Landscape Architects' Green Roof Kept Runoff Out of DC Sewers

WASHINGTON, DC, September 21, 2007 (ENS) - In July 2006, The American Society of Landscape Architects, ASLA replaced the conventional roof on its downtown Washington, DC headquarters with a green roof, and in the process installed equipment to gather data on stormwater runoff, water quality, and temperature.

When the figures were tallied in May, the new green roof was found to have retained thousands of gallons of stormwater, reduced building energy costs by hundreds of dollars a month, and lowered outdoor air temperature.

An ASLA report released Thursday shows that between July 2006 and May 200, ASLA's green roof prevented 27,500 gallons of stormwater - nearly 75 percent of all precipitation on the roof - from flowing into the Capital District'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 says.

"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 ASLA Executive Vice President and CEO Nancy Somerville. "The findings show that our green roof delivered significant economic and environmental benefits."

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 said.

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 use in the winter by 10 percent and has a potential to lower use of energy two to three percent in the summer.

The ASLA roof design includes two elevated "waves" formed from rigid insulation and covered with a green roof system. The waves create a pleasant foliage-enclosed space and block the view of the rooftop HVAC systems. The north-end wave cantilevers over one of the larger HVAC units.

Surrounding these waves is an extensive green roof system covered by aluminum grating that allows visitors to walk over the plant material without damaging it. The innovative use of grating over sedum for the central area and access path provides almost total green coverage for the roof while creating an area that can be enjoyed by the building occupants and accessed for maintenance.

One of the waves is planted primarily with sedums; the other wave, with slightly deeper soil, is planted with drought-resistant perennials and grasses as well as sedums.

When designing the green roof, ASLA experimented with a variety 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 performed well, but other sedum species did not.

On the intensive portion of the roof, flame sumac, smooth sumac, trumpet vine, and pasture rose were successful while Ceonanthus americanus, called New Jersey Tea, struggled.

Because of the small amount of water-retaining green space in urban areas, ASLA says, stormwater systems can be overwhelmed during periods of heavy rains. In the many cities that have combined sewer systems, the result is the release of untreated sewage and stormwater into rivers and lakes. Green roofs, which can retain up to 75 percent of a one-inch rainfall, alleviate pressure on city’s overburdened sewer systems caused by stormwater.

The full briefing report, the comprehensive water monitoring report and detailed planting information is online at http://www.asla.org/greenroof.

Founded in 1899, ASLA is the national professional association for landscape architects, representing more than 17,600 members in 48 professional chapters and 68 student chapters.

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