Landscape Architects Release Green Roof Performance Report

Roof Retained 27,500 Gallons of Stormwater in First Year


Washington, DC, September 19, 2007 — The American Society of Landscape Architects’ (ASLA) green roof retained thousands of gallons of stormwater, reduced building energy costs by hundreds of dollars a month, and significantly lowered outdoor air temperature according to a report today from the association. The report examined various components of ASLA’s green roof demonstration project, ranging from water and temperature monitoring to individual plant performance.

“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,” said Nancy Somerville, Executive Vice President and CEO of ASLA. “The findings show that our green roof delivered significant economic and environmental benefits.”

In 2006, ASLA replaced the conventional roof on its downtown Washington, DC headquarters with a green roof, installing equipment to gather data on stormwater runoff, water quality, and temperature.

From July 2006 to May 2007, ASLA’s green roof prevented 27,500 gallons of stormwater—nearly 75 percent of all precipitation on the roof—from flowing into Washington, DC’s overburdened sewer and stormwater system. Except during repeated heavy rains, the roof only created runoff during rainfalls that exceeded one inch. The water runoff itself contained fewer pollutants than typical water runoff.

ASLA’s green roof lowered air temperature by as much as 32 degrees in the summer when compared to a neighboring tarred roof, helping mitigate the urban heat island effect.

“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 continued.

The roof also reduced the building’s energy costs—especially in the winter. Engineering analysis showed that the green roof’s extra insulation lowered energy usage in the winter by 10 percent with a potential of two to three percent in the summer.

When designing the green roof, ASLA experimented with varying types of plants. The extreme nature of the rooftop environment allowed some to thrive while others struggled. On the extensive portion of the roof, hardy species of Sedum (Sedum album, Sedum reflexum, Sedum spurium, and Sedum sexangulare) performed well over other Sedum species (Sedum lanceolatum and Sedum stenopetalatum). Delosperma nubigenum (Ice Plant) did well in many areas but not well in the north terrace.

On the intensive portion, Rhus copallina (Flame Sumac), Rhus aromatica (Smooth Sumac), Campsis radicans (Trumpet Vine), and Rosa Carolina (Pasture Rose) were successful while Ceonanthus americanus (New Jersey Tea) struggled. Detailed information can be found at

The full briefing report can be found here. The comprehensive water monitoring report can be found here.


Kevin Fry
Director, PR and

JR Taylor
PR Coordinator