solar Increased efficiency (%)
Solar resource production or lower O&M

prepared by E E S C O N S U L T I N G 1 4 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Benefit Methodology and Data Sources
Colocation Benefits Green Increased food or plant production
Roof Greenhouse Gas sequestration
Other efficiencies
Food Production Separate qualitative analysis

Roof Service Life Useful life of liner based on literature review and/or manufacturer or installer warranty
LEED Credit Potential for LEED credit
Noise Reduction Qualitative Analysis based on literature research
Increased Comfort Qualitative Analysis based on literature research
Increased Aesthetic Qualitative Analysis based on literature research
Economic Development Input-Output modeling
Contractor survey input for job creation
O&M jobs
Labor wages available from RTF

TABLE 2.8: COST PARAMETERS AND PROPOSED METHODOLOGY
Cost Methodology and Data Sources
Green Roof Construction Useful life, capital, engineering, & installation source from literature and
local survey of contractors
Defined as incremental to baseline roof costs

Green Roof O&M Incremental to Baseline O&M
Define replacement period.
Solar PV + Storage Sizing
Construction
O&M
Useful Life
Data sourced from various sources
Program Administration Typically 20% of capital cost (NWPCC)
Depends on Program design/mandate
Other considerations?

2.5 C O LLABO RATIO N
The following table summarizes the entities that were contacted by the Project Team to ensure a comprehensive
analysis. The Project Team developed a list of 23 questions for installers about installation costs, O&M, best
practices, and potential barriers (see Appendix A). Green roof installers and partners were contacted and asked
to participate in a 20-30 minute telephone interview using thequestionnaire template.

prepared by E E S C O N S U L T I N G 1 5 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 2.9: COLLABORATION PLAN
Entity Objective
Northwest Power and Conservation Discussed work that may have been completed already or in
Council/Regional Technical Forum support of rooftop agrivoltaic measure evaluation
Energy Trust of Oregon Discussed work that may have been completed already or in
support of rooftop agrivoltaic measure evaluation
Green Roof Installers Sourced from green roof professionals map and
Landscape companies recommendations from industry professionals
(26 initial contacts) to collect cost and anecdotal data
1. Costs, O&M
2. Best practices
3. Barriers
4. Colocation of solar benefits/drawbacks
Architects of Washington State Obtain range of cost estimates for green roof/solar support for
Structural Engineering Firm/s coverage options in Table 2.2
Seattle City Light (Utility) Green Roof Program outcome, insights, coordination of efforts
Example sites
Puget Sound Energy (Utility) Green roof/solar program participation/findings
USGBC certification for LEED Identify LEED benefits

prepared by E E S C O N S U L T I N G 1 6 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

3 Existing Programs
The Project team developed a matrix of various mandatory and voluntary programs related to green roof or solar,
or both. This section of the report summarizes the key findings. Most programs are focused on either green roof
or rooftop solar. Few programs are designed to integrate both green roof and solar PV; however, these programs
are described in more detail followed by solar only or green roof only summaries.

3.1 G RE E N RO O F AND C O MBINE D SO LAR P RO G RAM S
3 .1 .1 San Fr anc is c o Bet t er Roofs O r d inanc e
Notably, San Francisco’s Better Roofs Ordinance combines green roof and solar technology for new building
construction. The Better Roofs Ordinance went into effect January 1st, 2017, and allows “living roofs” to meet
the requirements of a state law that mandates a portion of roofs be “solar ready.”

There are requirements for new building construction to facilitate the development of renewable energy facilities
and living roofs. These standards require that 15% of the roof space on most new construction is solar. These
requirements can also be met by providing 30% of the roof space as a living roof (i.e. green or vegetated roof), or
installing a combination of both solar and living roof. This option will allow a project sponsor to replace the
required solar with living roof at a ratio of 2 square feet of living roof for every 1 square foot of solar. Better Roofs
requirements apply to all projects proposing new construction that meet all of the following below:

 Non-residential with a gross floor area of 2,000 square feet or more, or residential of any size;
 Has 10 or fewer occupied floors; and
 Applies for a site permit or building permit on or after January 1, 2017.
 Commercial with 2,000 SF or more, any residential. Between 15-30% of roof space incorporate solar, living
roof, or both.

In total, 30% of roof space must be either solar, living roof, or a combination of both with any combination of the
following technologies, such that the performance requirements for each installed technology are met:

 PV, with a minimum 10 W DC per sq. ft. or roof area allocated to PV
 Solar Water Heat (SWH), with minimum 100 kBtu/SF of roof area allocated to SWH
 For buildings over 5,000 SF and subject to the SF Stormwater Management Ordinance (SMO), living roof,
such that 2 sq. ft. of living roof is installed to satisfy 1 sq. ft of minimum solar zone area

The program includes stormwater management and non-potable water ordinances within the Better Roofs
ordinance. Roof area is defined as all outside coverings of the building envelope. Planting emphasis on biodiversity

3 .1 .2 New Yor k C it y Ad minis t r at ive C od e and Build ing C od e
Another example of a city ordinance that applies to new buildings or new roofs on existing buildings. The
requirements for sustainable roofs were effective in 2019. The sustainable roofing zone must be either 100%
solar, 100% green roof, or a combination.

The ordinance allows for exceptions and design considerations regarding roof slope and solar PV capacity.
Specifically, on a high-slope roof (roof slope > 2:12), a solar photovoltaic system shall be provided. Where the

prepared by E E S C O N S U L T I N G 1 7 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

solar photovoltaic system cannot meet or exceed a capacity of 4kW, the roof is exempt. 7 Additionally, there are
several areas excluded from the sustainable roofing zone, including NYC Fire Code compliance areas, areas with
rooftop structures, areas occupied by stormwater management practices, building setbacks, recreational spaces,
pitched roofs, and areas where site conditions, as determined by the Department, are unfavorable to either solar
or green roof systems.

Tax credits are available to offset the cost of green roof installation based on soil depth. These tax credits apply to
roof areas ranging from 3,500 to 20,000 square feet.

3 .1 .3 C it y of C hic ag o G r een Roofs ; S olar E xp r es s P er mit P r og r am
The City of Chicago’s Express Permit Program applies to new or retrofit buildings with minimum area defined by
specific formula for either green roof or solar. To be eligible, the applicable roof area must be 50% or 2,000 square
feet. The City has implemented a user-friendly online portal to help administer the program.

3 .1 .4 Summar y G r een Roo f and C omb ined S olar P r og r ams
All three programs offer structure for an either/or approach to rooftop solar or green roof (or blue roof). This
flexibility allows for wider compliance opportunities where building characteristics vary widely. All mandated
programs allow for exceptions for small scale technical feasibility. The drafters of these ordinances seemed to
recognize that economies of scale were an important consideration to the cost-effectiveness of the building
upgrades.

TABLE 3.1: GREEN ROOF AND COMBINED SOLAR PROGRAMS SUMMARY
City Program Description Incentive Notes
San Francisco Green roof or solar PV to Alternative to solar Various program requirements
meet Building Code Ready, green roofs or including roof size, PV size and
requirement for Solar combination can meet 30% coverage
Ready buildings building code
requirement
New York City Sustainable Roof 100% Mandate, as a separate Exceptions for roof slope and
solar PV, green roof, or program, tax credits are solar PV capacity
combination available to offset the
cost of green roof
installation based on soil
depth
Chicago Express Permit Program Shorter permitting Coverage of 50% or at least 2,000
for solar PV or green roof process SF

3.2 G RE E N RO O F O NLY P RO G RAMS
The City of Seattle has a stormwater facility credit program (SFCP) that offers incentives to property owners for
installing systems that mitigate the amount and quality of stormwater released into the city’s stormwater
system. Besides incentives, reduced fees can be a motivation for developers. Fees for new development often
include a cost for the new impervious area of the project. Green roofs are considered pervious and can reduce or
eliminate this fee. In addition, the DOE Stormwater Manual for both sides of the state requires “Flow Control” to
reduce the impact of the additional impervious (impermeable) surface created by new construction or
redevelopment. The intent is to “control” the release rates for 2-, 10- and 100-year storms to protect downstream
waterways and habitat from increased erosions by mimicking natural conditions. The water is detained in a pond

7
Exempt from both green roof and solar.

prepared by E E S C O N S U L T I N G 1 8 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

or structure (like a concrete vault) which reduce the amount of land area available for that development and can
have an extraordinary cost (in the case of an underground vault).

The table below summarizes attributes of green roof only programs. Most programs for green roofs are focused
on the stormwater benefits. Consequently, incentives often include credit on municipal stormwater billing. The
first 3 examples are from Washington State in the areas selected for specific study.

TABLE 3.2: GREEN ROOF PROGRAMS SUMMARY
City Program Description Incentive Notes
Seattle Multiple ordinances and Stormwater bill credit Special height
programs for stormwater requirements can be
management adjusted to allow for
green roof vegetation
Seattle Green Factor Mandatory Requirements for new
buildings vary per their
zoning. A score must be
achieved through various
methods, one of which
are green roofs.
Yakima City encourages green None Found
roof practices for low
impact growth
Spokane Encourages green roofs None Found
to meet stormwater
requirements
Philedephia Green Roof on new or Tax Credit Up to 50% of the cost of
retrofit buildings green foor installation
Devens, MA Industrial Performance mandatory
Standards. Projects that
require air quality permit
are required to have a
vegetated roof that
covers at least 40% of
roof area
Portland In certain zones, For each square foot of The rooftop garden must
developments with rooftop garden area, a cover 50% of the roof
rooftop gardens receive bonus of one square foot area and at least 30% of
bonus floor area. of additional floor area is the garden area must
earned. contain plants.
The property owner must
execute a covenant with
the City ensuring
continuation and
maintenance of the
rooftop garden by the
property owner.
Denver Building Ordinance $999 fine or one year in Requirement does not
requires coverage prison per violation. Each apply to residential
minimums for green roof: violation accrues over 24 buildings less than 4
New buildings: 60% hours. stories or 50 feet in
Retrofit: 18% height, or greenhouses

prepared by E E S C O N S U L T I N G 1 9 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

City Program Description Incentive Notes
Coverage requirement and their related
Vegetation shall be 80% varies by roof area size structures.
of green roof area
Austin Credit for open space in Incentive is based on area The percent of vegetated
downtown area developed roof cover is calculated as
a portion of total roof
area excluding
mechanical equipment,
photovoltaic panels,
swimming pools, and
skylights.
Nashville Sewer rebate per square $10/square foot rebate Private property owners
foot for green roof per month until full only. Limit of $500,000
coverage of 50% or more amount is applied or 60 annually
months
Washington D.C. Multiple:
Stormwater Retention Stormwater Retention
credits
Clean Rivers Incentive Discount up to 20% for
green roof installation
River Smart Rewards Up to 55% discount based
on volume of stormwater
River Smart Rooftops reduction
Reduced water bill of
$15/SF

3.3 RO O FTO P SO LAR P RO G RA MS
Rooftop solar programs types are summarized in Table 3.3 by general type of program.

TABLE 3.3: ROOFTOP SOLAR PV PROGRAMS SUMMARY
Program Type Program Description Incentive Notes
Feed-in-Tariff Fixed payment for solar $/kWh production Incentive varies by utility.
PV output from utility Typically for in front of
company. the meter applications
Net Energy Metering (rooftop Net billing reduces energy Incentive level depends WA State NEM policy
solar PV) bills to consumers on State rules but can caps have been met for
range from full retail rate most utilities
to avoided power costs. Renewable energy credits
True-up period may owned by utility.
apply.
Expedited Building Permits For new or retrofit Shorter permitting
building permits meeting process
certain requirements for
energy efficiency or solar
PV, the permitting
process is streamlined.

Building Codes California building code Mandatory Program
requires all new

prepared by E E S C O N S U L T I N G 2 0 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Program Type Program Description Incentive Notes
residential buildings to be
solar ready
Solar Loan Programs Long-term, low-interest Incentive varies by
PACE loans for solar PV program
installation costs
Sales Tax Exemption Solar PV equipment and
installation is exempt
from state sales tax
Property Tax Exemption Solar energy systems not Struck down in Missouri
held for resale are Court in 2022 as
exempt from state, local, unconstitutional (State)
and county property
taxes.
Renewable Energy Credit Tariff Utility pays solar PV Fixed price $/kWh Program in Connecticut
owner for renewable has been discontinued
energy credits for a
period of 15 years
Federal Tax Credit Applies regardless of 30% of project cost Original installations only
system size

3.4 INTE RNATIO N AL P RO G RAMS
Programs from other countries are summarized in the table below. The most successful programs internationally
have promoted significant education and engagement. Cities with the highest green roof penetration have
provided support at multiple levels (regional, local, federal) for many years.

TABLE 3.4: INTERNATIONAL PROGRAMS SUMMARY
Country Program Description
Germany Germany was the first country to develop national green roof standards in the
1970s. More than 80 German cities offer green roof incentive programs and over
15% of all roofs in Germany are green roofs.
Denmark In Copenhagen, all new roofs of less than 30 degree pitch are required to be green
roofs. No financial incentives are available; however, there are no detailed
requirements for size or habitat developed.
Toronto, Canada Mandatory green roof law in 2009. Mandatory green roofs on industrial buildings
since 2012.
Sydney Defines green roof as 30% or greater coverage and can include renewable facilities.
Primarily educational programs including training and technical guidelines
England GRO Green Roof Code based on German model but consists primarily of intangible
incentives such as training and technical guidelines.

3.5 SUMMAR Y
Many of the jurisdictions surveyed included multiple programs. Where mandates exist, there were also incentive
programs to reduce implementation costs. Flexibility in mandates help buildings with differing characteristics
meet the requirement efficiently. Copenhagen was the exception in that no incentives are offered; however, the
lack of specific requirements inferrs project design flexibility and the potential for cost containment.

prepared by E E S C O N S U L T I N G 2 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

4 Regional Study Coordination
The Research Plan identified several contacts to coordinate study efforts. The efforts and results of this
collaboration are summarized in this section of the Report.

Northwest Power and Conservation Council/Regional Technical Forum (RTF)
The Power Council and RTF maintain the methodologies and multiple publicly available data sources used by
regional planners for estimating the potential for energy efficiency and demand savings. The RTF continually
evaluates new and existing energy and demand-saving measures including updating costs and savings information
in response to building code changes or planning environment. As a first step, the Project Team reviewed existing
and proposed measure files developed by the RTF and did not find any green roof related work. The Project Team
then reached out to Power Council Staff via email to ask about current or previous work on green roof measures.
Council Staff reported that there has been no history of green roof measures under evaluation. Council staff also
reached out to the Northwest Energy Efficiency Alliance (NEEA) and confirmed that NEEA has also not undertaken
research or projects related to green roofs.

Energy Trust of Oregon
The Energy Trust of Oregon administers various energy savings programs for Oregon investor-owned utilities.
Energy Trust frequently evaluates programs and new energy saving measures. The Project Team reached out to
the Energy Trust via phone to discuss any previous work that relates to green roofs. Energy Trust staff reported
that it has not looked into green roofs as energy savings measures. Energy Trust does offer renewable energy
programs to customers in its service area including community solar, residential solar plus battery, and income-
qualified solar.

Puget Sound Energy (PSE)
PSE is an investor-owned utility serving 1.2 million electric and 900,000 natural gas customers in Western
Washington. The Project Team contacted PSE to discuss the utility’s plans regarding potential green roof
programs. PSE staff reported that PSE was in the very beginning phases of evaluating tree plantings near buildings
to reduce summer cooling costs. PSE also reported that it was unaware of any analysis of combined solar and
green roofs within its service area, PSE has not been made aware of new commercial construction with green
roofs through its energy efficiency programs, and PSE’s understanding is that kWh savings is limited with regard
to cooling loads in the Pacific Northwest. The Project Team asked PSE staff about the types of information the
utility may find useful from this study. PSE noted that having savings and $/square foot for green roofs would be
useful to them.

Seattle City Light
SCL provides electric service to the City of Seattle including large industrial areas such as the Port of Seattle. Due
to building codes within the City, many of the green roof examples in Washington State can be found in Seattle.
The Project Team reached out to SCL multiple times, and, at the time of this draft report, have not received
feedback.

Architects of Washington State
Architects of Washington State shared the driving forces for designing and building green roof projects. It is often
the case that a green roof is an exciting design idea but rarely gets built due to costs and fears around maintenance
from other trades in the building industry. The most practical application of green roofs for developers are those

prepared by E E S C O N S U L T I N G 2 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

that allow occupiable space and where solar panels can be incorporated as a shade structure for the building
occupants.

4.1 G RE E N RO O F INSTAL LE R S
The Project Team contacted a total of 26 service providers believed to be engaged in Green Roof (GR) installations
in the State of Washington. The pool of service providers that were contacted, and the interview questions, are
included in this report as Appendix A.

A priority was placed on obtaining the following information from each of the service providers:

 Confirming that each company installs green roofs and the extent to which green roofs make up total
business for each company
 Geographical service area and type(s) of installations (i.e. new, retrofits, etc.)
 Building types (commercial, multi-unit residential, etc.)
 Average costs of green roof installation and of required operation and maintenance (O&M)
 Known/common incentives for building owners
 Impediments/challenges/lessons learned associated with green roofs
 Role of/involvement with architects or engineers

An e-mail was sent to each company with the survey questions. The Project Team then followed up with
telephone contact. A total of 8 parties ultimately responded and provided information. A communications record
is included as Appendix B. Responses to individual questions were significantly variable. Key findings from the
survey are provided below.

1. The limited amount of cost data gathered from this survey shows considerable variability and should be
viewed with caution; it was not feasible to standardize the criteria for cost information in the context of
the interviews.
2. Respondents opined that green roof incentives will increase the amount of green roof construction with
the caveat that the process to obtain the incentives is a simple process.
3. Respondents stated that policies encouraging green roofs need to be strong, but also need to motivate
users to not look for loopholes and workarounds.
4. Green roofs are currently implemented for occupiable space because it’s viewed by developers as
potentially bringing in more money or increasing asset value.
5. Developers are not interested in green roofs for sustainability because of the high overhead and
maintenance concerns around green roofs, including leaks.
6. Maintenance is the “make it or break it” to green roofs. Maintenance and installation companies need to
be properly trained, and probably certified.
7. Respondents reported their observations that the conventional roofing industry is not interested in
learning new green roof technologies and is generally skeptical of the entire concept.
8. Motivated building owners often lack the resources for green roof installations and are unwilling to
commit to the ongoing overhead associated with long term upkeep and maintenance.
9. There are mixed perspectives about combining green roofs and solar panels. Unless plant production is a
driving factor, green roof installers are skeptical of colocation.
10. The plant height to cool solar panels are thought to require an intensive green roof which is more
expensive and requires more structural support; it is not viewed as cost effective.
11. There is a desire from architects to design these buildings, but the architects observe a general reluctance
from the installers and contractors to get involved with implementation.

prepared by E E S C O N S U L T I N G 2 3 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

5 Costs and Benefits
The overall approach to collecting and summarizing benefits information focused preferentially on more recent
reports (i.e. last 10 years) specific to the Pacific Northwest, and supplementing that as needed (and when
available) with information originating from broader geographical area – namely, other areas in the U.S. Since so
much green roof (GR) work has been done in other countries (primarily western Europe and Australia), select
international information is also included in support of filling data gaps and identifying general trends.

Generally, most of the information reviewed is from academic research, with additional information provided by
various levels of government and sourced primarily from case studies, trade associations and service providers.

Multiple information sources have cautioned that many of the stated benefits of green roofs are highly dependent
on a variety of site-specific variables including those intrinsic to the green roof itself (i.e. design specifications, life
cycle) and extrinsic variables like climate, green roof operation and maintenance (O&M). Consequently, it is
difficult (and risky) to identify universal beneficial outcomes from all green roof installations; the research
summarized in this report focuses on generally reported trends and overall findings, and includes qualifiers
influencing resultant benefits, where these are identified.

5.1 STO RMWATE R Q UALITY
A very large body of information on the benefits of green roofs related to stormwater (SW) quality has been
generated over the last 20 years.8 Studies have evaluated different plant bed media and plant species. Many of
these studies have focused on plant nutrients and to a lesser extent, trace metals – both groups being prominent
in urban stormwater runoff. Depending on the airshed, rainwater is relatively pure in most cases; constituents are
limited to solids (dissolved or in suspension and possibly metallic components). Rainwater may also have an acidic
component.

The quality of SW runoff from a green roof can be considered both from a pollutant concentration and a total
pollutant amount (mass loading) standpoint. Green roofs are known to have the capacity to retain a significant
portion of precipitation, to slow the release of retained precipitation, and to release precipitation back into the
atmosphere through plant evapo-transpiration. Looking at SW quality in terms of mass loading, the mechanisms
described above can reduce the total quantity of SW that would otherwise be discharged, resulting in an overall
decrease in polluting constituents.

Looking strictly at water quality (WQ) however, reviewed information presents a “mixed bag.” Green roofs can be
either a sink or a source of water pollutants, depending on whether plants and the growing substrate take up or
release constituents as water passes through.

The primary reported9 determining factors influencing SW quality are as follows:

8
Akther et. al., “A Review of Green Roof Applications for Managing Urban Stormwater in Different Climatic Zones”,
Sustainability 2018, 10(8), 2864
9
Gnecco et. al. “The Role of Green Roofs as a Source/Sink of Pollutants in Storm Water Outflows”, Water Resou rces
Management Vol 27, pgs. 4715–4730 (2013)

prepared by E E S C O N S U L T I N G 2 4 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

 Climate, seasonal and hydrologic variables (seasonal and rainfall influences)
 Factors effecting constituent update or release from green roof growing beds
 Bed depth (intensive/extensive) and substrate (i.e. mineral and organic content)
 Vegetation type and health
 Fertilizer or other applications (nutrient and pH profile)
 Age of the green roof and maintenance practices

In the context of SW discharges from green roofs, the primary WQ constituents of concern, particularly in
developed watersheds, are nutrients (phosphorus (P) and nitrogen (N) compounds). The contributions from green
roofs of other constituents impacting stormwater discharges to an urban watershed (i.e. zinc, copper, petroleum
compounds) is typically assumed to be minimal.10

Research papers reviewed indicate that green roofs can be a source or a sink of P and N; for example,
concentrations of these nutrients were generally found to be higher in new green roof installations and to
decrease over time, as plant uptake stabilizes, and the growing medium ecosystem matures.11 However, site-
specific factors and short-term variations, typically caused by poor green roof management techniques, can result
in significant release of chemical constituents from a green roof. For example, excessive use of fertilizers,
particularly during wet seasons, can release concentrated “slugs” of nutrients to the local watershed. Given that,
a scenario involving a number of these poorly maintained green roofs discharging excessive nutrients to nearby
surface waters (i.e. a nearby pond or recreational lake) could contribute to eutrophication problems. This
underscores the importance of continued care and maintenance of green roofs throughout the duration of their
useful life.

The information reviewed generally concludes that healthy, well-maintained green roofs either benefit SW quality
or have a net neutral effect on the quality of discharged SW over the long term, the overall benefit of green roofs
to SW quality is generally positive, primarily due to stormwater retention and moderation of flow volumes over
time, and to a lesser extent, to the filtering and natural attenuation of water-borne constituents by plants and the
growing substrate. Stormwater quality impacts can vary significantly across specific green roof sites and over time
periods at any specific site, however, the overall impact to the runoff WQ within a given watershed, however, may
be minimal.

5.2 IMP RO V E D AIR Q UALITY (AQ ) AND RE DUC E D G RE E NHO USE G AS (G HG )
E MISSIO NS
Vegetation can improve air quality in the local environment. Larger shrubs and trees can intercept air borne solid
particles (such as dust, diesel soot particles, etc.) directly onto their foliage. Rainwater picks up airborne
contaminants directly as it falls through the atmosphere, and deposits it on plant surfaces. Plants can directly trap
aerosol particulates potentially containing contaminants. By these mechanisms, contaminants are transferred
from the atmosphere and deposited within or on the plants or are transported down into the growing medium
(substrate).

10
Okita, et. al., “Effect of Green Roof Age on Runoff Water Quality in Portland, Oregon”, Journal of Green Building (2018) 13
(2): 42–54
11
Lim, “What happens to nitrogen and phosphorus nutrient contributions from green roofs as they age? A review”,
Environmental Advances, Vol. 12, July 2023, 100366

prepared by E E S C O N S U L T I N G 2 5 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

In addition to direct air pollution removal mechanisms, plants improve air quality by converting carbon dioxide
(CO2) into oxygen through photosynthesis. CO2 is a major greenhouse gas (GHG) emission, so plants play a major
role in countering global warming. Green roofs can contribute to reducing GHG emissions as plants take up CO2
and sequester carbon within plant growth through photosynthesis. Green roofs could be credited as a carbon sink
in a cap-and-trade system, which provides a mandatory cap on carbon emissions.

Green roofs indirectly reduce GHG emissions through their contribution to cooling and insulating a building’s heat
envelope. Insulation provided by green roofs can reduce the amount of energy needed to moderate building
temperatures, since roofs are the site of the greatest heat loss in the winter and the hottest temperatures in the
summer. The specific amount of energy saved depends on a variety of site and setting -specific factors. The
temperature moderating effects of green roofs can reduce demand for electrical power generation needed for
building heating and cooling, thereby potentially decreasing the amount of CO2 and other pollutants being
released into the air. For example, research published by the National Research Council of Canada found that an
extensive green roof reduced the daily energy demand for air conditioning in the summer by over 75%. 12 (Liu,
2003)

Numerous studies over the past 20 years reveal that green roofs directly sequester substantial amounts of carbon
in plants and soils through photosynthesis, countering the effects of global warming by reducing GHGs. 13 The two
mechanisms – direct carbon sequestration and reduction in building power demands – are represented graphically
in the figure below.

FIGURE 5.1: GREEN ROOF CARBON FOOTPRINT IMPACTS ( SEYEDABADI , 2021)
14

12
Liu et. al., “Thermal Performance of Green Roofs Through Field Evaluation”, In Proceedings: The First North American Green
Roof Infrastructure Conference, Chicago, IL, 2003 (pgs. 1-10)
13
Shafique, et. al., “An overview of Carbon Sequestration of Green Roofs in Urban Areas”, Urban Forestry & Urban Greening,
Vol. 47, January 2020, 126515
14
Source: Seyedabadi et. al., “Plant Selection for Green Roofs and Their Impact on Carbon Sequestration and the Building
Carbon Footprint” Environmental Challenges, Vol. 4, August 2021, 100119

prepared by E E S C O N S U L T I N G 2 6 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

As an example of the research conducted in this area, a 2021 study15 modeling buildings with green roofs found
that popular sedums resulted in sequestration of 0.029 to 0.424 lbs of CO 2 per square foot per year. For
perspective, a 2009 report on green roofs in Seattle WA estimated the total square footage at 359,375.16 Using
the lower carbon sequestration factor (0.029) from above, this amount of green roof area would correspond to
5.2 tons of sequestered CO 2. Using the same lower factor, a national survey in 2018 estimated the total square
footage of green roof in the US to be 3,112,818, 17 corresponding to 45 tons of sequestered CO 2.

Using the carbon sequestration metric from the building simulation study, and the social cost of carbon, each
square foot of extensive green roof has a carbon sequestration value of approximately $0.01 per year
($2023). Intensive green roofs store more CO2 resulting in an average value of $0.018/square foot per year using
the carbon sequestration values estimated in a 2017 study in Mexico City.18

Building development density impacts surface temperatures in many cities around the world where urban
properties experience markedly warmer temperatures compared with the surrounding countryside. These areas
are called “urban heat islands” (UHI). The effect occurs because most urban surfaces are typically buildings, roads
and sidewalks that tend to absorb heat from the sun. Rooftops typically make up 5 to 35 percent of the urban
landscape.19 In addition, urban areas have much less “green canopy” provided by trees and other plants to provide
cooling through evapotranspiration, when heat energy is dissipated as water evaporates and transpires from
vegetation. In addition, light absorbed by vegetation would otherwise be converted into heat energy.

The U.S. Environmental Protection Agency (EPA) reports that over 90% of the rooftops in the United States are
dark in color. Since dark surfaces reflect very little sunlight, they absorb all the incoming heat. As a result, city roof
surfaces can reach temperatures of 150°-190°F (66°-88°C) during the summer.20 At these elevated temperatures,
cooling interior building spaces requires much more electricity. Urban air pollution, especially photochemical
smog is intensified by the heat island effect. Consequently, heat island effects increase heat-related illness and
mortality, in addition to increasing building energy consumption and related GHG generation.

Green roofs help reduce the urban heat island effect by replacing black heat-absorbing surfaces with vegetation
that can significantly reduce local temperatures in cities during the hottest times of the year, through the natural
functions of plants, during the diurnal dew point and evaporation cycle, plants on vertical and horizontal surfaces
are able to cool cities during hot summer months and reduce the UHI effect. One study found that green roof
temperatures are within ambient air temperatures whereas conventional roofing temperatures are 7 degrees
Celsius higher than ambient temperatures.21

15
See id.
16 McIntosh, “Green Roofs in Seattle - A Survey of Vegetated Roofs and Rooftop Garden”, prepared for the City of Seattle,

Dept of Planning & Development and the University of Washington Green Futures Lab, August 2010
17
Green Roofs for Healthy Cities, “2018 Annual Green Roof Industry Survey Executive Summary”, (www.greenroofs.org),
October 2019
18
Collazo-Ortega, Margarita, Ulises Rosas and Jeronimo Reyes-Santiago. Towards Providing Solutions to the Air Quality Crisis
in the Mexico City Metropolitan Area: Carbon Sequestration by Succulent Species in Green Roofs. March 2017.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5400495/#:~:text=The%20species%20displayed%20their%20typical,meter
s%20in%20the%20short%20term.
19
U.S. Environmental Protection Agency. 2008. Green Roofs, in “Reducing Urban Heat Islands: Compendium of Strategies”.
Draft. https://www.epa.gov/heat-islands/heat-island-compendium
20
See id.
21
William Reshmina et al. An Environmental cost-benefit analysis of Alternative Green Roofing Strategies. Ecological
Engineering Volume 95, October 2016. https://www.sciencedirect.com/science/article/abs/pii/S0925857416304165

prepared by E E S C O N S U L T I N G 2 7 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Intensive green roofs have the potential to provide more cooling and energy savings benefits than extensive green
roofs. The increased soil depth and plant density of intensive green roofs prevents heat from moving down to
interior ceiling heights. Studies at the National Research Council of Canada (2003) have shown that green roofs
can reduce the heat flow through the roof by 70% to 90% in the summer and 10% to 30% in the winter. 22 This
effect is more significant in hot, dry climates than in humid or temperate climates.

Data was not available for quantifying the non-building value of reduced UHI effects in urban areas. Anecdotally,
reduced surface temperatures will have greater community health benefits in lower-income communities due to
the higher share of impermeable surfaces found in those communities. A California study found that ambient
temperatures were as much as 5 degrees warmer in low-income neighborhoods compared with median income
neighborhoods.23 Green roof temperature effects on very warm days have the potential to positively impact
comfort and reduce heat related illness and mortality. This potential is particularly important in low income or
other disadvantaged communities where temperatures are generally warmer compared with temperatures in
median income or more affluent communities.

5.3 FO O D P RO DUC TIO N
Green Roofs as spaces for urban agriculture is not a new concept but in general has not yet seen wide- spread
application, despite the potential to provide new opportunities for local urban food production, a nd for creating
potential economic and social benefits not available on conventional roofs. Depending on the size and scale of
food production, fresh produce could be marketed to local businesses and restaurants or could help meet the
needs of the building occupants.

Green roof food production provides a number of potential benefits, including the following:

 Increased property values and added marketability
 Potential venue to educate urban residents about food production
 Revenue and reduced costs associated with providing fresh produce locally
 Environmental benefits from reduced transportation, potentially less packaging and localized control of
agricultural chemicals
 Increased security of the growing operation and isolation from damage by vermin
 Support of the local economy in growing, processing and distributing, and through potential job creation

Although some crops such as herbs can be grown in extensive green roofs, growing food crops would typically
require deeper growing media which requires specifically designed and engineered green roofs that can
structurally support intensive beds and ancillary systems for plant nutrition and irrigation. 24 Many (if not most)
existing large flat roofs may not have the loading capabilities to support food crop production, some roofs will,
and more new roofs will be appropriately designed as the concept enters the mainstream. In addition, many
existing balconies in urban areas are ideal for small-scale food production.

22
Tolderlund, “Design Guidelines and Maintenance Manual for Green Roofs in the Semi-Arid and Arid West”, University of
Colorado, Denver CO, 2010
23 Dialesandro, John et al. Dimensions of Thermal Inequity : Neighborhood Social Demographics and Urban Heat in the

Southwestern U.S. Environmental Public Health 2021.
24
United States General Services Administration, “The Benefits and Challenges of Green Roofs on Public and Commercial
Buildings”, May 2021

prepared by E E S C O N S U L T I N G 2 8 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Several vegetable growing rooftop gardens have been implemented in Seattle 25 including single family
residences. The Stack house Apartments is home to a 1,000 square foot roof top garden. The garden is
maintained by a third party and produce is provided free to the apartment residents. These apartments are
considered premium value with monthly rent currently in the range of $1,700-$12,000. Similarly, the Angeline
apartments includes a 1,000 square foot edible garden that offers residents u-pick foods 10 months out of the
year. A studio apartment at Angeline starts at $2,100 per month. These rent levels are typical of Seattle
apartments suggesting that edible gardens add value without increasing the monthly cost.

In another example, Quality Athletics in Pioneer Square has implemented a rooftop edible garden where the
produce is served in the restaurant. Restaurant seating and gathering space complement the rooftop garden.

5.4 BIO DIV E RSITY, E C O SYS TE M HE ALTH AND HUMAN Q UALIT Y O F LIFE
C O NSIDE RATIO NS
Healthy green roofs can sustain a variety of plants and invertebrates, and provide new habitat in urban areas,
increasing local biodiversity. They provide excellent urban habitat for various bird species, and by acting as a
transitory habitat for migrating birds they contribute to linking sustaining habitat that would otherwise be
fragmented.

The most important factors in encouraging biodiversity in a green roof are vegetation type, growing medium depth
and variation in plant height and spacing.26 In addition, green roof topography and adjacent local landscape areas,
can affect a roof’s ability to enhance biodiversity.

Increasing biodiversity and available habitat can provide significant benefits in three fundamental areas:

Ecological – Diverse ecosystems are better able to support higher and more robust levels of productivity and
provide resilience to potential changes or disturbances compared with those hosting fewer species.
Economic – Healthy and stable biodiversity supports economic activity and provide effective and ecosystem
services (maintains hydrological cycles, air and water quality, storage and cycling nutrients)
Social-Environmental diversity and visual aesthetics can have positive impacts on community health and
human well-being.

Green roofs provide additional benefits to humans. The ability of green roofs to reduce air pollution and improve
water quality has direct positive influence on overall human health and associated demands for healthcare. Green
roofs can serve as community hubs, increasing social cohesion, sense of community, and public safety.

Green roofs are more effective at noise reduction especially in areas with heavy motor or air traffic, and at
countering urban heat island effects than convention roofs. Green roofs are an attractive space for tenants (and
for occupants of neighboring buildings), providing a place of refuge recreation and relaxation, potentially reducing
stress and improving worker productivity. They can offer quality outdoor spaces that may add value for building
owners. Green roofs are an important component of “urban greening” and the larger strategy for beautifying the

25
https://www.seattleurbanfarmco.com/
26
Kessling et. al., “Feasibility of Combining Solar Panels and Green Roofs on the Activities and Recreation Center”

prepared by E E S C O N S U L T I N G 2 9 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

built environment and increasing investment opportunity. Aesthetic and quality of life benefits from green roofs
are tangible but difficult to quantify.27

5.5 STO RMWATE R Q UANTITY AN D P E AK FLO W BE NE FI TS
Stormwater benefits include reduced total runoff, reduced peak flow and contribution to Combined Sewer
Overflow (CSO) events. The Seattle climate (and much of Western Washington) experiences periods of extensive
rainfall followed by very dry summer months. During storm events the wet weather results in CSO events where
the partially separated sewer and storm drain systems become overloaded. The City of Seattle plans its capital
outlay to reduce these events to 1 outfall per year for each of the 82 sewer outfalls. Reducing or delaying
stormwater runoff for new buildings could impact the City’s capital improvement expense. Additionally, reducing
stormwater flows through treatment facilities will have a direct impact on the variable cost of operating the
stormwater utility.

A study by the University of Washington28 provides some specific data for green roofs in the Seattle area. This
study reiterated the difficulty in quantifying the hydrologic performance of these structures. Over a 12-month
period, 5 test panels were used of varying growing depths, plant types, drainage media and layers. The study
resulted in a wide variation of rainfall retention (30-56 percent) over one year, with better performance by panels
with deeper soil and a layer of moisture retention fabric. The best retention occurred in all panels during the
month of August when the least amount of rainfall occurred, with the worst retention happening, when back-to-
back rainstorms occurred in November. The results mirror what is found in other studies: the factors that
determine the amount of stormwater retained include the surface area of the roof, the depth and type of growing
medium, the slope and type of plants selected. 29

Reduced or delayed runoff will be impacted by several site-specific conditions including the following:

 Size, shape, and slope of roof
 Antecedent soil moisture conditions
 Magnitude, duration, and distribution of rainfall event
 Vegetative conditions, and
 Runoff travel path.30

The capacity of green roof systems to provide seasonal runoff volume reduction varies. The effectiveness is
reduced for sustained or repeated storm systems during the rainy season. However, the reduction in peak flow
was found significant in a Seattle Study. The peak flow reduction for three of the sampled green roofs varied from
-15-63%.31 The percentage of rainfall measured in runoff was also measured. The findings are summarized in
Table 5.2.

27
United States General Services Administration, “The Benefits and Challenges of Green Roofs on Public and Commercial
Buildings”, May 2021
28
Yocom, Ken and Ben Spencer. Greenroof Performance Study: Puget Sound Region. Seattle WA. 2012
/https://thecela.org/wp-content/uploads/GREENROOF-PERFORMANCE-STUDY.pdf
29 U.S. Environmental Protection Agency. Stormwater Best Management Practice. Green Roofs. December 2021. Available

at: www.epa.gov/system/files/documents/2021-11/bmp-green-roofs.pdf
30
Cardno TEC, Inc. Green Roof Performance Study. Prepared for Seattle Public Utilities. June 2012. Available at :
https://www.seattle.gov/documents/Departments/OSE/Green-Roof-Performance-Study-2012.pdf
31
See id.

prepared by E E S C O N S U L T I N G 3 0 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 5.2: SUMMARY OF FINDINGS GREEN ROOF PERFORMANCE STUDY, 2012
Peak Reduction Runoff Volume as a
Roof Estimate Percentage of Rainfall Other Details
Seattle Facilities: Fire 7% - 40% 32% - 100% 2” drainage later + 4” growth layer
Station 10 (FS10) 6,400 square feet.
Slope 1:24
Shorter runoff path and impervious surfaces
compared with EOC
Seattle Facilities: 35% - 65% 27% - 93% 2” drainage later + 4” growth layer
Emergency 7,475 square feet
Operations Slope 1:24
Center (EOC)
Zoomazium -15% - 63% 7% - 87% 6” soil, vegetation quality high
Woodland Park Zoo 8,000 square feet
Slope variable: 0:12 to 3:12

Another study estimated 10-25% annual runoff reductions could be achieved when only 10% of the Seattle
watershed is converted to green roof. This study found these results based on simulation analysis of the Seattle
area.32

The DOE manual for Western WA refers to a runoff model using modeling software to represent the changes to
stormwater “flow control” when utilizing a green roof. This model, along with any possible credits from local
agencies, is what is used by engineers to determine if a green roof is viable for new development.

Simlar DOE screening criteria for Eastern Washington is determined by the wet load capacity of the roof due to
wet soil and snow loads customary to that part of the state. Models of two different-sized roofs (average 34,355
and office 22, 970 sq ft) were undertaken using the Western Washington Hydrology Model;33 the percentage of
mitigation from a flow and percentage perspective were relatively the same for both sizes (about 51.6%
reduction). The model output did not change when the roof slope or depth of soil were increased; these variances
have more impact to ongoing maintenance than to stormwater mitigated.

5 .5 .1 St or mw at er Benefit Mod eling
Arizona State University has developed a calculator for measuring energy and stormwater savings resulting from
green roofs.34 The calculator results were the same for any size building, similar to the modeling efforts previously
discussed. In addition, it only had Seattle and Spokane, so Fresno CA was used since it has similar rainfall to
Yakima. While most installations recommend irrigation of some kind, drought-tolerant species of plants in
Western WA can prosper once established without irrigation. Vendors surveyed for this project stated that
irrigation in Western Washington is recommended in the first 1-2 years. All plant mediums will require some

32
Barnhart, Brad et al. Modeling the hydrologic effects of a watershed-scale green roof implementation in the Pacific
Northwest United states. Journal of Environmental Management Volume 277, 1 January 2021.
33
Solutions, Clear Creek. Western Washington Hydrology Model Version 4.2.19. Accessed April 2023.
34
Arizona State University. https://sustainability-innovation.asu.edu/urban-climate/green-roof-calculator/ NOTES:
Over the course of a simulation year the net water inflow may not balance outflow due to changes in soil moisture. Also,
water balance dynamics are sensitive to growing media composition, compaction, etc. As these variations are not captured in
the present tool, the runoff results should be considered as order-of-magnitude estimates.

prepared by E E S C O N S U L T I N G 3 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

degree of maintenance and re-planting over the life of the roof. This would offset the reduced maintenance of
the roof itself as the membrane, when protected by sunlight, has a longer service life than a typical roof.

TABLE 5.3: CHANGES IN STORMWATER RUNOFF WITH GREEN ROOFS
Seattle Spokane Yakima*
Rainfall, Inches 34.8 14.5 7.6
Scenario 1: 70, 4, Y
Evapotranspiration 18.8 15.2 12.2
Irrigation 11.7 11.7 11.7
Net Runoff 26.4 11.1 8.2
Reduced Runoff 8.4 3.4 -0.6
gal/year 62.748 25.398 -4.482
Scenario 2: 30,4,Y
Evapotranspiration 8 6.5 5.2
Irrigation 5 5 5
Net Runoff 31.2 13 7.9
Reduced Runoff 3.6 1.5 -0.3
gal/year 26.892 11.205 -2.241
Scenario 3: 50, 6, Y
Evapotranspiration 13.4 10.9 8.7
Irrigation 8.4 8.4 8.4
Net Runoff 28.8 12.1 8
Reduced Runoff 6 2.4 -0.4
gal/year 44.82 17.928 -2.988
Scenario 4: 70, 6, Y
Evapotranspiration 19.7 15.6 12.8
Irrigation 11.7 11.7 11.7
Net Runoff 25 10.4 8
Reduced Runoff 9.8 4.1 -0.4
gal/year 73.206 30.627 -2.988
Scenario 6: 70, 6, N
Evapotranspiration 14 9.7 6.9
Irrigation 0 0 0
Net Runoff 19.1 4.7 2.3
Reduced Runoff 15.7 9.8 5.3
gal/year 117.279 73.206 39.591
Scenario 7: 50, 6, Y
Evapotranspiration 14.1 11.1 9.1
Irrigation 8.4 8.4 8.4
Net Runoff 27.9 11.6 7.9
Reduced Runoff 6.9 2.9 -0.3
gal/year 51.543 21.663 -2.241
Scenario 8: 30, 6, Y
Evapotranspiration 8.5 6.7 5.5
Irrigation 5 5 5
Net Runoff 30.7 12.7 7.8
Reduced Runoff 4.1 1.8 -0.2
gal/year 30.627 13.446 -1.494

prepared by E E S C O N S U L T I N G 3 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

*Fresno, CA used to model Yakima. Annual rainfall in Yakima is 8.0 inches.
Scenario legend (Green Roof Coverage %, Media Depth, Irrigated Y/N)
All scenarios were given a “leaf index” value of 2. (Bare ground =0 Dense Forest = 6)

The above analysis does not appear to consider the seasonality of irrigation in each of the regions. Therefore,
the Project Team evaluated how irrigation systems would operate in each month to determine the net runoff
where irrigation is needed only in dry months. The charts below illustrate the monthly rainfall days.35 It was
assumed that months with less than 1.5% rainfall days would require irrigation. The irrigation contribution to
annual run off is then adjusted based on that assumption. Figure 5.1 shows the months where irrigation would
impact runoff volumes (yellow shaded months). Tables 5.4 through 5.6 show the impact of the seasonality
adjustment for irrigation. Spokane soil depths were adjusted to 6 inches for extensive and 10 inches for intensive
so that irrigation systems were no longer needed.

35
Data sourced from Weather-us.com.

prepared by E E S C O N S U L T I N G 3 3 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

FIGURE 5.1: MONTHLY RAINFALL DAYS DATA

Jan 8.8 2.41%
Feb 7.8 2.14%
March 8.9 2.44%
April 8.1 2.22%
May 7 1.92%
June 5.4 1.48%
July 4.2 1.15%
Aug 2.8 0.77%
Sept 6 1.64%
Oct 7.2 1.97%
Nov 9.1 2.49%
Dec 8.9 2.44%

Jan 9.2 2.52%
Feb 8.4 2.30%
March 15.9 4.36%
April 16.3 4.47%
May 13.4 3.67%
June 11.7 3.21%
July 4.9 1.34%
Aug 5.8 1.59%
Sept 7.5 2.05%
Oct 12.9 3.53%
Nov 12.2 3.34%
Dec 9.6 2.63%

Jan 10.3 2.82%
Feb 7.8 2.14%
March 10.8 2.96%
April 7.8 2.14%
May 6.8 1.86%
June 4.8 1.32%
July 2.1 0.58%
Aug 2 0.55%
Sept 3.9 1.07%
Oct 8 2.19%
Nov 8.8 2.41%
Dec 9.1 2.49%

prepared by E E S C O N S U L T I N G 3 4 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

T AB LE 5 .4 : SE AT T LE G RE EN ROOF N ET RUN OFF
Seattle

Rainfall 34.8 Based on rainfall days for the
Scenario 1a: 70, 4, Y Seattle region, months with the
Evapotranspiration 18.8
least amount (less than 1.5% was
Irrigation 1.95 (2/12 of 18.8)
Net Runoff 17.95 chosen arbitrarily for all cities)
Scenario 2a: 30,4,Y are July and August or 2/12
Evapotranspiration 8.0 months of the year. Irrigation
Irrigation 0.83 (2/12 of 5.0) rates from earlier scenarios were
Net Runoff 27.63 adjusted to reflect irrigation for
Scenario 3a: 50, 4, Y only two months. The net runoff
Evapotranspiration 13.4
Irrigation 1.4 (2/12 of 8.4)
was then approximated. The
Net Runoff 22.8 resulting runoff is considerably
Scenario 4a: 70, 6, Y reduced for the roofs with the
Evapotranspiration 19.7 largest area.
Irrigation 1.95 (2/12 of 11.7)
Net Runoff 17.05

TABLE 5.5: SPOKANE GREEN ROOF NET RUNOFF
Spokane
Rainfall 14.5
Scenario 6: 70, 6, N (from above) Based on rainfall days for the
Evapotranspiration 9.7 Spokane region, there is
Irrigation 0 consistent rain each month (only
Net Runoff 4.7 one month less than 1.5%). For
Scenario 6a: 30, 6, N this reason, the model was
Evapotranspiration 4.1
Irrigation 0
adjusted by increasing the
Net Runoff 10.3 amount of growing media in the
Scenario 6b: 70,10,N soil from 6-inches (Scenario 6
Evapotranspiration 9.8 above) to retain the rainfall
Irrigation 0 instead of irrigating. The end
Net Runoff 4.4 result had a very slight reduction
Scenario 6c: 30,10,N
in net runoff.
Evapotranspiration 4.2
Irrigation 0.0
Net Runoff 10.2

prepared by E E S C O N S U L T I N G 3 5 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 5.6: YAKIMA GREEN ROOF NET RUNOFF
*Yakima
Rainfall 7.6 Based on rainfall days for the
Scenario 1a: 70, 4, Y
Yakima region, there are 4
Evapotranspiration 12.2
Irrigation 3.9 (4/12 of 11.7) months with the least amount
Net Runoff 0 (less than 1.5% was chosen
Scenario 2a: 30,4,Y arbitrarily for all cities) rainfall,
Evapotranspiration 5.2 or 4/12 months of the year.
Irrigation 1.7 (4/12 of 5.0) Irrigation rates from earlier
Net Runoff 4.1
scenarios were adjusted to
Scenario 3a: 50, 4, Y
Evapotranspiration 8.7 reflect irrigation for only four
Irrigation 2.8 (4/12 of 8.4) months. The net runoff was then
Net Runoff 1.7 approximated. The resulting
Scenario 4a: 70, 6, Y runoff is considerably reduced
Evapotranspiration 12.8 for the roofs with the largest
Irrigation 3.9 (4/12 of 11.7) area.
Net Runoff 0
*Modeled based on Fresno, CA annual rainfall inches .

The resulting stormwater reduction estimation is presented in gallons per year below. The irrigation impacts
results in negative gallons or an increase in runoff. This finding will be analyzed further in the next phase of our
analysis. The modeling above does not consider the seasonality of irrigation vs. stormwater storage or delayed
runoff.

TABLE 5.7: CHANGE IN RUNOFF PER SQUARE FOOT OF GREEN ROOF PER YEAR
70% Green
Roof 30% Green Roof 50% Green Roof
Gallons/year Extensive Extensive Intensive Extensive Intensive
Commercial Commercial Multifamily Commercial Multifamily
Yakima 5 2 0 4 4
Spokane 6 3 3 7 7
Seattle 10 4 5 7 8

Valuing the change in stormwater runoff is challenging since stormwater systems are sized to handle specific
maximum flow events. The majority of stormwater costs are fixed; however, reduction in peak flows and
quantities will reduce the capital investment needed in future years. Stormwater utility infrastructure in each of
the three selected regions will vary depending on total rainfall, watershed size, development level, and severity
of storm events As mentioned above, the stormwater infrastructure in Seattle is partially combined with the
sanitary sewer system. This is the case for many urban areas originally designed and contucted with combined
systems. City stormwater budgets were reviewed and compared with the quantity of stormwater handled by the
stormwater system. Treatment costs for stormwater are considered variable costs. Spokane and Seattle both
treat some of the stormwater runoff captured in the combined sewer systems. Yakima does not currently treat
stormwater. The table below summarizes the annual stormwater treatment water expenses for each of the cities
and estimates the unit cost of one gallon of stormwater. The values for Seattle are highest given the amount of
annual rainfall and population density. The quantity of treated stormwater in gallons was not available from each

prepared by E E S C O N S U L T I N G 3 6 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

City. Therefore, the resulting variable values should be considered indicative of the relative importance of
stormwater management rather than the true cost of avoiding 1 gallon of stormwater.

TABLE 5.8: ANNUAL STORMWATER COSTS, PER GALLON
Stormwater Gallons of Stormwater
Treatment Budget per Year $/Gallon of
(2024) (treated and untreated) Stormwater
Seattle $9,800,000 36 400,000,000 37 $0.0245
Spokane $8,798,550 38 1,000,000,000 39 $0.0088
40 41
Yakima $0 275,000,000 $0.0000

Combining the above values results in the value of stormwater reduction per square foot of green roof for each
city as shown in Table 5.9.

TABLE 5.9: ANNUAL STORMWATER BENEFITS, PER SQUARE FOOT ROOF AREA
70% Green
Roof 30% Green Roof 50% Green Roof
Gallons/year Extensive Extensive Intensive Extensive Intensive
Commercial Commercial Multifamily Commercial Multifamily
Yakima $0.00 $0.00 $0.00 $0.00 $0.00
Spokane $0.05 $0.02 $0.02 $0.06 $0.06
Seattle $0.26 $0.11 $0.12 $0.18 $0.19

Deferred capital investments due to reduced stormwater quantities were not estimated. Current stormwater
capital budgets address current needs that are unlikely to change in the near term are the result of green roofs
on new construction. In order to estimate the deferred capital investment, detailed studies would need to be
conducted that evaluate planned infrastructure, new development vs existing issues, and flow impacts from green
roofs on new development. These studies are not readily available; therefore, the value is not included in the
benefit-cost analysis.

36
City of Seattle Drainage Wastewater Rate Study.
https://www.seattle.gov/documents/Departments/SPU/Services/Rates/2022-2024_DrainageWastewater-RateStudy.pdf
37
Seattle Public Utilities and King County. Green Stormwater Infrastructure 2017-2018 Overview and Accomplishment
Report. https://www.seattle.gov/documents/Departments/SPU/Documents/Reports/GSI-ProgressReport2018.pdf
38
City of Spokane 2023 Adopted Budget. https://static.spokanecity.org/documents/budget/2023/2023-adopted-budget-
detail.pdf
39
City of Spokane notes 1 billion gallons of stormwater runoff feeding the Spokane River.
40
City of Yakima 2024 Preliminary Budget. https://www.yakimawa.gov/services/finance/files/2023-2024-Revised-
Preliminary-Budget-Qtr-Amend.pdf
41
Estimated based on annual rainfall and land area.

prepared by E E S C O N S U L T I N G 3 7 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

5.6 BUILDING P E RFO RMANC E E NE RG Y IMP AC TS
Our analysis of the energy effects on agrivoltaic buildings began with a literature review of applicab le studies,
primarily scientific journal studies and white papers. It was observed that the prevalence of available data focuses
on the thermal resistance (R-value) of a green roof assembly, often in cooling climates where the insulative
benefits would be greater as compared to a black commercial roof, but some data did exist for temperate or
heating climates. From these reports, there did not seem to be a universally aligned expectation for the green
roof impact on the building energy due in large part to the variation in design, installation practices, and limited
number of projects with available data. Rather, there was a range of estimated impacts on the savings, from
minimal/zero to 30%. Further, as the primary focus of the available reports was on the thermal transfer through
the green roof assembly itself, little analysis was performed on the overall energy impacts to the building heating
and cooling systems.

Some reports noted a calculable energy usage reduction with no real utility billing or building energy consumption
data to support the claims, often only discussing the thermal transfer benefits of the green roof components and
substrates as having energy reduction benefits.42

FIGURE 5.2: TEMPERATURE DATA FOR VEGETATED COMBINATIONS (CELIK, PURDUE 2010)

Following this initial research on market data, our Team selected the DesignBuilder 3-D modeling program for use
in performing energy models of the various building types in our study. The program has developed a solid
reputation in the building design industry for variability and is known to have a very reliable green/eco roof feature
that can be adjusted to fit almost any type of building. Backed by an EnergyPlus (Department of Energy) engine,
the program produced annual consumption outputs for electricity, gas, and water by end use so that EES Team
could compare the conventional roof performance to the intensive and extensive green roof coverage models at
30%, 50%, and 70% coverage. Temperature balance and heat transfer by component can be reviewed in these
models, as shown below in the Multifamily (low rise) 50% green roof model:

42
Besbes, Karim et al. Green Roofs Impact on Buildings Cooling Load. Purdue University 2012.
https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1068&context=ihpbc

prepared by E E S C O N S U L T I N G 3 8 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

FIGURE 5.3: MULTIFAMILY GREEN/ECO ROOF PARAMETER OUTPUT FOR SUMMER COOLING (JUN-SEPT)

Initial comparisons of the 50% coverage green roof models vs. standard construction commercial buildings in the
Seattle area showed a small but measurable difference in both heating and cooling energy. The differences are
more pronounced in areas with the higher heating degree days, with some heating loads actually increasing
notably due to the absence of a low albedo (reflective) roof decking. For example, the modeled green roof (50%)
“Assembly” facility in the Seattle area draws approximately 104 kBtu more annually to heat than its conventional
roof counterpart, but the heating energy differential for the modeled building in Yakima expands to approximately
3,074 kBtu more to heat the green roof version of the same building.

Similar differentials can be seen amongst the other building types, such as the low-rise Multifamily structure
(shown below as the 50% green/eco roof) where the Yakima based building has a 6,109 kBtu higher heating load

prepared by E E S C O N S U L T I N G 3 9 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

for the green roof than the standard dark albedo roof but a lower difference between the buildings in the Seattle
weather zone of 2,260 kBtu. As expected in both cases, the cooling load is higher for the conventional roof but by
a nominal amount.

FIGURE 5.4: LOW-RISE MULTIFAMILY WITH 50% GREEN/ECO ROOF

For purposes of modeling, the Project Team looked at the energy impact per square foot of building space and
determined that the heating and cooling energy differentials were not significant, as the registered values hovered
around .0001-.01 MMBtu/sf for any of the modeled scenarios. This is to be expected, given that the green/eco
roof installation technically sits outside of the building envelope, supported by a concrete roof decking in most
situations. Additionally, the climate zone impact is less remarkable in the relatively temperate cooling and heating
zones and more pronounced in the high temperature variability areas. Finally, conventional building science
theory points to the naturally low impact of the roof component of the commercial building shell in any scenario,
as the energy impacts of the vertical components (walls, windows, doors) and the building activity type are
traditionally more influential to the overall HVAC consumption.

5.7 LE ADE RSHIP IN E NE RG Y AND E NV IRO NME NTAL D E SIG N (LE E D) C RE DITS
LEED certification is recognized as a symbol of sustainability achievement and leadership. LEED buildings have
higher resale value and use less energy and water than non-LEED counter parts. At least 40 LEED credits are
required to become certified. A 2021 study found that a LEED certified office building was rented at a rate 11.1%

prepared by E E S C O N S U L T I N G 4 0 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

higher than a non-LEED counterpart and experiences less vacancy. Additionally, LEED-certified assets held value
21.4% higher than average market rates over non-LEED buildings.43

The value of LEED credits is estimated based on a marginal cost analysis. Green buildings cost on average just 2%
more than their non-green counterparts.44 These costs are primarily attributed to soft costs such as increased
architectural and engineering design time, modeling costs, and time needed to integrate sustainable building
practices. This is estimated to add $5 to $10 per square foot to building construction costs. LEED Certification
level can vary; however, the first 50 LEED credits (approximately Certified/Silver level) can provide significant
benefits in terms of energy and water use savings and building comfort.

This one-time benefit is realized in year 1. Afterward, a return on equity calculation is used to value the sustained
earnings attributable to the original investment. A return on equity value of 10% is used consistent with private
equity returns in the state of Washington. The annual benefit for each LEED credit is $0.04/square foot over the
study period of 15 years. This benefit assumes that the building owner installing the rooftop solar or green roof
has already decided to obtain LEED certification. The cost to become LEED-Certified can range significantly from
a few thousand to $60,000.45

5 .7 .1 G r een Roof LE E D C r ed it s
Depending on building design and integration with the building systems, green roofs could contribute as much
as 15 credits. Direct credits for green roofs may be obtained for the following:

 Reducing site disturbance, protect or restore open space (2 points)
 Landscape design that reduces urban heat islands (2 points)
 Stormwater management (2 points)
 Water efficiency landscaping (2 points)
 Innovative wastewater technologies (2 points)
 Regional materials (2 points)
 Building Efficiency (up to 19 points)
 Innovation in design (up to 5 points)

This study assumes that the average green roof would meet some but not all of the above requirements. To
avoid double counting benefits, where values are estimated elsewhere, such as stormwater, LEED credits for
stormwater management are not included. The following LEED credits are assumed based on the measure
design and construct of the benefit-cost analysis:

 Reduce Heat Islands (2)
 Regional Materials (2)
 Innovation in Design (intensive roof 5, extensive roof,1)

The total LEED credits attributable to extensive green roofs is 4 where the total LEED credits for intensive green
roofs is 9. The above credits are applied regardless of green roof area coverage. More points may be available

43
Cushman & Wakefield. Green is Good: Sustainable Office Outperforms in Class A Urban Markets. August 2021. https://cw-
gbl-gws-prod.azureedge.net/-/media/cw/americas/united-states/insights-pdfs/green-is-good-
spotlight_final1.pdf?la=en&rev=e26315797d7d49faa6c58ca7762f91a6&hash=7D9FDF170FDB5E46A373233AD8F2999F
44
Kats, Gregory H. Green building Costs and Financial Benefits. Massachusetts Technology Collaborative. 2003.
https://www.wbdg.org/files/pdfs/green_bldg_costs_kats.pdf Figure 1.
45
USGBC. Leed Certification Fees. https://www.usgbc.org/tools/leed-certification/fees

prepared by E E S C O N S U L T I N G 4 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

for higher coverages or additional features; however, for the purposes of this study, a conservative approach to
LEED credits is applied as a proxy for value related to the unquantifiable benefits. These unquantifiable benefits
include reduced heat islands, regional sourcing of materials, and aesthetic and use value for innovative design
(intensive only). Depending on the building type, the LEED value for green roofs is estimated to range between
$0.25 and $5 per square foot of total roof area.

5 .7 .2 Solar LE E D C r ed it s
Builders also accrue LEED credits for both rooftop solar panels and ground-mounted systems located at the project
site. The proposed rooftop solar measures would be considered Tier 1 (on-site generation) and could earn up to
5 credits:

 1 point = 2% electricity generation
 2 points = 5% electricity generation
 3 points = 10% electricity generation
 4 points = 15% electricity generation
 5 points = 20% electricity generation46

Based on the above point system and the solar generation as a share of estimated building energy use, Table 5.10
shows the build-up of the LEED credits. Because the study already values energy production from solar, the value
for the below LEED credits is not included in the benefit/cost analysis.

TABLE 5.10: SOLAR LEED CREDITS
Annual Solar kWh
Building
Building EUI Energy Use, 10% Solar 25% Solar 50% Solar
Size, SF kWh/SF Annual kWh Coverage Coverage Coverage1
Assembly 36,333 7.9 288,121 56,985 142,974 284,824
Grocery 23,625 40.2 950,434 55,867 138,506 278,123
Hospital 207,115 32.3 6,698,099 188,769 471,350
Lodging 51,923 10.9 565,441 44,696 111,723 223,393
Mixed Commercial 44,515 15.2 676,183 55,867 140,740 280,357
Office 58,308 11.8 690,367 67,039 168,664 337,320
Other 13,000 6.4 83,070 25,703 63,688 127,337
Residential Care 61,360 14.7 904,446 88,265 222,276 442,310
Retail / Service 78,842 9.6 756,095 176,482 440,076
School 73,000 7.4 539,470 145,208 363,009 726,007
Warehouse 71,838 3.7 267,237 139,623 348,489 698,083
Multifamily Low Rise (1-3) 15,897 9.7 154,201 2,236 4,471 8,941
Multifamily Mid Rise (4-6) 14,501 8.1 117,458 7,824 18,998 36,876
Multifamily High Rise (7+) 17,250 5.9 101,775 3,353 10,059 18,998
1. 1. Rooftops for hospitals and very large retail buildings are excluded from maximum solar coverage
scenarios.

46
https://unboundsolar.com/blog/do-you-need-leed-credits-heres-what-we-can-do

prepared by E E S C O N S U L T I N G 4 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 5.11: SOLAR LEED CREDITS
% Annual Usage LEED Credits: Solar
10% Solar 25% Solar 50% Solar 10% Solar 25% Solar 50% Solar
Coverage Coverage Coverage Coverage Coverage Coverage1
Assembly 19.8% 49.6% 98.9% 4 5 5
Grocery 5.9% 14.6% 29.3% 5 3 5
Hospital 2.8% 7.0% 0.0% 1 2
Lodging 7.9% 19.8% 39.5% 2 5 5
Mixed 8.3% 20.8% 41.5% 2 5 5
Commercial*
Office 9.7% 24.4% 48.9% 2 5 5
Other 30.9% 76.7% 153.3% 5 5 5
Residential Care 9.8% 24.6% 48.9% 2 5 5
Retail / Service 23.3% 58.2% 0.0% 5 5
School 26.9% 67.3% 134.6% 5 5 5
Warehouse 52.2% 130.4% 261.2% 5 5 5
Multifamily Low 1.5% 2.9% 5.8% 0 1 2
Rise (1-3)
Multifamily Mid 6.7% 16.2% 31.4% 2 3 5
Rise (4-6)
Multifamily High 3.3% 9.9% 18.7% 1 2 3
Rise (7+)
1. Rooftops for hospitals and very large retail buildings are excluded from maximum solar coverage scenarios.

5.8 G RE E N RO O FS INSTALL A TIO N C O STS AND O P E RATIO N & MAINTE N ANC E (O & M)
C O STS
Installation of a green roofs as an alternative to conventional roofs adds additional cost, but over time, savings
can be realized over the life of the roof through stormwater management benefits, increased roof life span and
building energy efficiency and improved real estate value.

Unit cost ranges quoted by service providers and found in published information vary widely, for both installation
and O&M. Unit cost varies with the size and type of GR, with larger installations costing less per square foo t, and
with extensive (“shallow”) systems costing less than intensive (deeper) systems. Other important cost drivers
include the following:

 New vs. retrofit green roof
 Materials and installation equipment
 Labor market
 Configuration of building and ease of access – for both installation and O&M
 Required O&M – for the life of the roof

The vendor survey conducted as part of this project indicated cost per square foot ranges of $5-$15 up to $50 -
$75. Reviewed published information (primarily from government agencies) indicated ranges of costs,
summarized in the table below.

prepared by E E S C O N S U L T I N G 4 3 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

prepared by E E S C O N S U L T I N G 4 4 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 5.12: LITERATURE REVIEW OF GREEN ROOF CAPITAL AND O&M COSTS
US General Services Administration Installed cost premium (incremental cost over a conventional roof) for multi-
(2011) course extensive (3-inch depth) green roofs ranges from $10.30 to $12.50 / sq.ft.
Installed cost premium (incremental cost over a conventional roof) for semi-
intensive (6-inch depth) green roofs ranges from $16.20 to $19.70/ sq. ft.
Annual maintenance for a green roof is typically higher than for a conventional
black roof, by $0.21 to $0.31 per sq. ft.

USEPA Office of Water, 4203M, “Sources often cite the total cost of a green roof as $15 to $35 per square foot,
Stormwater BMPs - GR (2021) with cost per square foot decreasing as size increases
USEPA The costs of green roofs vary depending on the components, such as the growing
Reducing Urban Heat Islands: medium, type of roofing membrane, drainage system, use of fencing or railings,
Compendium of Strategies (Draft), and type and quantity of plants. A 2001 report estimated that initial costs start at
Chapter 3 - Green Roofs (2008) $10 per square foot (0.09 m2) for the simpler, extensive roof and $25 per square
https://www.epa.gov/heat- foot for intensive roofs. Other estimates assume $15 to $20 per square foot.
islands/heat-island-compendium Costs in Germany, where green roofs are more prevalent, range from $8 to $15 per
square foot. Prices in the United States may decline as market demand and
contractor experience increase.
City of Portland OR. EcoRoofs New GR construction (includes structural support): $10 - $20/SF
Handbook (2009) Re-roofing (GR retrofit): $6 - $40/SF

Minnesota Pollution Control Agency Extensive GR: $10 to $30/sq.ft. for the components above the waterproofing
(MPCA), Minnesota Stormwater assembly and a simple irrigation system.
Manual, Cost-benefit considerations
for green roofs (2022)
Greenroofs.com, Extensive –Common range, $14 – $25/sq. ft., (including roofing membranes).
FAQs - Greenroofs.com
(2023) Intensive – $25 – $40 and up

Greenroofs.org An installed extensive green roof with root repellent/waterproof membranes may
Green Roofs for Healthy Cities, About be installed for $10-$24 (USD) per square foot.
Green Roofs — Green Roofs for
Healthy Cities (2023)

Based on the above research and the vendor survey, capital costs of $20 and $35 per square foot were selected
for extensive and intensive green roofs respectively. These costs are incremental to conventional roofing. O&M
costs reported by the surveyed landscapers varied from $0.30/square foot to about $1.50/square foot. Based on
this range O&M costs are estimated at $0.86/square foot per year. For very large green roofs (>30,000 square
feet) the O&M cost is reduced to $0.50/square foot.

5.9 E LE C TRIC SYSTE M BE NE FITS
Electric system benefits of rooftop agrivoltaics include both electric building efficiency savings as well as the local
solar resource energy production. The regional electric system benefits are modeled within the Power Council’s
ProCost worksheets leveraging some of the avoided cost assumptions from the 2021 Power Plan.47 The ProCost
model combines the annual kWh savings, or resource production in the case of solar, based on an hourly
load/savings shape. Peak demand savings are valued according to the load shape value in Peak Period 1 or 2. The

47
Northwest Power and Conservation Council. Regional 2021 Northwest Power Plan. March 10, 2022.

prepared by E E S C O N S U L T I N G 4 5 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Project Team updated the wholesale price of energy based on Q1 2023 forward prices for electricity at Mid-C. The
peak values are taken directly from the most recent Power Plan. The value of GHG savings associated with energy
are calculated based on WAC-194-40-100 and updated to $2023 using the GDS deflator. Table 5.13 summarizes
the electric system assumptions. Note that 2021 Plan values are adjusted to 2023 dollars using the GDP deflator.
Each is described in more detail below the table.

TABLE 5.13: SUMMARY OF ELECTRIC SYSTEM COST AVOIDANCE, $2023
Assumption Source
Wholesale Electricity Price, 15-year Levelized $/MWh $ NWPCC Baseline
Market Price
Forecast April
2023 48
Social Cost of Carbon, $/metric ton, 15-year levelized $30/MWh WAC-194-40-100
Carbon Intensity Generic Resources, lbs/kWh 0.874 WA Dept. Ecology
Bulk System T&D Loss Factor 2.3% 2021 Plan49
Bulk System T&D Credit ($/kw-yr) $4.47 2021 Plan
Bulk System T&D Credit - Applicable Peak Period (1,2, or 1 and 2) Peak Periods 1 and 2 2021 Plan
Bulk System T&D I2R Loss Component (%) 90% 2021 Plan
Local System Distribution Loss Factor 4.74% 2021 Plan
Local System Distribution Credit ($/kw-yr) $8.74 2021 Plan
Local System Distribution Credit - Applicable Peak Period (1,2, or both) Peak Periods 1 and 2 2021 Plan
Local System Distribution I2R Loss Component (%) 70% 2021 Plan
Deferred Resource Capacity Credit ($/bulk kW-year) $152.14 2021 Plan

5 .9 .1 Avoid ed C os t O ver view
From a total resource cost perspective, energy efficiency and rooftop solar provide multiple benefits beyond the
avoided cost of energy. These include deferred capital expenses on generation, transmission, and distribution
capacity; avoided social costs of carbon emissions, and the reduction of utility resource portfolio risk exposure.
Since energy efficiency measures provide both peak demand and energy savings, these other benefits are
monetized as value per unit of either kWh or kW savings.

FIGURE 5.5: OVERVIEW OF PORTFOLIO REQUIREMENTS

Energy-Based Capacity Based

• Energy Price at Mid-C • Generation Capacity Deferral
• Social Cost of Carbon • Transmission Capacity Deferral
• Renewable Energy Credits • Distribution Capacity Deferral
• GHG-Free or Neutral Resources

48 Northwest Power and Conservation Council. Energy Forecasts. Wholesale Electricity Market Price and Avoided
Emission Rates Forecasts (April 2023). https://www.nwcouncil.org/energy/energy-forecasts/
49
See id.

prepared by E E S C O N S U L T I N G 4 6 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

The estimated values and associated uncertainties for these avoided cost components are based on the 2021
Power Plan, Washington-specific requirements such as WAC-194-40-100 and the Clean Energy Transformation Act
(CETA), and updated market price forecasts. The timeline below summarizes the relevant milestones for
Washington electric utility portfolio planning. The type of energy Washington utilities will need to procure is
based on these requirements; therefore, the requirements set the avoided cost as it relates to capacity,
renewable, and GHG-free power supply.

FIGURE 5.6: OVERVIEW OF ELECTRIC UTILITY PORTFOLIO REQUIREMENTS

2030
2045
2020 100% GHG
100% GHG
Neutral, up to
15% RPS Free, no
20% from
Offsets
Offsets

5 .9 .2 Soc ial C os t of C ar b on
The social cost of carbon is a cost that society incurs when fossil fuels are burned to generate electricity. Both the
Energy Independence Act (EIA) rules and CETA require that utilities include the social cost of carbon when
evaluating cost effectiveness using the total resource cost test (TRC). CETA further specifies the social cost of
carbon values to be used in resource analytics such as conservation potential assessments and integrated resource
plans. These values are shown in the table below.

TABLE 5.14: SOCIAL COST OF CARBON VALUES 50

According to WAC 194-40-110, values may be adjusted for any taxes, fees or costs incurred by utilities to meet
portfolio mandates.51 For example, the social cost of carbon is the full value of carbon emissions, which includes
the cost to utilities and ratepayers associated with moving to non-emitting resources. Rather than adjust the
social cost of carbon for the cost of RECs or renewable energy, the values for RECS and renewable energy are
excluded from the analysis to avoid double counting.

50 WAC 194-40-100. Available at :https://apps.leg.wa.gov/wAc/default.aspx?cite=194-40-100&pdf=true
51 WAC 194-40-110 (b).

prepared by E E S C O N S U L T I N G 4 7 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

The emissions intensity of the marginal resource (market) is used to determine the $/MWh value for the social
cost of carbon. Ecology states that unspecified resources should be given a carbon intensity value of 0.437 metric
tons of CO 2e/MWh of electricity (0.874 lbs/kWh).52 This is an average annual value applied in all months. The
resulting levelized cost of carbon is $30/MWh over 15-years.

5 .9 .3 Avoid ed Renew ab le E ner g y P urc has es
Renewable energy purchases need to meet both RPS and CETA and can be avoided through conservation. Utilities
may meet Washington RPS through either bundled energy purchases such as purchasing the output of a wind
resource where the non-energy attributes remain with the output, or they may purchase unbundled RECs.

As stated above, the value of avoided renewable energy credit purchases resulting from energy efficiency or
distributed renewable generation is accounted for within the social cost of carbon construct. The social cost of
carbon already considers the cost of moving from an emitting resource to a non-emitting resource. Therefore, it
is not necessary to include an additional value for renewable energy purchases prior to 2045 when all energy must
be non-emitting or renewable.

5 .9 .4 E lec t r ic Wholes ale Mar ket P r ic e
The Council has developed an updated wholesale electricity market price forecast since the 2021 Power Plan was
finalized. The Baseline forecast is used to value energy production (solar) or savings (green roof) in this study.
The levelized cost of energy is estimated at $0/MWh over the next 15 years..

5 .9 .5 Defer r ed Tr ans mis s ion and Dis t r ib ut ion Sys t em Inves t ment
Energy efficiency measure savings, demand response, and behind-the-meter solar reduce capacity requirements
on both the transmission and distribution systems. Capacity expansions due to load growth through electrification
may be avoided in part due to investments in energy efficiency and behind-the-meter resources. Capacity
expansions may include investments in transformers, conductors, and substations. The Council’s 2021 Power has
estimated these avoided costs at $3.08/kW-year and $6.85/kW-year for transmission and distribution systems,
respectively ($2016).53

5 .9 .6 Defer r ed Inves t ment in G ener at ion C ap ac it y
Local resources such as rooftop solar and energy efficiency reduce the capacity need from generating resources.
From a regional perspective, generating resource capacity is developed to meet peak demand at any given point
in a year. Depending on the resource, different capacity credits can be attributed based on the time of year. A
natural gas resource that can quickly ramp up or down to meet changes in loads is considered to provide its full
capacity value regardless of the time of year. Other resources such as hydropower are credited capacity value
depending on its operational constraints such as fish and wildlife mitigation, water rights, or water availability.
Similarly wind or solar resources are intermittent based on sun or wind availability. The 2021 Power Plan bases
the deferred investment in generation resources that provide peak capacity benefits on natural gas resources.

52 WAC 173-444-040 (4)
53 Northwest Power and Conservation Council Memorandum to the Power Committee Members. Subject; Updated
Transmission & Distribution Deferral Value for the 2021 Power Plan. March 5, 2019. Available at:
https://www.nwcouncil.org/sites/default/files/2019_0312_p3.pdf

prepared by E E S C O N S U L T I N G 4 8 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

In the Council’s 2021 Power Plan, 54 a generation capacity value of $123/kW-year was explicitly calculated ($2016).
This value is adjusted to 2023 dollars using the GDP deflator.

5 .9 .7 Nor t hw es t P ow er Ac t C red it
In accordance with the Northwest Power Act, a 10% adder is included as a bonus to the avoided costs for energy
efficiency.55

5.10 RO O FTO P SO LAR
Energy production from rooftop solar is valued at local market prices based on the energy production profile in
each solar zone. Not surprisingly, solar production for each of our solar zones was extremely consistent across
building types, with the exception of the smallest systems (under 5kW) which seem to produce slightly less energy
per kW. This is likely due to the very small size of the PV systems compared to systems on all other building types
which were all over 5kW and predominantly over 20kW. Overall, solar generation in zone 1 is about 29.4 kWh/sq.
ft. of solar panel coverage, whereas in zone 3 generation is estimated at about 37.7 kWh/sq. ft. of solar panel
coverage (when not including systems under 5kW).

Solar PV production estimates do not take into account any increased production due to installation of a green
roof, which may lead to an increase in production due to the cooling effects of the plants. Currently there are
limited studies showing additional energy generation due to the inclusion of plants under the panels. However,
according to Cavadini and Cook, for a flat rooftop PV installation near Zurich, Switzerland (temperate climate),
results show that, compared to a conventional roof, green roofs can increase annual PV energy yield, on average,
by 1.8%, whereas cool roofs can increase it by 3.4%.56 In addition, a study done at the University of Technology in
Sydney, Australia comparing how much energy two solar systems produced over an 8 month period found “the
“green roof” improved performance by as much as 20 per cent at peak times and by 3.6% over the length of the
experiment.”57

The expected monthly generation for each solar zone 1 and zone 3 can be seen in the figures below. Note
production is based on a 100kW PV system using bifacial panels, standard efficiency modules, 96% efficient
inverters, system losses of 14.08%, tilted at 20°, and an azimuth of 180°.

54 https://www.nwcouncil.org/energy/powerplan/7/home/
55 in 16 U.S.C. § 839a of the Pacific Northwest Electric Power Planning and Conservation Act
56 Cavadidi, Giovan and Lauren Cook. (2021). Applied Energy 296 (2021) 117082. Green and cool roof choices integrated into

rooftop solar energy modeling.
57
Irga, Peter et al. Green Roof & Solar Array – Comparative Research Project. Final Report July 2021. University of Technology
Sydney.

prepared by E E S C O N S U L T I N G 4 9 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

FIGURE 5.7: ESTIMATED SOLAR GENERATION FOR YAKIMA (LEFT) AND SEATTLE (RIGHT)

5 .1 0 .1 Solar O & M
Operation and maintenance of a PV system can have a significant impact on its productivity and profitability.
According to National Renewable Energy Laboratory (NREL), O&M costs can range from $18-$28/kW/year and
putting a commercial system closer to the $18/kW/year estimate. Considering a lifetime of at least 25 years for
the system, O&M costs can be significant. O&M includes module cleaning, system inspections, component
replacements (such as inverters at 15 years), and overall system monitoring.

5 .1 0 .2 Solar C ap it al C os t s
The cost of solar PV systems has decreased dramatically over the last 10 years. In 2010 a commercial rooftop PV
system cost approximately $5.57/watt, but by March 2022, NREL estimated the price of installing a commercial
PV system to be about $1.84 per watt DC. 58 Estimates from CoveTool from May 2023 show commercial PV costs
at about $175,000 for a 100 kW system, or about $1.75/watt and provide and overall estimate of $1.83/watt for
all commercial systems.59 These costs include panels, racking, inverters, etc. and do not include any available tax
credits. When accounting for tax credits, overall system costs can be reduced. Commercial rooftop PV costs since
2010 can be seen in the figure below.

58
Feldman, David et al. U.S. Solar Photovoltaic System and Energy Storage Cost Benchmark: Q1 2020. National Renewable
Laboratory Technical Report. January 2021. Available at: https://www.nrel.gov/docs/fy21osti/77324.pdf
59
Cove Tool. Solar Panel Cost – Residential + Commercial. https://help.covetool.com/en/articles/4529706-solar-panel-cost-
residential-commercial

prepared by E E S C O N S U L T I N G 5 0 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

FIGURE 5.8: U.S. SOLAR PV SYSTEM COSTS 2010-202060

5.11 BATTE RY STO R AG E
When battery storage is combined with solar PV, on-peak wholesale power costs can be avoided when the solar
resource is not generating. A typical battery analysis would consider the building load profile, retail electric rates,
total size of the solar PV resource.

To illustrate the impact of a short duration battery on rooftop agrivoltaic economics, a representative building
was selected for battery optimization. The selected representative building is a hospital. This was selected in
order to support a larger battery installation. The battery optimization included the following assumptions:

 The dispatch model applied was tuned for peak reduction.
 Load assumed was 200 kW peak with a residential/commercial mixed load profile
 The battery was sized at 50% of the solar PV nameplate and is 4-hours in duration.
 Solar PV max output was 137 kW ac, thus Battery size is 69 kW ac
 Charging efficiency is 90%
 194 annual equivalent cycles on the battery (or 53,000 kWh/year)
 This example served 25% of the annual building load

Figure 5.9 illustrates an example generation profile of the solar resource with and without the battery. The battery
is charged during the first part of the day and discharged in the afternoon when wholesale prices for electricity
are higher.

60
National Renewable Energy Laboratory. Documenting a Decade of Cost Declines for PV Systems. February 10, 2023.
Available at: https://www.nrel.gov/news/program/2021/documenting-a-decade-of-cost-declines-for-pv-systems.html

prepared by E E S C O N S U L T I N G 5 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

FIGURE 5.9 AVERAGE DAILY GENERATION PROFILE SOLAR AND BATTERY MONTH OF JULY
45
40
35
30
25
kW
20
15
10
5
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour

Solar+Battery Solar PV

5 .1 1 .1 Bat t er y O & M

For both rooftop agrivoltaic and conventional building solar+battery systems, an effective Operation and
Maintenance (O&M) program or plan provides certainty that a system will perform at or above its production
estimate and costs over the useful life. Therefore, a long-term performance management strategy will assure the
expected benefits of an asset and should be properly considered in the budgeting phase of the project and
sustained over time through monitoring and performance guaranties. Key considerations for building operators
are listed below:

O&M Plans: Strategies exist to provide O&M services to manage the desired performance expectations. The
ownership model largely determines the maintenance approach of the system over the useful life. Solar and
energy storage service and maintenance programs are well developed throughout industry which should be
presented by the contractor including regular maintenance tasks to maintain the warranties of equipment.

Lifecycle Management strategies are necessary to deal with the certainty of decaying battery performance.
Battery degradation occurs every time there is a charge or discharge cycle, and it is a function of depth of
discharge, current rate, and average state of charge. The battery’s energy capacity, the duration that the battery
can serve the load, fades over time, which can be managed by proper initial sizing and a strategy to maintain the
system performance over the life. Typically, the battery is slightly oversized to account for degradation and allow
the system to provide the appropriate duration over the contract term. Tier One lithium-ion suppliers offer up to
10-year warranties and LTSAs with contractors can help manage those warranties and act as the guaranteeing
party for the term of the agreement.

Mechanisms of Degradation in lithium-ion batteries are primarily due to the innate chemical reactions that occur
between the electrolyte and anode as well as mechanical breakdown of the electrodes due to usage. The two
primary mechanisms of battery aging are resultant of usage and the passage of time. It is worth noting that cycle
life and discharge throughput through a lithium-ion battery are directly proportional, but not in a linear fashion,
and the relationship differs by cell based on the exacerbating variables.

prepared by E E S C O N S U L T I N G 5 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Cycling Degradation from charging and discharging of a lithium-ion battery results in the repeated intercalation
and deintercalation of lithium ions into and out of the crystalline electrode structures forcing repeated expansion
and contraction of such electrodes with each cycle. As with any mechanical movement, over time the underlying
electrode materials or the composite electrode structure will become fatigued from stress and fail and/or a
current collector can separate from its electrode. The rate at which mechanical damage occurs increases under
certain conditions. Under controlled cycling, the mechanical damage develops over time and manifests as an
increase in internal resistance and reduced charge-carrying capacity; however, accelerated degradation can occur
if such fatigue breaks through the solid electrolyte interface (SEI) layer and wear through the separator shorting
the electrodes.

FIGURE 5.9: BATTERY DEGRADATION FACTORS

5 .1 1 .2 Bat t er y C ap ital C os t s
Understanding the financial implications of a power system installation is crucial for understanding the feasibility.
The cost analysis examines the project costs based on the agreed upon system specification as well as local and
federal financial incentives such as the investment tax credits, federal and state incentives, and depreciation
benefits that may apply to the project.

An agreed upon set of performance requirements is the most influential component of the solar/storage
development process. It influences the installation requirements, integration costs, operational requirements,
and timeline. The cost of the equipment from a specific vendor will likely not change very much from site to site.
However, the cost of interconnection, development, and acquisition of physical space to accommodate the
solar/storage system can vary significantly.

prepared by E E S C O N S U L T I N G 5 3 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Our Team utilized some public data sources such as the Lazard report on Levelized Cost, published in April of 2023,
as well as proprietary vendor quotations for solar+storage equipment as a part of recent EES Team project work.61
Following the analysis, we concluded that the current average for installed storage system cost is approximately
$873/kWh and this was used as a basis for capital installation cost. The operation and maintenance costs for the
average storage system size used in this study, roughly 275 kWh, hovered around $1.00/kWh annually.

5.12 C O LO C ATIO N BE NE FITS O F G RE E N RO O FS WITH P HO TO V O LTAI C ARRAYS
As demand for space on flat roofs increases, along with the awareness of potential opportunities associated w ith
making good use of rooftop space, colocation of photovoltaic (PV) panels and green roofs on the new
developments is increasing. The two systems are no longer regarded as mutually exclusive, and if designed
correctly, both can occupy the same roof space and function as intended.

PV panels are not typically integrated into the green roof system but are mounted on it via a metal frame secured
to the structural (non-green roof) components of the roof. This integrated mounting design allows the green roof
vegetation and substrate to provide the ballast and support to the PV array and eliminates the need to penetrate
the layer waterproofing to secure the mounting units directly to the roof itself. 62

The key issue in co-locating green roofs and solar panels is the distance between each solar array. Maximizing
available roof area attributed to solar panels can have detrimental effects on both systems. In addition, proper
staging of the installation is critical to ensure that both systems (including the vegetation) are not damaged.
Consistent coordination with both green roof and solar contractors during both the installation and for periodic
O&M is essential.63

“Solar green roofs” have been implemented in various ways for decades. Colocation has a number of advantages:

 increased efficiency of the solar system through temperature moderation
 protection of the waterproof layer by avoiding penetration and providing cover from thermal and
mechanical stress
 providing a degree of shelter and shading to plants
 increase in biodiversity (over solar-only roofs)

The electrical output of PV panels is most efficient within an operating temperature range; outside this optimal
range, electricity generation becomes less efficient. Colocation with a green roof maintains a more constant
temperature, resulting in PV panels working more efficiently throughout the day. An Australia study64 found that
solar PV efficiency increased by up to 20% and by an average of 3.6%. This study assumes a 3.6% adder for kWh
for the case where the solar PV and green roof are collocated. The adder amounts to an additional 877 kWh per
year for a multifamily mid-rise building (25% solar PV coverage), or enough to provide total electricity demand to
one apartment for one month.

61 Lazard. 2023 Levelized Cost of Energy+. April 12, 2023. Available at: https://www.lazard.com/research-insights/2023-
levelized-cost-of-energyplus/
62
European Federation of Green Roofs and Walls, https://efb-greenroof.eu
63
Green Roof Organization (GRO), “The Gro Green Roof Code, Green Roof Code of Best Practice - Incorporating Blue Roofs
and BioSolar Applications” 2021
64 Irga, Peter et al. Green Roof & Solar Array – Comparative Research Project. Final Report July 2021. University of
Technology Sydney.

prepared by E E S C O N S U L T I N G 5 4 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

The solar array in turn creates wind and sunshade if orientated correctly, improving vegetation productivity and
the potential for increased biodiversity by creating microenvironments with varying solar exposure.65

5.13 E C O NO MIC DE VE LO P ME NT
Green roof programs have the potential to promote economic growth directly through job creation. Effective
programs could create additional jobs in architecture, building/green roof installation, and operation and
maintenance. Based on interviews with installers, about half the cost of green roof is labor expense. If more green
roofs are installed there is potential for job growth and additional economic investment. Multiplier impacts are
not calculated in this study since any rooftop agrivoltaic program would need to be evaluated as a whole compared
with current conditions.

5 .1 3 .1 Solar P V E c onomic Imp ac t s
Similar to green roofs, solar PV installation and operation creates local jobs. The Project Team used NREL’s Jobs
and Economic Development Impact Model (JEDI) to estimate the jobs and economic impacts for rooftop solar
installations. The model can be customized for Washington state and average project size. Commercial rooftop
solar was selected and the sales tax value modified to reflect Washington State’s exemption for solar PV. Each
system supports 1.2 jobs during construction and installation. Operation and maintenance for the same system
supports 0.024 jobs. The wages paid to these workers are accounted for in the benefit-cost analysis under the
O&M and capital cost assumptions.

TABLE 5.15: JEDI OUTPUT SOLAR PV 100 KW SYSTEM IN WASHINGTON STATE
LOCAL ECONOMIC IMPACTS
Jobs Earnings Output Value Added
During construction and installation period $000 (2023) $000 (2023) $000 (2023)
Project Development and Onsite Labor Impacts
Construction and Installation Labor 0.2 $13.0
Construction and Installation Related Services 0.3 $19.7
Subtotal 0.5 $32.7 $66.5 $51.0
Module and Supply Chain Impacts
Manufacturing Impacts 0.0 $0.0 $0.0 $0.0
Trade (Wholesale and Retail) 0.1 $8.0 $27.2 $17.6
Finance, Insurance and Real Estate 0.0 $0.0 $0.0 $0.0
Professional Services 0.1 $5.0 $16.1 $10.2
Other Services 0.1 $15.1 $42.2 $26.8
Other Sectors 0.2 $5.0 $14.8 $9.2
Subtotal 0.4 $33.2 $100.3 $63.8
Induced Impacts 0.2 $14.6 $51.4 $32.5
Total Impacts 1.2 $80.5 $218.2 $147.3

Annual Annual Annual
Annual Earnings Output Output
During operating years Jobs $000 (2023) $000 (2023) $000 (2023)

65 Walston et. al., “Opportunities for Agrivoltaic Systems To Achieve Synergistic Food -Energy-Environmental Needs and

Address Sustainability Goals”, Frontiers in Sustainable Food Systems, 2022

prepared by E E S C O N S U L T I N G 5 5 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Onsite Labor Impacts
PV Project Labor Only 0.0 $1.0 $1.0 $1.0
Local Revenue and Supply Chain Impacts 0.0 $0.3 $1.0 $0.6
Induced Impacts 0.0 $0.1 $0.5 $0.3
Total Impacts 0.0 $1.4 $2.5 $1.9

Jobs: Full Time Equivalent (FTE) 2,080 hours/year
Earnings: Earnings refers to wage and salary compensation paid to workers and benefits.
Output: Output refers to economic activity or the value of production in the state or local economy.

Value Added: Value added is the difference between total gross output and the cost of intermediate inputs. It is
comprised of payments made to workers (wages and salaries and benefits), proprietary income, other property
type income (payments from interest, rents, royalties, dividends, and profits), indirect business taxes (excise and
sales taxes paid by individuals to businesses, and taxes on production and imports less subsidies.

The above modeling can be leveraged to evaluate future rooftop agrivoltaic programs and their impact on the
local economy. As with green roofs, economic development impacts for solar PV would need to be estimate in
the context of a new program and compared with the business as usual case.

5.14 SUMMAR Y O F Q UANTI FIE D BE NE FITS AND C O STS
Table 5.16 summarizes the costs and benefits for green roofs paired with solar PV. Some of the benefits have
multiple quantitative measures. For example, green rooftop agrivoltaics both sequester carbon within the plants
and also reduce carbon emissions through solar PV production. Each quantitative benefit is defined separately
and included in the benefit-cost model.

TABLE 5.16: SUMMARY OF ROOFTOP AGRIVOLTAIC BENEFITS AND COSTS
Perspective Quantitative Qualitative
Building Owners/Occupants
Reduced energy use through Efficiency
Noise reduction
and Solar PV generation plus colocation
Increased Comfort
Benefits benefits
Food production
LEED Credits for Innovative Design, Local
Increased aesthetic
Materials, Reduced Heat Islands
Architecture/Engineering
Installation
Costs
Capital
O&M
Public
Reduced air pollutants
Reduced Stormwater Quantities
Health Benefits
Reduced or Sequestered GHG
Natural Habitat
Local generation decreases long-term
Benefits Fire Safety
need for electricity transmission and
Increased Stormwater Quality
distribution
Reduced Surface Temperatures
Economic Development

Costs Program administration

prepared by E E S C O N S U L T I N G 5 6 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

6 Benefit-Cost Analysis Results
The benefits and costs for green roofs are evaluated using the Power Council’s methodology for evaluating energy
efficiency measures. The Power Council has developed a Pro-Cost tool to value the energy savings that can be
applied to rooftop agrivoltaics. ProCost was modified to evaluate the following:
1. Rooftop agrivoltaic measure definitions including green roof type, building type etc. in $/square foot.
2. Energy savings from behind-the-meter solar pv production. Solar PV generation profiles can be compared
with market prices on an hourly basis or within segments as defined by the Power Council.
3. Local generation provides distribution system and transmission system benefits. Deferred investment in the
electric system will be valued and applied to regional and local coincident peak demand savings estimates.
4. Colocational benefits of green roofs and solar PV are quantified based on available research.
5. Reductions in greenhouse gas emissions due to energy savings and solar PV production can be valued when
compared with the carbon intensity of the electricity market.
6. Non-energy benefits such as water savings, impacts to stormwater treatment systems, and others identified
in the study are included.
7. Costs include capital costs for equipment, materials and labor needed to install rooftop agrivoltaic systems,
annual operation and maintenance costs (O&M).
8. The useful life of the technology was evaluated based on the shortest useful life of green roof and solar
components. Once the useful life expires, significant capital investment would be needed to continue project
operation (discussed below).

The benefit-cost model will value the costs and benefits of each measure from a total resource cost perspective.
Costs and benefits will be included regardless of to whom they accrue. We recommend the use of the Power
Council framework since it is consistent with existing state legislation regarding resource evaluation for electric
utilities.66 The results of the benefit-cost modeling include net present value of benefits and costs over the life of
the measure.

6.1 FINA NC IAL ASSUMP TIO N S
The benefit cost analysis requires a time period for the analysis as well as discount rate assumptions to convert
costs and benefits to current dollars. These assumptions are discussed first then results are provided.

6 .1 .1 Meas ur e Life
Rooftop agrivoltaics is essentially a bundle of measures including some or all three parts: green roof, solar PV, and
battery. Each individual measure has a unique useful life. The useful life of the measure is defined as the period
of time the technology or measure is expected to exist before needing replacement or major repairs. Table 6.1
below summarizes the useful lives for the main components for rooftop agrivoltaics.

66
Energy Independence Act and Clean Energy Transformation Act both reference NWPCC regional power plans.

prepared by E E S C O N S U L T I N G 5 7 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 6.1: USEFUL MEASURE LIFE
Useful Life, Years
Solar PV 15
Cells 20
Inverter 15
Green Roof 40
Membrane 40
Other Components 40
Battery 10

The Project Team evaluated the time period over which to calculate the costs and benefits. To combine all of the
above components, the useful lives would need to be similar. In order to have similar useful lives, a few options
were considered:

1. Use the shortest useful life so that all components are in good working order without major repair and
replacement
2. Add replacement costs for each component to extend the useful life to the longest time period as defined
in Table 6.

Neither of these methods are likely to produce perfect agreement between replacement costs and useful life.
Some repair and replacement costs may extend the useful life of a particular component beyond the maximum
useful life being evaluated. Therefore, the Project Team selected the first option to evaluate the benefits and
costs. This option is preferred for the following reasons:

1. The longer the planning period, the greater the uncertainty associated with forecasted values.
2. Building owners are likely more willing to accept shorter payback periods for building investments
3. Similar to (1) above, changes in technologies, climate, or regulatory have a greater chance of interrupting
the benefit and cost analysis when evaluated over a longer period of time.

Therefore, the measure life of 15 years is used in the analysis. After 15 years, inverters on the solar PV resource
will need to be replaced. When a battery is included, the measure life drops to 10 years.

6 .1 .2 Dis c ount Rat e
The discount rate is used to adjust future value streams to current 2023 dollars. The 2021 Power Plan used a
discount rate of 3.75% which reflects the real weighted average cost of capital. This discount rate is assumed in
the benefit-cost analysis as well.

6.2 RE SULTS: G RE E N RO O F A ND SO L AR P V NO C O LO C ATIO N
The benefit-cost analysis results in mixed cost-effectiveness levels. The solar analysis and green roof analysis are
prepared separately since the solar generation profile differs from the energy efficiency profiles for building HVAC.
Because the analyses are prepared separately, their results can also be viewed separately. Mainly, solar PV was
not cost-effective in any of the cases when a 15-year lifecycle is assumed. This result is not all that surprising from
a wholesale energy perspective. Most rooftop solar installations are deemed cost effective when compared to
retail utility rates which include costs that are considered transfers in this benefit cost analysis.

In some instances green roofs alone were cost-effective. This is the case when stormwater benefits are quantified
such as in Seattle or Spokane. Green roofs in Yakima are not cost effective, which is not surprising considering

prepared by E E S C O N S U L T I N G 5 8 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

the low average annual rainfall and related stormwater utility infrastructure. Table 6.1 summarizes the BCA
results by combining all roof coverages for solar and green roof. Detail by coverage share is provided later in this
section.

TABLE 6.1: BCA RESULTS GREEN ROOF PLUS SOLAR PV ALL COVERAGES
Building Type Seattle Spokane Yakima
Assembly 0.82 0.50 0.41
Grocery 0.81 0.46 0.37
Hospital 0.91 0.72 0.65
Lodging 0.88 0.56 0.50
Mixed Commercial* 0.88 0.52 0.46
Multifamily High Rise (7+) 0.96 1.59 NA
Multifamily Low Rise (1-3) 0.92 0.56 0.51
Multifamily Mid Rise (4-6) 0.92 0.81 0.75
Office 0.88 0.51 0.45
Other 0.87 0.46 0.40
Residential Care 0.87 0.48 0.41
Retail / Service 0.88 0.52 0.46
School 0.86 0.46 0.40
Warehouse 0.86 0.47 0.40

Green roof installations on multifamily buildings have been most common in the literature and many examples of
green roofs on multifamily buildings have been identified in Seattle. Table 6.2 shows the BCA results for Mid-Rise
multifamily buildings (4-6 stories).

TABLE 6.2: BCA RESULTS MID-RISE MULTIFAMILY BUILDINGS INTENSIVE GREEN ROOF, $/SQUARE FOOT
ROOF AREA
Solar GHG
Production Sequestration,
Electric LEED Credit, Total
System Stormwater NPV Total NPV Benefit/Cost
Benefits Management Benefits Costs Ratio
Seattle 10% Solar 30% Green Roof $4.69 $32.72 $37.41 $26.37 1.42
25% Solar 50% Green Roof $12.10 $43.06 $55.16 $50.32 1.10
50% Solar 70% Green Roof $24.20 $47.14 $71.34 $76.87 0.93
Spokane 10% Solar 30% Green Roof $5.77 $28.02 $33.79 $26.37 1.28
25% Solar 50% Green Roof $14.43 $31.98 $46.41 $50.32 0.92
50% Solar 70% Green Roof $28.86 $23.35 $52.21 $76.87 0.68
Yakima 10% Solar 30% Green Roof $6.20 $26.87 $33.07 $26.37 1.25
25% Solar 50% Green Roof $15.50 $26.92 $42.42 $50.32 0.84
50% Solar 70% Green Roof $31.00 $17.15 $48.15 $76.87 0.63

In each city, the green roof is cost-effective at 30% coverage, but when adding additional solar and green roof,
the rooftop agrivoltaic measure cost-effectiveness decreases. Figure 6.1 compares the share of cost for solar and

prepared by E E S C O N S U L T I N G 5 9 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

green roof measures. Figure 6.1 also shows that the cost per square foot of roof area increases as coverage
increases for solar and green roof installations.

FIGURE 6.1: INTENSIVE GREEN ROOF ON MID-RISE MULTIFAMILY BUILDINGS, ALL CITIES, NPV $/SF ROOF
AREA
$90.00

$80.00

$70.00

NPV Costs, $/SF Roof Area
$60.00

$50.00

$40.00

$30.00

$20.00

$10.00

$0.00
10% Solar 30% Green Roof 25% Solar 50% Green Roof 50% Solar 70% Green Roof

Capital Green Roof O&M Green Roof Capital Solar PV O&M Solar PV Program Admin

When looking at just Seattle, the benefit-cost ratios in Table 6.3 show that adding additional solar (increased from
10% to 25% coverage) reduces the cost-effectiveness across all building types. Conversely, increasing the green
roof coverage in Seattle from 50% to 70% increases the benefit-cost ratio in all cases.

prepared by E E S C O N S U L T I N G 6 0 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 6.3: SEATTLE ROOFTOP AGRIVOLTAIC BENEFIT-COST RATIOS
10% Solar 25% Solar
50% Green 50% Green 10% Solar 70%
Roof Roof Green Roof
Assembly 0.76 0.73 0.90
Grocery 0.70 0.68 0.85
Hospital 0.97 0.88 NA
Lodging 0.92 0.85 1.02
Mixed Commercial* 0.81 0.76 0.93
Multifamily High Rise (7+) 2.09 1.58 1.74
Multifamily Low Rise (1-3) 0.84 0.77 0.94
Multifamily Mid Rise (4-6) 1.13 0.95 1.12
Office 0.83 0.78 0.95
Other 0.72 0.70 0.87
Residential Care 0.78 0.74 0.91
Retail / Service 0.70 0.68 NA
School 0.72 0.70 0.94
Warehouse 0.72 0.70 0.94

The benefit-cost analysis for Spokane shows the opposite. In Spokane, adding more solar increases the benefit-
cost ratio. In most cases, however, the benefit-cost ratio is below 1.0.

TABLE 6.4: SPOKANE AGRIVOLTAIC BENEFIT-COST RATIOS
10% Solar 25 Solar
50% Green 50% Green 10% Solar 70%
Roof Roof Green Roof
Assembly 0.49 0.56 0.40
Grocery 0.43 0.51 0.36
Hospital 0.68 0.70
Lodging 0.65 0.67 0.44
Mixed Commercial* 0.54 0.59 0.36
Multifamily High Rise (7+) 1.86 1.41 1.16
Multifamily Low Rise (1-3) 0.57 0.60 0.36
Multifamily Mid Rise (4-6) 0.90 0.78 0.55
Office 0.56 0.60 0.38
Other 0.46 0.53 0.30
Residential Care 0.51 0.57 0.34
Retail / Service 0.44 0.51
School 0.45 0.52 0.32
Warehouse 0.46 0.53 0.32

prepared by E E S C O N S U L T I N G 6 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Table 6.5 shows that the benefit-cost ratios for Yakima are always below 1.0. Solar PV in Yakima improves the
cost-effectiveness of rooftop agrivoltaics but without the stormwater benefit in this city, none of the measure
permutations are cost-effective.

TABLE 6.5: YAKIMA ROOFTOP AGRIVOLTAIC BENEFIT-COST RATIOS
10% Solar 25% Solar
50% Green 50% Green 10% Solar 70%
Roof Roof Green Roof
Assembly 0.37 0.47 0.28
Grocery 0.30 0.42 0.23
Hospital 0.54 0.61 NA
Lodging 0.52 0.59 0.40
Mixed Commercial* 0.41 0.51 0.31
Multifamily High Rise (7+) NA
Multifamily Low Rise (1-3) 0.51 0.52 0.32
Multifamily Mid Rise (4-6) 0.79 0.70 0.50
Office 0.43 0.52 0.33
Other 0.33 0.44 0.25
Residential Care 0.38 0.49 0.29
Retail / Service 0.31 0.43 NA
School 0.33 0.44 0.27
Warehouse 0.33 0.45 0.27

6.3 RE SULTS: G RE E N RO O F A ND SO L AR P V WITH C O LO C ATIO N
The literature review concluded that colocation benefits of rooftop agrivoltaics could increase solar production by
up to 20% during very warm periods and by 3.6% on average. Adding colocation benefits of solar to Yakima or
Spokane rooftop agrivoltaic measures will not increase the cost-effectiveness above the 1.0 threshold. However,
Table 6.6 shows the impact to selected measure permutations of adding in the colocation benefits.

prepared by E E S C O N S U L T I N G 6 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

TABLE 6.3: SEATTLE ROOFTOP AGRIVOLTAIC BENEFIT-COST RATIOS WITH COLOCATION BENEFITS
10% Solar 25%Solar
50% Green 50% Green 10% Solar 70% 50% Solar 70%
Roof Roof Green Roof Green Roof
Assembly 0.76 0.74 0.90 0.80
Grocery 0.70 0.69 0.85 0.77
Hospital 0.97 0.89 NA NA
Lodging 0.92 0.85 1.02 1.31
Mixed Commercial* 0.81 0.77 0.94 0.83
Multifamily High Rise (7+) 2.09 1.59 1.74 0.94
Multifamily Low Rise (1-3) 0.85 0.78 0.94 0.82
Multifamily Mid Rise (4-6) 1.13 0.96 1.13 0.83
Office 0.83 0.79 0.95 0.78
Other 0.73 0.71 0.87 0.81
Residential Care 0.78 0.75 0.91 1.31
Retail / Service 0.71 0.69 NA NA
School 0.72 0.71 0.94 0.82
Warehouse 0.73 0.71 0.95 0.82

Because the colocation benefits are relatively small, the benefit-cost ratios did not improve significantly when
they are included.

6.4 RE SULTS: G RE E N RO O F, SO L AR P V , AND SHO RT -TE RM BATTE RY STO RAG E
Adding short-term battery storage to the solar PV increases the cost-effectiveness given the modeled
assumptions. The Procost model assigns value to peak load reductions at the time of the regional system peak.
The production from the solar+battery is valued higher from primarily a capacity perspective compared with solar
alone. Table 6.4 illustrates the results of the benefit-cost analysis for 10% solar coverage on the representative
hospital.

TABLE 6.4 SOLAR + BATTERY BENEFIT-COST RESULTS,10% ROOFTOP COVERAGE HOSPITAL, $/SF ROOF
AREA
Electric System Electric System
Benefits Energy Benefits Capacity Reduced GHG Total Benefits Total Costs BCR
Seattle $2.16 $1.43 $1.67 $5.26 $17.17 0.31
Yakima $3.08 $0.33 $2.75 $6.15 $17.17 0.36
Spokane $2.76 $0.27 $2.45 $5.48 $17.17 0.32

prepared by E E S C O N S U L T I N G 6 3 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Table 6.5 shows the results when solar+battery is combined with a 30% coverage green roof.

TABLE 6.5 30% GREEN ROOF, 10% SOLAR + BATTERY BENEFIT-COST RESULTS, HOSPITAL $/SF ROOF AREA
Total Total
Benefits Costs BCR
Seattle $22.25 $33.74 0.66
Yakima $17.67 $33.74 0.52
Spokane $16.65 $33.74 0.49

prepared by E E S C O N S U L T I N G 6 4 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

7 Key Findings
7.1 STUDY FI NDING S
The research and modeling revealed several key findings, some of which are specific to Washington State.

1. Increasing soil depths on green roofs in Spokane results in not needing irrigation systems due to consistent
monthly rainfall.
2. Stormwater benefits in this study are dependent on the variable cost of stormwater treatment. Not all
areas in Washington State will experience a reduction in water treatment cost due to green roof
installations. Yakima is an example from this study where wastewater treatment facilities are impacted
more significantly from irrigation leakage during the summer months than in winter from stormwater.
3. The primary benefit of green roofs is stormwater retention. Energy savings from the added R-value from
green roof minimally impacted new building energy efficiency when the most recent Washington building
codes are factored into the analysis. Stormwater retention potentially reduces flooding and severity of
storm damage.
4. The primary quantified benefit of green roofs is the stormwater impact, and this benefit varies widely
depending on the local stormwater infrastructure, operating characteristics, and annual precipitation.
Studies that have shown positive cost/benefit results for green roofs have relied on quantifying softer
benefits such as increased occupant productivity, comfort, or energy savings based on buildings that are
not as efficient as required in Washington State. Utilizing green roofs to mitigate stormwater in Western
WA also has an economic development component as it leaves more land area available for development
or reduces the cost of underground storage when required.
5. In practice, rooftop agrivoltaic projects are not common. Most often when solar and green roofs are
combined on the same roof, they exist separately leaving colocation benefits unrealized.
6. Rooftop solar programs are well-defined in Washington State. The cost of the infrastructure is also well-
known. Washington has already implemented several programs to promote increased adoption of rooftop
solar including solar-ready buildings, sales tax rate exemptions, and grant monies for qualifying projects.
Additionally, significant incentives are available at the Federal level from the American Recovery and
Reinvestment Act.
7. The colocation benefits of solar PV and green roofs related to energy production is not significant. The
study identified qualitative colocation benefits for green roofs from solar PV shading; however, the data
available was insufficient for quantifying those benefits.

7.2 BARRIE R S A SSE SSME NT
The barriers assessment includes both cost-related barriers as well as implementation barriers.

Based on the interviews with green roof installers and architects implementation barriers exist for implementing
green roof and green roofs paired with solar.

1. Many green roof installations are motivated by the permitting process in Seattle that expedites buildings
with green roof features. Without an incentive or mandate, most building developers choose conventional
roofing.
2. There is a perception that co-locating solar and green roofs would interfere with O&M for the green roof
and solar panels separately.
3. Green roofs on multifamily buildings are most popular due to the real estate value added.

prepared by E E S C O N S U L T I N G 6 5 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

4. There is a perception held by some roofing companies that green roofs will eventually leak and cause
damage to the building.
5. The variability in climate conditions across the State makes it difficult to establish a best practice for green
roof or solar coverage. Examples of green roofs in the East side of the state are limited.
6. Washington’s commercial building codes already require a high level of building energy efficiency. Even
in relatively colder/hotter climates like Spokane or Yakima, green roofs provide minimal additional energy
efficiency savings.
7. The number of landscape companies implementing green roofs does not appear to be limited in the
Seattle Area. If a program is implemented in Washington State, there is opportunity for landscape
installers to expand their business to green roof work.

Cost related barriers include the up-front capital cost for implementing the green roof plus the solar resource. The
incremental cost of the green roof outweigh the benefits quantified in this study especially in drier regions such
as Spokane and Yakima. When combined, the cost of rooftop agrivoltaics is as much as twice the cost of the
benefits quantified. When battery storage is added to rooftop agrivoltaics, electric system capacity benefits are
realized; however, those benefits do not outweigh the cost of capital for the battery.

7.3 RE C O MME NDATIO NS
From this study it can be concluded that rooftop agrivoltaics is not wholly cost-effective. Despite the economic
viability of rooftop agrivoltaics, the State may wish to promote rooftop agrivoltaics for economic reasons that are
difficult to quantify based on the current literature. If the State were to move forward with a rooftop agrivoltaic
program, the Project Team makes the following general recommendations:

1. Develop certification for O&M providers specific to green roof needs. Green roof maintenance should
address erosion, roof drains, gravel stops, utilities, dead plants, fertilization, weeds, irrigation system,
summer watering, mosquitos, and nuisance animals.
2. Green roofs should be given priority as a method to satisfy local stormwater management requirements
in regions such as the Greater Puget Sound where stormwater runoff costs can be marginally impacted.
3. Scoring for increased square footage, tax abatements, and stormwater credits can be based upon depth
of soil media and green square footage.
4. The value of stormwater reductions in terms of reduced long-term infrastructure costs can be passed onto
developers in the form of tax reductions, utility fee reductions, or direct incentives. These programs are
best suited for stormwater district administration which would include cities and counties.
5. If green roof mandates are considered, it is recommended to create multiple solutions for developers to
meet the requirements. An example is the San Francisco model which lets builders choose between solar,
green roof, or both.
6. Evaluate program spending or rooftop agrivoltaic potential based on specific parameters (mandate,
incentive structure, grant funding level) for economic development impacts.
7. Carve out additional incentives for development incorporating rooftop agrivoltaics near highly impacted
communities.

prepared by E E S C O N S U L T I N G 6 6 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

8 References
Arizona State University. Green Roof Calculator. https://sustainability-innovation.asu.edu/urban-climate/green-
roof-calculator/
Akther et. al., “A Review of Green Roof Applications for Managing Urban Stormwater in Different Climatic Zones”,
Sustainability 2018, 10(8), 2864;
Barnhart, Brad et al. Modeling the hydrologic effects of a watershed-scale green roof implementation in the
Pacific Northwest United states. Journal of Environmental Management Volume 277, 1 January 2021.
Besbes, Karim et al. Green Roofs Impact on Buildings Cooling Load. Purdue University 2012.
https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1068&context=ihpbc
Cardno TEC, Inc. Green Roof Performance Study. Prepared for Seattle Public Utilities. June 2012.
Cavadidi, Giovan and Lauren Cook. (2021). Applied Energy 296 (2021) 117082. Green and cool roof choices
integrated into rooftop solar energy modeling.
Collazo-Ortega, Margarita, Ulises Rosas and Jeronimo Reyes-Santiago. Towards Providing Solutions to the Air
Quality Crisis in the Mexico City Metropolitan Area: Carbon Sequestration by Succulent Species in Green
Roofs. March 2017.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5400495/#:~:text=The%20species%20displayed%20their%2
0typical,meters%20in%20the%20short%20term.
Cove Tool. Solar Panel Cost – Residential + Commercial. https://help.covetool.com/en/articles/4529706-solar-
panel-cost-residential-commercial
Cushman & Wakefield. Green is Good: Sustainable Office Outperforms in Class A Urban Markets. August 2021.
https://cw-gbl-gws-prod.azureedge.net/-/media/cw/americas/united-states/insights-pdfs/green-is-good-
spotlight_final1.pdf?la=en&rev=e26315797d7d49faa6c58ca7762f91a6&hash=7D9FDF170FDB5E46A373233A
D8F2999F
Dialesandro, John et al. Dimensions of Thermal Inequity : Neighborhood Social Demographics and Ubran Heat in
the Southwestern U.S. Environmental Public Health 2021.
European Federation of Green Roofs and Walls, https://efb-greenroof.eu
Feldman, David et al. U.S. Solar Photovoltaic System and Energy Storage Cost Benchmark: Q1 2020. National
Renewable Laboratory Technical Report. January 2021. Available at:
https://www.nrel.gov/docs/fy21osti/77324.pdf
Getter et. al., “Carbon Sequestration Potential of Extensive Green Roofs”, Environ. Sci. Technol. 2009, 43, 19,
7564–7570
Gnecco et. al. “The Role of Green Roofs as a Source/Sink of Pollutants in Storm Water Outflows”, Water
Resources Management Vol 27, pgs. 4715–4730 (2013)
Green Roofs for Healthy Cities, “2018 Annual Green Roof Industry Survey Executive Summary”,
(www.greenroofs.org), October 2019
Green Roof Organization (GRO), “The Gro Green Roof Code, Green Roof Code of Best Practice - Incorporating
Blue Roofs and BioSolar Applications” 2021
Irga, Peter et al. Green Roof & Solar Array – Comparative Research Project. Final Report July 2021. University of
Technology Sydney.

prepared by E E S C O N S U L T I N G 6 7 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

opus.lib.uts.edu.au/bitstream/10453/150142/2/City%20of%20Sydney%20Final%20Report%20EPI%20R3%20
201920005.pdf
Lazard. 2023 Levelized Cost of Energy+. April 12, 2023. Available at: https://www.lazard.com/research-
insights/2023-levelized-cost-of-energyplus/
Lim, “What happens to nitrogen and phosphorus nutrient contributions from green roofs as they age? A review”,
Environmental Advances, Vol. 12, July 2023, 100366
Liu et. al., “Thermal Performance of Green Roofs Through Field Evaluation”, In Proceedings: The First North
American Green Roof Infrastructure Conference, Chicago, IL, 2003 (pgs. 1-10)
Kats, Gregory H. Green building Costs and Financial Benefits. Massachusetts Technology Collaborative. 2003.
Available at : https://www.wbdg.org/files/pdfs/green_bldg_costs_kats.pdf
Kessling et. al., “Feasibility of Combining Solar Panels and Green Roofs on the Activities and Recreation Center”
McIntosh, “Green Roofs in Seattle - A Survey of Vegetated Roofs And Rooftop Garden”, prepared for the City of
Seattle, Dept of Planning & Development and the University of Washington Green Futures Lab, August 2010
National Renewable Energy Laboratory. Documenting a Decade of Cost Declines for PV Systems. February 10,
2023. Available at: https://www.nrel.gov/news/program/2021/documenting-a-decade-of-cost-declines-for-
pv-systems.html
Northwest Power and Conservation Council Memorandum to the Power Committee Members. Subject; Updated
Transmission & Distribution Deferral Value for the 2021 Power Plan. March 5, 2019. Available at:
https://www.nwcouncil.org/sites/default/files/2019_0312_p3.pdf
Northwest Power and Conservation Council. Regional 2021 Northwest Power Plan. March 10, 2022.
Okita, et. al., “Effect of Green Roof Age on Runoff Water Quality in Portland, Oregon”, Journal of Green Building
(2018) 13 (2): 42–54
Paladino & Company, Inc., Green Roof Feasibility Review. February 2006. Available at:
https://kingcounty.gov/~/media/depts/dnrp/solid-waste/green-building/documents/KC_Green_Roof_case-
study.ashx?la=en
Reshmina, William et al. An Environmental cost-benefit analysis of Alternative Green Roofing Strategies.
Ecological Engineering Volume 95, October 2016. Available at:
https://www.sciencedirect.com/science/article/abs/pii/S0925857416304165
Seattle Urban Farm Co. https://www.seattleurbanfarmco.com/
Seattle Public Utilities Drainage Wastewater Rate Study. Available at :
https://www.seattle.gov/documents/Departments/SPU/Services/Rates/2022-2024_DrainageWastewater-
RateStudy.pdf
Seattle Public Utilities and King County. Green Stormwater Infrastructure 2017-2018 Overview and
Accomplishment Report. Available at:
https://www.seattle.gov/documents/Departments/SPU/Documents/Reports/GSI-ProgressReport2018.pdf
Seyedabadi et. al., “Plant Selection for Green Roofs and Their Impact on Carbon Sequestration and the Building
Carbon Footprint” Environmental Challenges, Vol. 4, August 2021, 100119
Shafique, et. al., “An overview of Carbon Sequestration of Green Roofs in Urban Areas”, Urban Forestry & Urban
Greening, Vol. 47, January 2020, 126515
Solutions, Clear Creek. Western Washington Hydrology Model Version 4.2.19. Accessed April 2023.

prepared by E E S C O N S U L T I N G 6 8 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Spokane 2023 Adopted Budget. Available at : https://static.spokanecity.org/documents/budget/2023/2023-
adopted-budget-detail.pdf
Tolderlund, “Design Guidelines and Maintenance Manual for Green Roofs in the Semi-Arid and Arid West”,
University of Colorado, Denver CO, 2010
Unbound Solar. Available at: https://unboundsolar.com/blog/do-you-need-leed-credits-heres-what-we-can-do
August 10, 2023.
United States General Services Administration, “The Benefits and Challenges of Green Roofs on Public and
Commercial Buildings”, May 2021
U.S. Environmental Protection Agency. 2008. Green Roofs, in “Reducing Urban Heat Islands: Compendium of
Strategies”. Draft. Available at: https://www.epa.gov/heat-islands/heat-island-compendium
U.S. Environmental Protection Agency. Stormwater Best Management Practice. Green Roofs. December 2021.
Available at: www.epa.gov/system/files/documents/2021-11/bmp-green-roofs.pdf
USGBC. LEED Certification Fees. https://www.usgbc.org/tools/leed-certification/fees
WAC 194-40-100 through 110. Available at :https://apps.leg.wa.gov/wAc/default.aspx?cite=194-40-
100&pdf=true
WAC 173-444-040
Walston et. al., “Opportunities for Agrivoltaic Systems To Achieve Synergistic Food-Energy-Environmental Needs
and Address Sustainability Goals”, Frontiers in Sustainable Food Systems, 2022
Weather-US.com
Yakima 2024 Preliminary Budget. https://www.yakimawa.gov/services/finance/files/2023-2024-Revised-
Preliminary-Budget-Qtr-Amend.pdf
Yocom, Ken and Ben Spencer. Greenroof Performance Study: Puget Sound Region. Seattle WA. 2012
/https://thecela.org/wp-content/uploads/GREENROOF-PERFORMANCE-STUDY.pdf

prepared by E E S C O N S U L T I N G 6 9 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

9 Appendix A Green Roof Installation Provider Contacts
and Inter view Questions

Sol Terra, info@SolTerra.com (206) 462-1103 https://solterra.com/ (Seattle)

McDonald & Wetle Roofing AnnaB@mcdonaldwetle.com 253-589-8999
https://www.mcdonaldwetle.com/seattle-metro-area/ (Seattle)

Cobalt Exteriors, (253) 844-5141 (serves Seattle, Bellevue, Bothell, Kirkland, Redmond, Renton, Tacoma,
Woodinville) https://www.cobaltexteriors.com/roofing/green-roofing/

Element Smartroofing info@elementsmartroofing.com 425- 968-3000 and (253) 642-7669
https://elementsmartroofing.com/services/roofing/green-roofs-2/
(Serves Eastern WA and Bellevue/Tacoma)

Green Roofs Olympia, 360-968-6120, jraines@olympiagreenroofs.com
https://www.olympiagreenroofs.com/ (Landscape Architect, messaged about leads on green roof
installers/contractors)

Bark Kings (Blower truck service for green roof soils) 425-814-6444
https://www.barkking.com/services/green-roof-soil-installation/
(They’re based in Kirkland but serve eastern WA)

Transblue, inquiries@transblue.com (844) 482-2583 https://transblue.com/locations/WA/monroe
https://transblue.com/locations/WA/tacoma
(Monroe, Tacoma and surrounding areas)

Pacific Landscape Management (503) 648-3900 info@pacscape.com
https://www.pacscape.com/sustainability/green-roofs.php (serves Puget Sound)

Terra Firma NW LLC 360-201-1366 http://www.tfnwllc.com/about_firm.html
(Orcas Island)

All Weather Roof 425-258-4400, info@allweatherroof.net http://allweatherroof.net/
(Based in Everett.)

Nussbaum Group 206-545-0111 michellep@nussbaum-group.com (PM)
https://nussbaum-group.com/ (Based in Redmond, serving regionally)

Alliance Landscape Services 425-359-7544, scott@alliancelandscapeservices.com
https://www.alliancelandscapeservices.com/home.html (Based in Snohomish, serving regionally)

prepared by E E S C O N S U L T I N G 7 0 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Pacific Earthworks info@pacificearthworks.com 206-625-1749 and 360-794-7579
https://www.pacificearthworks.com/ (Based in Monroe, serving regionally)

PCL Construction tbkautz@pcl.com Tyler Kautz, Seattle District Manager (425) 454-8020
https://www.pcl.com/us/en/who-we-are/our-offices/seattle-buildings

Snyder Roofing (425) 402-1848 https://snyder-builds.com/ (Based in Snohomish.)

Northwest Landscape Services https://www.nlswa.com/ (They are now a part of Monarch Landscape
Companies)

Monarch Landscape Companies 833-652-7233 info@monarchlandscape.com
https://www.monarchlandscape.com/landscape-development (Washington)

MIG (SvR merged with them) 206-223-0326 info@migcom.com https://www.migcom.com/green-
infrastructure Audrey West (Seattle Director of Landscape Architecture awest@migcom.com)

Berschauer Group (360) 539-7252 admin@berschauergroup.com https://www.berschauergroup.com/
(Based in Tacoma and Olympia.)

Weston Solutions (888) 404-4743 info@greengridroofs.com

prepared by E E S C O N S U L T I N G 7 1 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

Interview Questions

Introduction: Hello my name is Rachel with Peak Sustainability Group. We were hired by WA dept of Commerce
to evaluate the benefits and costs of agrivoltaic projects on new commercial buildings sized 10,000 square feet or
larger. We’re specifically interested in the installation, O&M costs and considerations of green roof applications.
If you have about 30 minutes we would like to interview you as of several companies we are looking to as we
gather green roof industry information. Is there a time that works for you to have a chat? In the meantime, feel
free to answer the list of questions below. This will help make us of our time and keep our call short.

1. Does your company install green roofs? If so, what services do you offer? (design, installation, O&M)
a. What other types of companies, if any, do you interface with? (Architects, engineers, etc.)
b. Can you recommend any individuals or firms to speak with from an architectural or engineering
standpoint?
2. To what specific cities or regions do you provide these services?
3. What are the estimated number of green roof projects completed by your company per year?
4. Are green roof installations done for existing or new buildings? (one more than the other? Only new
buildings?) – What is the estimated percentage split between existing and new?
5. For green roof projects, what are the primary motivations for building owners? (if known)
6. Are you aware of any green roof incentive programs?
a. Which programs seem to be the most successful, or lead to more work for you?
7. Please describe the most requested projects in terms of the following:
a. Building type (Retail, restaurant, multi-family, office, warehouse, municipal)
b. Building size (is there a minimum or maximum, average SF roof size)
c. Green roof design (extensive and intensive, food growing?)
i. How often is irrigation required?
ii. Are there standard irrigation specifications?
iii. How do costs and maintenance for irrigation vary?
d. Typical coverage for each type
8. What is the cost of green roof installation? Rough estimate in $/SF, or range of estimates. Cost to
include materials, labor, design.
a. Are there supply chain impacts on design, cost, or implementation?
9. What is the cost for O&M in $/SF/year (range or by building installation type?)
10. How long does it take for green roof to be established/successful?
a. What contributes to variability in establishment?
11. Is there a specified plant palette that is used for extensive applications? How does the plant palette
vary?
12. What is the lead time for a client from the time they contact you to when the project is finished?
13. What is the typical crew size for each project? (IE: 1 foreman, 4 laborers)
14. In your experience, what attributes/considerations make for successful green roof installation and
maintenance?
15. What is the typical crew size for each project? (i.e. 1 foreman, 4 laborers)
16. What is the roof size to number of crews ratio? i.e. A 10,000 SF roof gets 2 crews working full-time and
the job is finished in a week.
17. At what point would you need to hire more staff? How many projects would you need scheduled to
expand your staff?
18. In the regions you provide services, can you install green roofs during all seasons or only in some
months?
19. What Best Management Practices do you observe in your practice?

prepared by E E S C O N S U L T I N G 7 2 
W A S H I NG T O N ST A TE DE P AR T ME NT O F CO MME R CE Green Roof and Rooftop Agrivoltaic Benefit-Cost Study 6.01.23

a. Is there a standard followed?
20. Have you completed any projects that also include solar panels?
a. What are the considerations for paired systems (green roof + solar?).
b. When solar is included, does green roof design factor in co-benefits of solar shading plantings?
If so, what assumptions/best practices are used?
c. Are there specific maintenance considerations on roofs with PV’s?
21. Is there anything else we should know about?
a. Can specify examples such as policy, incentives, best practices, what not to do, etc.
22. Is there a modeling software or 3d modeling that you can share to help with energy savings analysis?
a. File extensions xml idf
23. Is it ok if we follow up with other questions, if we have any?

prepared by E E S C O N S U L T I N G 7 3 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

10 Appendix B Interview Responses
1. Does your company install green roofs? If so, what services do you offer? (Design, installation, O&M)
a. What other types of companies, if any, do you interface with? (Architects, engineers, etc.)
b. Can you recommend any individuals or firms to speak with from an architectural or engineering
standpoint?
(GreenGrid Weston Solutions – Michal Krol, in Glastonbury, CT, with several other offices across the
nation.)
We used to do installations but not anymore. However, installation is easy, and we help customers decide
on a proper system. We don’t have enough people to install all the projects we do so we work closely with
installers throughout the US.
The services we offer are design consulting, O&M, installation support, and onsite manufacturer’s
representative.
a.) Architects, engineers, general contractors, roofers, landscapers, homeowners, and green roof specialty
companies.
b.) Depends on location and goals of project

(LMN Architects – Kjell Anderson, Office located in Seattle). We are architects and design green roofs.
a.) Landscape Architects, Engineers, etc.
b.) (we didn’t have time for this) (McDonald & Wetle Roofing) we provide roofing installation; we’re
qualified to lay green roofs, but we don’t because of the inevitability of leaking and other issues, and most
landscapers won’t do the maintenance. (Though they were encouraged to, McDonald & Wetle Roofing
was not interested in answering many questions because they are not fond of green roofs. Their next and
last response is to question 19.)

(MIG-SvR – located in Seattle) We are a group of landscape architects and urban planners, we work with
other fields such as engineers and architects, landscape, and general contractors. Cerna Landscape is their
contact for green roof installations. (Cerna has been called by phone. They do not have a website or listed
email address online.)

(Olympia Green Roofs – Jana Raines, located in Olympia) We design green roofs, we don’t do the
installation or O&M. Other’s that Jana interfaces with are Olympia Master Builders, contractors, and
developers.
Jana recommends speaking with American Hydrotech (listed for future contact), Columbia Green (listed
for future contact), Elizabeth Morris of Green Up Roofing in Portland (listed for future contact), and Olyssa
Starry (a professor at Portland State University, expert in green roofs and storm water management.)

(Pacific Landscape Management – CJ, located in Seattle) We do maintenance and renovation work, and
re-landscape. CJ interfaces with various engineers and architects and recommends contacting Malones
Landscape and Green Effects for these interviews (listed for future contact).

(Landscape Professionals – Brian Mazzola, located in Seattle) We provide a variety of landscape
installation services, and we install green roofs on apartments in Seattle (True Green often does the
maintenance on them, but they are not educated or know what they’re doing.)
a.) Landscape architects, suppliers and their representatives, warranty companies (mainly Suprema,
who provides a 20-year coverage i.e. if veg penetrates membrane they will cover it, but he’s never

prepared by E E S C O N S U L T I N G 7 4 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

seen it happen.) He also provides his own warranty on projects. He occasionally interfaces with
someone from Seattle Public Works when tray systems are used in storm calculations for the City
of Seattle.
b.) He does not have any recommendations of who to speak with.

prepared by E E S C O N S U L T I N G 7 5 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

2. To what specific cities or regions do you provide these services?

(GreenGrid Weston Solutions) Entire U.S., everywhere from Seattle, San Diego, Omaha, to the east coast.
We even have done projects in both HI and AK.

(LMN Architects)
Pacific Northwest
(McDonald & Wetle Roofing) Seattle
(MIG-SvR) Seattle

(Olympia Green Roofs) South Puget Sound Region, if policy were created where people were installing
them then they would offer to all of western Washington.

(Pacific Landscape Management) Throughout Portland and Salem, Oregon and Puget Sound (Issaquah,
Mukilteo)

(Landscape Professionals) He provides service to the Puget Sound area.

prepared by E E S C O N S U L T I N G 7 6 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

3. What are the estimated number of green roof projects completed by your company per year?

(Green Grid Weston Solutions) >100

(LMN Architects) –

(McDonald & Wetle Roofing) –

(MIG- SvR) They have designed 2, but they were value engineered off the projects.

(Olympia Green Roofs) 4-5 at best per year; there are more conversations than actual work – she had to
get another job.

(Pacific Landscape Management) The company is dominant in Oregon and gets 40-50 thousand projects
a year (primarily residential and multifamily). Their recent expansion to Seattle is the result of an
acquisition.

(Landscape Professionals) We do 2-3 green roof projects per year.

prepared by E E S C O N S U L T I N G 7 7 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

4. Are green roof installations done for existing or new buildings? (One more than the other? Only new
buildings?) – what is the estimated percentage split between existing and new?

(GreenGrid Weston Solutions) Both, ~75% on new buildings and ~25% on existing buildings. It used to be
retrofits but has shifted over the years to new construction.

(LMN Architects) –

(McDonald & Wetle Roofing) –

(MIG- SvR) We have only designed for new buildings.

(Olympia Green Roofs) We mostly design for new buildings but have done retrofits. It’s about 60% new
buildings and 40% retrofits.

(Pacific Landscape Management) We’re only getting work on existing buildings right now.

(Landscape Professionals) We never work on existing buildings, only new buildings.

prepared by E E S C O N S U L T I N G 7 8 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

5. For green roof projects, what are the primary motivations for building owners?

(Green Grid Weston Solutions) Amenity spaces, stormwater management, tax abatements, regulatory
compliance, ESG reporting, and general sustainability that led to LEED certification.

(LMN Architects) Occupiable space or anything that allows the developer or building owner to “sell”

(McDonald & Wetle Roofing) –

(MIG- SvR) creating open space for use by the building occupants (residential, office, commercial, etc.)

(Olympia Green Roofs) doing the right thing and having occupiable space if possible.

(Pacific Landscape Management) Residential, commercial, mixed use; higher apartment condos are for
tenant use, mixed use is for creating usable space, commercial for employee break areas.

(Landscape Professionals) Green Roofs help the developer get their building built, but they wouldn’t if
they didn’t have to. Heating/cooling bills are often less overtime as well but that’s a secondary motivation.

prepared by E E S C O N S U L T I N G 7 9 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

6. Are you aware of any green roof incentive programs?
a. Which programs seem to be the most successful, or lead to more work for you?

(Green Grid Weston Solutions) Yes; Upfront capital cost reductions, others that allow for enhanced
building footprint, tax credits, sewer fee (credits back since you are discharging less)

(LMN Architects) Stormwater mitigation program/Policy in Seattle to mitigate stormwater (blue roofs and
green roofs); for example, the Convention Center in Seattle, has 11 strategies with tiered options to
mitigate stormwater (green factor); King County has a few as well. It doesn’t pay back to reuse stormwater
in the building until 10-15 years, but it does help get a high-level LEED certification for the building.

(McDonald & Wetle Roofing) –

(MIG- SvR) No, we haven’t designed green roofs by the help of policy or incentives. Money talks, and
developers (even those who are focused on affordable housing) are using Evergreen Sustainable
Development Standards.

(Olympia Green Roofs) No policies or incentives are leading to more work in the south Puget Sound region.

(Pacific Landscape Management) Not aware of any incentives or policy. CJ has been working for this
company for many years in Portland and recently (in the last 1-2 years) PLM expanded to the Seattle area.

(Landscape Professionals) Seattle has a “Green Factor” where the Landscape Architect must fill out a point
sheet and a green roof adds a lot of points. Green Factor is the reason they get green roof projects.

prepared by E E S C O N S U L T I N G 8 0 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

7. Please describe the most requested projects in terms of the following:
a. Building type (Retail, restaurant, multi-family, office, warehouse, municipal)
b. Building size (is there a minimum or maximum, average SF roof size?)
c. Green roof design (extensive and intensive, food growing?)
i. How often is irrigation required?
ii. Are there standard irrigation specifications?
iii. How do costs and maintenance for irrigation vary?
d. Typical roof coverage for each type in 7a. above (what percent roof coverage is optimal)?

(Green Grid Weston Solutions) a. Mixed-Use Buildings, b. Not really, they can be very small (~500sf) or
large in size (>100,000sf); c. Extensive, i) TBD, depends on many factors (region, plants, etc.) but most
extensive pre-grown modular systems don’t require irrigation, ii) Surprisingly overhead spray is more
effective than drip. Green roof media is very porous so in order to get water to reach every plant you’d
have to space drip irrigation very close. More success with overhead spray. iii) Standard sedum green
roofs are low maintenance but not “no maintenance”. The biggest disappointment in the long run is the
lack of maintenance. Sometimes all it takes is fertilizer once a year and hand weeding throu ghout the
growing season. Well maintained = low effort. Green roof restoration is a lot more expensive than proper
maintenance. D) Full coverage is optimal, lifecycle cost (covering entire roof), energy savings, stormwater
(more linear as it’s based on SF). Larger green roofs (more coverage) mean more energy savings and a
positive effect on stormwater.

(LMN Architects)

a.) Seattle – multifamily primarily and some office projects are the most requested project’s building
types, because people feel like being outdoors (especially with Covid) and typically PV’s as are used
as trellis’s.
b.) Building size is often 25,000 sf on the roof. The Vancouver Convention Center is 1,000,000 sf + has
some PV some blue roof, some planters on the side that are stacked and mitigating storm water with
Silva Cells (an underground tree rate that allows roots to spread out and down, and the structure
serves as an underground bioretention area in tight urban spaces, assisting with stormwater
management.). Townhomes in Seattle (3-4 stories) have occupiable roofs with greenery but no PV.
c.) Usually, green roofs are not growing food, but the case for food is potentially viable if, for example,
herbs are grown for restaurants (since they are expensive and chef’s want them fresh) or tomatoes
are grown, but only for very motivated clients. The Vancouver Convention Center has been keeping
on it and uses the honey in the restaurant and catering for the Convention Center. Food production
is not happening as much though since the depth is 4” trays and there is only 4” of soil above the
root blocker with some water retention texture.
i) Not sure Vancouver conv. Center, changed the plantings, using tall grasses now - those are
successful and shallow;
d.) N/A for ii and iii

(McDonald & Wetle Roofing) –

(MIG- SvR) Multifamily residential where creating open space on the structure can be done sustainably
(providing multiple benefits).

prepared by E E S C O N S U L T I N G 8 1 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

(Olympia Green Roofs) Family, multifamily. It’s been very difficult to find any work designing green roofs.
Many contractors and builders are not familiar with new technology for green roofs and shoot down the
idea. She hasn’t had wide enough installation experience to answer this set of questions.

(Pacific Landscape Management) a.) Multifamily; b.) range between 1500-5000 sf; c.) mostly using 4”
trays, i.) Irrigation is always required. Green roofs are inhospitable spaces, not even xeriscapes do well
due to sunlight and harsh conditions (he’s been doing this for 15 yrs in the pacific northwest). ii.) regarding
standard irrigation specifications, it depends on who designed and installed it. Drip is more efficient due
to runoff and drainage. If the design and layout are not good for irrigation, then they use spray because it
is more forgiving. D.) typical roof coverage is no more than 1/3 of the roof. Due to maintenance and
weight, and possibly price. It depends on the building (the architect would know)

(Landscape Professionals)
a.) We install green roofs on apartment buildings.
b.) The typical roof size is about 3,000 SF, but we install projects from 200 SF to 13,500 SF and the buildings
are midrise from 5-7 stories.
c.) Extensive green roofs are typical, but we have installed intensive roofs for growing grasses and vegetation.
Some architects like to do a mix of both, but there is not much food production. Strawberries and
raspberries would be ideal, and plants with shallow roots that aren’t aggressive growers. They installed
an Aegis senior living building that had galvanized steel 2’x6’ planters for gardens.

d.) i.) Irrigation is always required for at least the first two years. It remains in place after that period in case
it is needed again.
e.)
f.)
ii.) He tries to use spray when he can. Drip is used when the green roof is installed in late fall, so the
plants can root through the winter and then in the spring they can get away with drip irrigation rather
than spray. He typically uses spray heads with rotary nozzles because they work with lower pressure and
distribute water more evenly than other sprinklers. If there is a serious lack of water pressure, he will use
drip instead. This is due to building elevation since they lose ½ pound of water pressure per every foot of
elevation. The higher buildings are better off having extensive green roofs for this reason, but drip is rarely
specific in architectural plans.
iii.) Typically, maintenance costs for irrigation aren’t much. In the spring the irrigation gets turned on
and things are adjusted and fixed as needed every month.
d.) The optimal roof coverage is 1/3 - 1/2 of the roof and no more than that.

prepared by E E S C O N S U L T I N G 8 2 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

8. What is the cost of green roof installation? Rough estimate in $/sq. ft., or range of estimates. Cost to
include materials, labor, design.
a. Are there supply chain impacts on design, cost, or implementation?

(Green Grid Weston Solutions) $5-$15/sf, N/A

(LMN Architects) –

(McDonald & Wetle Roofing) --

(MIG- SvR) Doesn’t remember rough estimate in $/SF but did say that irrigation costs are constantly
changing based on the price of oil.

(Olympia Green Roofs) Extensive green roof with intention of agriculture (new construction) $50-75/sf,
over-head cost; on a structurally secure building around $25-40/sf. Green roofs for healthy cities and other
professional websites with numbers. Regarding supply chains: irrigation costs change when the price of
oil changes.

(Pacific Landscape Management) Installation costs about $200-500/SF but it varies a lot, considering
things like pavers or amenities. Everybody on all phases of the industry right now are experiencing supply
chain issues, likely due to the war in Ukraine for fertilizer, and maybe effects of COVID.

(Landscape Professionals) Roughly speaking, a built-up system (that with no trays, just membranes,
aluminum edging, soil and sedum mats) is $25-35 /sf; and extensive application using sedum trays is $32-
33/SF.
a.) Supply chain impacts include soil, and truck drivers, but right now there aren’t any supply chain
impacts. Columbia Green used to ship soil from Oregon but now Cedar Grove is supplying soil so they are
experiencing any supply delays.

prepared by E E S C O N S U L T I N G 8 3 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

9. What is the cost O&M in $/sq. ft./year (range or by building installation type)?

(Green Grid Weston Solutions) $0.50-$1.50/sf but it’s driven by size and logistics to access. Intensive roof
gardens are going to be very similar to ground maintenance which is driven by plant types and irrigation.

(LMN Architects) Doesn’t remember

(McDonald & Wetle Roofing) --

(MIG- SvR) Doesn’t remember

(Olympia Green Roofs) Extensive green roof with intention of agriculture (new construction) $50-75 per
sf, over-head cost; on a structurally secure existing (?) building likely $25-40; Gr roofs for healthy cities
and other professional websites with numbers;

(Pacific Landscape Management) $5 -$10/SF, but could be as low as $1, and approximately $8-15k/yr for
a commercial building of 50,000 SF coverage with sedums plantings.

(Landscape Professionals) O&M costs are roughly $1000 per year. The biggest issues are weeds and bird
poop and it’s harder to maintain a bigger roof. Generally, 1 hour per week would do it for maintenance.

prepared by E E S C O N S U L T I N G 8 4 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

10. How long does it take for a green roof to be established/successful?
a. What contributes to variability in establishment?

(Green Grid Weston Solutions) ~4-6 months pre-grown from cuttings at the nursery, option to reduce lead
times to ~6 weeks with sedum mat option. ~6-9 months if pre-grown from seed.
a.) Region, weather, plant selection, etc.

(LMN Architects) 6 months – 1 year, it depends on the plants and growing conditions.

(McDonald & Wetle Roofing) --

(MIG- SvR) 6 months - 1 year

(Olympia Green Roofs) 6 months

(Pacific Landscape Management) Plant health is the determining factor for timing.

(Landscape Professionals) It takes 2 years to be established, and irrigation is discontinued after that. a.)
Plant health is the biggest contributor to variability in establishment.

prepared by E E S C O N S U L T I N G 8 5 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

11. Is there a specific plant palette that is used for extensive applications? How does the plant palette
vary?

(GreenGrid Weston Solutions) We use a mix of sedum that can vary from 6-16 different species. Plant
palette can vary based on color specifications and sun exposure.

(LMN Architects) It depends on what is growing well and may need to be adjusted. Typically, plants that
do well in the wind and rain and those that can handle some drought.

(McDonald & Wetle Roofing) –

(MIG- SvR) Sedums (variety for visual interest) grasses; Dianella (a New Zealand native) holds up well in
the wind and is drought tolerant, keeps its shape and form well.

(Olympia Green Roofs) sedums and grasses but can also be shade plants.

(Pacific Landscape Management) Sedums, the top 35 plants in PNW.

(Landscape Professionals) Extensive applications only use sedums. Intensive applications require drought
tolerant grasses (blue fescue, pennesetum, juncus, etc.)

prepared by E E S C O N S U L T I N G 8 6 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

12. What are the challenges for commercial green roof installations you’ve come across?

(GreenGrid Weston Solutions) Lead times, construction delays, improper phasing (green roof installed too
early), Health and Safety (H&S; retrofits might not have accommodations for meeting H&S requirements,
post-installation maintenance)

(LMN Architects) It costs a lot of money and there is no pay back, the client needs to think the project will
be sellable (or incentives need to be provided), maintenance issues including access and proper timing.

(McDonald & Wetle Roofing) Green roofs prone to leakage, and finding landscape maintenance crews is
very difficult.

(MIG- SvR) Long term risk of project failure; it’s a costly installation; developers are concerned with owning
a building long term, and that they have to deal with the issues that arise.

(Olympia Green Roofs) Lack of roofing companies being willing to learn to trade old for new.
These projects bring in another insurance component. People who were willing to have the conversations
were young businesses, altruistic, but didn’t have the overhead.

(Pacific Landscape Management) Moving material through surface elevators, cost of cranes depending on
building height, logistics.

(Landscape Professionals) 12th floor – the wind was a big challenge. It’s like a tunnel. We had to use special
pieces to hold down sedum mats.

prepared by E E S C O N S U L T I N G 8 7 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

13. In your experience, what attributes/considerations make for successful green roof installation and
maintenance? What have been lessons learned?

(GreenGrid Weston Solutions) Proper lead times and planning, maintenance piece is owner commitment.
Project schedules and timing that don’t add additional costs.

(LMN Architects) Occupiable roofs that have greenery and then green roofs that are not occupiable,
occupiable roof can have a ton of people on it, and if it’s not occupiable it needs to have maintenance
workers on it and there needs to be a door, smokers have to be 25’ away from the doors, client pays for
it.

(McDonald & Wetle Roofing) --

(MIG- SvR) Plant performance considering harsh conditions. Installers for solar panels need to be very
careful to not puncture the green roof membranes. Creating occupiable space is where projects get built,
and using pots and 4” trays is the easiest for maintenance. Many don’t want to pay the high overhead and
those who are willing to go the extra mile are usually young and can’t afford the high overhead costs.

(Olympia Green Roofs) Using the right products with the latest technology and having skilled labor to do
the work. Projects are rare due to costs and lengthy waits for benefits to building owners. Maintenance
and fear of failure deter the trades from being interested.

(Pacific Landscape Management) Craftmanship and design are important. Not everyone is as good at
building things and often the outcome of the installation depends on the crew.

(Landscape Professionals) The biggest indicator of success is maintenance.

prepared by E E S C O N S U L T I N G 8 8 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

14. What is the typical crew size for each project? (IE: 1 foreman, 4 laborers)
(GreenGrid Weston Solutions) 1 foreman, 4 laborers

(LMN Architects) N/A

(McDonald & Wetle Roofing) --

(MIG- SvR) N/A

(Olympia Green Roofs) N/A

(Pacific Landscape Management) It depends on the project. It could even be just 2 laborers.

(Landscape Professionals) 1 foreman, 4 laborers.

prepared by E E S C O N S U L T I N G 8 9 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

15. How many hours of labor per 100 sq. ft.?

(GreenGrid Weston Solutions) 2,500 sf/day to start, +5,000 sf/day for multiple days. With GreenGrid
products it takes 15-30 minutes to install 100SF.

(LMN Architects) N/A

(McDonald & Wetle Roofing) –

(MIG- SvR) N/A

(Olympia Green Roofs) N/A

(Pacific Landscape Management) 20-40 hours

(Landscape Professionals) 1,000 SF per day with a crew of 5 working 8 hours. Or, 2.5 hours per 100 SF.

prepared by E E S C O N S U L T I N G 9 0 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

16. In the regions you provide services, can you install green roofs during all seasons or only in some
months?

(GreenGrid Weston Solutions) Depends on region, some allow for yearly installations other only during
the warmer months (temps above freezing and no snow cover)

(LMN Architects) N/A

(McDonald & Wetle Roofing) –

(MIG- SvR) N/A

(Olympia Green Roofs) – We did not have time for this question.

(Pacific Landscape Management) We install in all seasons.

(Landscape Professionals) We install in all seasons.

prepared by E E S C O N S U L T I N G 9 1 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

17. What Best Management Practices (BMPs) do you observe in your practice?
a. Is there a written standard or set of referenced specification followed?

(GreenGrid Weston Solutions) Sum of everything that has already been said

(LMN Architects) Several Rules of thumb for solar, keep penetrations out of major areas of the roof,
plumbing penetrations can result in losing several panels minimum. Laying it out, add 4-10 lbs to roof load
per SF to support ballast and other systems to hold PV down. Keep plumbing penetrations out of the green
roof medium. Yes, there is a standard to follow.

(McDonald & Wetle Roofing) –

(MIG- SvR) Using planters for plants on roofs makes easier maintenance and less goes wrong
BMP – making sure waterproof layer is protected – biggest issue is accidental penetration of the
membrane.

(Olympia Green Roofs) Following the guidelines put out by the German Commission with those standards
and protocols by Green Roofs for Healthy Cities; don’t try to reinvent it.

(Pacific Landscape Management) We focus on staff training and follow all specifications on architectural
drawings.

(Landscape Professionals) Columbia Green provides a great maintenance packet. The general
maintenance person is the one in charge of maintaining the green roof.

They installed a gated dog run, but people didn’t use it and they let their dog wander wherever cau sing
them to pee and poo on the green roof area rather than the dog run. Most have a railing to keep people
from falling off a building. The dog excrement affects the performance of the green roof.

a.) Columbia Green’s package has good specifications and Brian can send it over if needed.

prepared by E E S C O N S U L T I N G 9 2 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

18. Have you completed any projects that also include solar panels?
a. What are the considerations for paired systems (green roof + solar?).
b. Are there specific solar-green roof configurations that are typically favored?
c. When solar is included, does green roof design factor in co-benefits of solar shading plantings?
If so, what assumptions/best practices are used?
d. Are there specific maintenance considerations on green roofs with PV’s?

(Green Grid Weston Solutions)

a.) They are complimentary, green roof keeps the roof cooler which makes the solar panels more efficient
when they run cooler.
b.) Green roof in one area and solar in another or alternating.
c.) Project location and climate specific.
d.) Not typically, maybe in high snowfall areas maybe you wouldn’t want green roof under where snow
would pile up high.

(LMN Architects)

a.) Occupiable space, creating enough space for maintenance, proper installation.
b.) Trellis PV’s for shading occupiable areas.
c.) No specific plants to report but yes there are co-benefits.
d.) 1/3 area is mechanical space, green roof is more rare than using PV, PV is inexpensive and screams
sustainability, historically more green roof than PV but economics are changing and WA State energy code
requires PV on roof although there are exceptions for it.

(McDonald & Wetle Roofing) No

(MIG- SvR) Yes; a.) There was a project where about 20% roof coverage were vines and 50% roof coverage
was solar; b.) this was for an overhead parking structure; c.) It can, but our design wasn’t focused on using
co-benefits; d.) Proper access space.

(Olympia Green Roofs) Hot completed any projects, but have researched and designed. (The following
answers are based on Jana’s research.) a.) They work symbiotically together; the cooling environment is
sustainable for colocation.

The engineering and intensive green roofs are quite heavy. 25-30% has to be green roof or solar and the
weight can be dispersed on the structure (Denver policy). Areas that can’t handle weight load must be
non-vegetated.

b.) Contact Jennifer Buffalo from Colorado State University, she’s an ag-voltaic expert.

c.) Sedums and grasses do well with shaded area; can get a greater diversity of plants to grow in shade
d.) Be careful to not damage the membrane and the solar panels.
There are many leakage options, and to avoid them everything needs to be kept in-place and well-sealed.
The maintenance person needs education around the needs of both PV’s and green roofs. Right now,
anyone can do the work that wins the bid, and they aren’t necessarily educated with the latest technology
and installation methods. We need the labor field to be educated and standardized.

prepared by E E S C O N S U L T I N G 9 3 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

(Pacific Landscape Management) No, we have not done PV work.

(Landscape Professionals) Yes, we worked on a project with both green roofs and solar panels. It was in
the lower Queen Ann area of Seattle and the PVs were outside of the green roof area.
a.) It’s best if green roofs and solar panels are kept separate for maintenance.
b.) He would say that it’s favorable to have the two systems kept separate and not integrated.
c.) N/A

d.) No maintenance considerations, just keep green roofs and solar panels separate.

prepared by E E S C O N S U L T I N G 9 4 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

19. Is there anything else we should know about?
a. Can specify examples such as policy, incentives, best practices, what not to do, etc.
(GreenGrid Weston Solutions) No.

(LMN Architects) Don’t puncture the green roof membranes when installing PV’s or plumbing. Provide
adequate space for maintenance workers and mechanical needs (roughly 30% of roof space) be sure to
include a door that maintenance folks can use to access the roof, but that others can’t use in the case it’s
not an “occupiable space”. Be sure there is space for smokers to be 25’ away from the door if it is
occupiable space.

Solar panels and green roofs are not mutually exclusive, especially with maintenance.
When wanting wind or rain protection, it makes sense to have a terrace use of PV on roofs.

A good example of green roof and solar project is the Austin Central Library in Austin, Texas – has an
occupiable terrace for the public and a trellis of PV that blocks some sun and is compelling from an
architectural point of view.

There is a case for using blue roofs in the pacific northwest (which stores water on the roof and delays
stormwater.)
Green roofs are often used in urban areas because cities have less greenery and taller buildings where
people want to add to a view or want to see the existing view with a specific space for viewing (a case for
a green roof).
Other apt buildings around Seattle have an occupiable terrace on the roof with hardscape and landscape
but none have combined landscape and PV, it’s primarily hardscape under PV.
A project in Madison, Wisconsin had some PV on the terrace and used a green-blue roof to capture water
for use in toilets, contributing to stormwater management.
Green roofs are best used in occupiable areas and for capturing rainwater, which could be used in the
building for toilets only. If people are on the roof, then the water needs to be treated before it is used in
the building, even for toilets.

(McDonald & Wetle Roofing) Policy and incentives would just anger the industry because suppliers are
responsible for the technology. The problem lies in extreme amounts of water on the roof and it’s not
easy to clean. Environmentally it’s a great idea, but they are hard to maintain. More of these are done in
Portland due to their policies.

(MIG- SvR) - Money talks, Developers are using evergreen design standards (developers that focus on
affordable housing). One of the deciding factors of a project that kept the green roof was having the roof
space for use for residents and events, rather than the sustainability factor (insulation and stormwater).
The last green roof project MIG-SvR had was 7 years ago and the green roof didn’t go through the
beginning phase of the design. We designed an agrivoltaic project that had vine trellises and solar panels
over the parking garage of a government building, but it got value engineered out of the installation.)

(Olympia Green Roofs) Seattle’s policy gets worked around and there’s a handful of failures in the policy
terms which aren’t being lived up to, or developers have found ways around it (like the case in Denver
where folks can pay their way out of it.)

prepared by E E S C O N S U L T I N G 9 5 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

Recommendations for policy (like Denver) needs to be a voter-driven policy change, we need signatures
to put it on a ballot because the building industry has shot it down a few times and it needs to be a voter
driven policy. A strong policy is needed, where there’s a task force to figure out how to make it work.
Another person to contact is Anna Thurston who wrote a thesis on green roof failures at Colorado State
University’s College of Agricultural Sciences.

Commercial scale projects should specify using American Hydrotech’s waterproofing membrane systems
for the ease of instruction and technology that simplifies installation.
Cities don’t know what to do regarding permitting and what to inspect. The City or State needs codes that
address problems like lengthy applications and proper practices (even just how to get a permit for a green
roof). For example, a retrofit falls under re-roof and that doesn’t require a permit, but new roofs do
require a permit (at least for residential projects in Thurston, Mason, Clallam, and Jefferson counties.)

(Pacific Landscape Management) Anytime you give a rebate structure, you’ll win as long as it’s simple and
not lengthy.

(Landscape Professionals) Green roofs and solar panels look good and have benefits. Bryan wishes the
owners or management would want to take better care of them. There needs to be an emphasis on
maintaining the landscape.

prepared by E E S C O N S U L T I N G 9 6 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

20. Is there a modeling software or 3d modeling that you can share to help with energy savings analysis?
(File extensions in .xml .idf)
(GreenGrid Weston Solutions) No, but GreenGrid has system specific energy performance data that can
be modeled using standard modeling software.

(LMN Architects) PV Watts for standard modules

(McDonald & Wetle Roofing) ----

(MIG- SvR) The engineers may have used modeling software for cost savings on government projects, but
they don’t typically use models. Usually when MIG is the lead they have a consultant doing to modeling.

(Olympia Green Roofs) SketchUp, but the Green Roofs for Healthy Cities website has cost-savings models.

(Pacific Landscape Management) No.

(Landscape Professionals) No

prepared by E E S C O N S U L T I N G 9 7 
W A S H I NG T O N ST A TE Green Roof and Rooftop Agrivoltaic Potential: Assessment for Washington 10.17.22

21. May we contact you with further questions?
(Green Grid Weston Solutions) Yes
(LMN Architects) Yes
(McDonald & Wetle Roofing) No
(MIG- SvR) Yes
(Olympia Green Roofs) Yes
(Pacific Landscape Management) Yes
(Landscape Professionals) Yes

prepared by E E S C O N S U L T I N G 9 8 

File and source

File: 2023_commercereports-agrivoltaics-benefit-cost-analysis-final-report-final-.pdf
Size: 2,926,214 bytes
SHA-256: e7399dbdd52ed656e5c654505128ceca6beab6d20bd351189d41130644836595
Our copy: 2023_commercereports-agrivoltaics-benefit-cost-analysis-final-report-final-.pdf
Original: app.leg.wa.gov

Green Roof and Rooftop Agrivoltaic Benefit-Cost Analysis — Washington State Department of Commerce (June 22, 2023)

Summary

Cite this document

Full text

File and source

Related records