Green Roofs Prevent Runoff, Save Money in Energy Costs: Study

Washington, D.C., USA – The green roof installed atop the headquarters of the American Society of Landscape Architects (ASLA) saved the group hundreds of dollars each month in energy costs and avoided thousands of gallons of stormwater runoff.

The organization released a report Wednesday detailing the environmental benefits of its green roof demonstration project. It installed the green roof in 2006, as well as monitoring equipment to measure stormwater runoff, water quality, temperature and plant growth.

“Because landscape architects are leading in the design of green roofs across the country, it was important for us to build a demonstration project and measure the impact green roofs have on their surrounding communities,” Nancy Somerville, ASLA executive vice president and CEO, said in a statement. “The findings show that our green roof delivered significant economic and environmental benefits.”

The results indicate the roof avoided nearly 75 percent of the stormwater, or 27,500 gallons, from joining the capital’s sewage and stormwater system. Runoff occurred only when rainfall exceeded an inch, and it contained fewer pollutants from ordinary runoff. Air temperature dropped by as much as 32 degrees in the summer compared to a nearby tarred roof.

The green roof’s extra insulation decreased the building’s energy usage by 10 percent in the winter, and by 2 percent to 3 percent in the summer.

“Collectively, green roofs can save billions of dollars in urban infrastructure costs, which is why more and more cities are encouraging them through tax and other incentives,” Somerville said.

Green roofs initially cost between $10 and $20 per square foot, compared to $5 to $10 per square foot for a conventional roof. Aside from the energy savings, green roofs significantly extend the lifespan of the roof membrane because of sunlight and temperature variance protection.

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