October 26, 2007—The American Society of Landscape Architects (ASLA) has released a report with the results of the first year of its green roof demonstration project, ranging from water and temperature monitoring to individual plant performance. ASLA says its green roof retained thousands of gallons of stormwater, reduced building energy costs by hundreds of dollars a month, and significantly lowered outdoor air temperature.
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, says the organization. Except during repeated heavy rains, the roof only created runoff during rainfalls that exceeded one inch. The water runoff itself reportedly contained fewer pollutants than typical water runoff.
ASLA says its 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.
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.
For more information visit ASLA’s Green Roof Project Web site.
