The decision of whether to install a green roof should be considered on a case-by-case basis. Variability in structure, municipality, ownership, tenant, investment, technology, climate and other aspects requires specific attention to ensure accuracy.
It can be difficult to make the financial case for green roofs due to the higher installation costs compared to conventional roofs. A lifecycle analysis of each can counter these concerns. The added cost of installing a green roof is mostly offset by its increased longevity; however, the added maintenance costs can be significant. Over a 50-year period, the stormwater, energy, carbon dioxide equivalent (CO2e, which measures the potential global warming effect of a greenhouse gas) and community earnings of green roofs often more than make up for the increased premium of installing and maintaining them.
Green roofs provide an estimated payback (based on a 50-year average annual savings) of approximately 6.2 years nationally (internal rate of return of 5.2%) and 6.6 years in Washington DC (internal rate of return of 4.2%). Conservative analysis puts the average life expectancy of a green roof at 40 years, versus 17 for a conventional roof.1
When making roofing project decisions, you should compare the lifecycle cost of the proposed green roof to the conventional option for your facility (often a white or black roof). See Life Cycle Approach for additional guidance. A generic cost-benefit analysis of green roofs versus black roofs, at both the national level and Washington, DC level, is available from GSA's Green Roof page.
When comparing roof options, remember to include the following in both lifecycle analyses:
- Growth rates for labor and materials, energy, and stormwater
- Diminishing returns (based on expected increase in supply)
- Discount rate
- Standardized timeline (recommended 50 years)
- Cost of the roof membrane (required for all new roofs and most replacements)
It is also important to consider the increase in real estate value associated with installing a green roof. A green roof can count toward certifications such as LEED and Energy Star. A 2011 CoStar study found that "Non-LEED buildings had 4.81% lower average rents than the broader market while LEED buildings were 7.38% higher."2 The Rocky Mountain Institute found that LEED and Energy Star buildings claim an 11-26% sales price premium.3
Be sure to consider the impact of a green roof on your facility’s risk, as this is often not captured in a lifecycle analysis. For example, an extensive green roof can function as a preventive fire break, which lowers the risk of fire spreading. This may be reflected in the lifecycle analysis if it also lowers insurance premiums.