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September 19, 2007, 01:56 PM ET
Landscape Architects Review the Benefits of Green Roofs
The American Society of Landscape Architects has been studying its green roof, shown here (Photograph by the American Society of Landscape Architects)
Green roofs might be on top of buildings, but landscape architects have long regarded them as part of their domain. The American Society of Landscape Architects, often known simply as ASLA, will release data from a study of the green roof on its building in Washington, D.C. (When we have a link for the report, we’ll post it here.)
The study should be of interest to facilities managers at colleges and universities, which have been installing green roofs on buildings in growing numbers.
Green roofs are often touted for the additional insulation (and subsequent energy savings) they provide for the roof, for their ability to retain storm water and the pollutants that water might carry, and for their ability to reduce the “heat-island effect” — the heat radiating from pavement and rooftops, which leads to higher temperatures in urban areas.
In terms of energy, ASLA’s study showed that the green roof led to a 10-percent reduction in energy use in the winter and a negligible difference in the summer. The researchers determined that the building was overcooled; they will fix this problem and continue to monitor the energy savings. The surface temperature on the roof was as much as 32 degrees cooler than nearby roofs. The parts of the roof with thicker vegetation were coolest.
The roof retained much of the water that fell on it — 100 percent in a one-inch shower, but less if it rained more. The green roof did not have excess nitrogen in its runoff, but it did have excess phosphorous. The researchers expect less excess nutrients in the runoff as the plants on the roof continue to grow.
The researchers also tested different species of plants. Sedum grew well, as did sumacs, grasses, and milkweed. Black-eyed Susans and New Jersey tea struggled.
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