materials, stormwater costs, energy prices, carbon (if valued),
community benefits (inflation), rent, absorption and retention,
were included in the cost-benefit analysis.
Stormwater equipment costs (equivalent BMP/equipment cost
for stormwater management) and surcharges (stormwater utility
fee and water impervious charge in the District of Columbia),
heating and cooling savings (and carbon savings associated with the
energy savings), and carbon sequestration were included in the
cost-benefit analysis. As a result, a 12,000-square-foot green roof
on a building in Washington, D.C. resulted in a net present value of
-$7,075, a payback of 6.5 years and a return on investment (ROI)
of 193.8%.
Additionally, a real estate assessment analyzing the risk (capitalization rate), average rent, average vacancy, absorption and tenant retention in Washington, D.C. were included in the
cost-benefit analysis.
Finally, the cost-benefit analysis reviewed the affect on the
community by valuing the benefit green roofs have on biodiversity and habitat, air quality, heat island (energy and peak demand),
and the wastewater treatment plant.
The assumptions and results of the analysis are shown in Table
1 and tabulated in Table 2.
Sensitivity analysis
A sensitivity analysis was performed to identify the more important variables based on their impact on the total net present
value (see Figure 1). Table 3 ranks the most important variables
affecting the net present value of the green roof, where the roof
life and installation cost have the greatest affect on the net present value.
The cost of a green roof is a major factor for a developer or
building owner. Capital costs, maintenance costs, and expected
roof membrane longevity are the major factors affecting the net
present value. It is important to understand that this analysis
was conducted using current data. As more and more green
roofs are installed and their performances are monitored, the
installation costs, maintenance fees and perceived risk of the
technology should all come down, while stormwater fees and
energy prices are predicted to rise more than inflation. As a result, green roofs currently look to be a net neutral investment
when looking at only the basics (installation, maintenance, replacement, stormwater and energy). However, by broadening
the scope of financial return (i.e. carbon, real estate impacts,
community benefits, productivity), the financial viability of
green roofs as compared to conventional black roofs is bound
LI
to become increasingly attractive.
Adam Friedberg, PE, LEED AP, is senior sustainability consultant
for Arup in New York, New York.
Jordan O'Brien, LEED AP, is a senior management consultant, Sustainability & Real Estate, for Arup in San Francisco.
Article courtesy of Green Roofs for Healthy Cities (GRHC), and
was originally published in GRHC's Living Architecture Monitor:
www.livingarchitecturemonitor.com.
GHRC's GreenSave Calculator is a life cycle cost-benefit tool available to members online.
www.landscapeirrigation.com
Landscape and Irrigation 11
