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The U.S. Environmental Protection Agency installed a green roof this past March on its Denver headquarters. Photos: Green Grid Green Roofs
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The American Red Cross of Greater Chicago building has a modular green roof. The 2800-square-foot roof was installed in the summer of 2004.

Scientists predict that the salmon population in Seattle's Puget Sound will be extinct within the next 60 years, according to an article in the Oct. 11, 2006, Seattle Post-Intelligencer. The paper reports that “the scenic bay is under assault ... people do not understand that Puget Sound is turning into a cesspool.”

Even though more than $100 million has been spent to clean up the area, the pollution problems persist, with the main culprit being stormwater runoff. With each rainstorm, a hefty dose of algae-fertilizing nutrients along with assorted toxic filth washes off streets, yards, and building roofs and eventually flows into the harbor.

Government officials and scientists have known for many years that polluted storm-water poses a potential threat to the health of the area. They now realize that the problem is not years away, but on their doorsteps.

Problems such as this are not found just in Seattle. In New York City, as much as 30% of the stormwater runoff after a major rainfall event is released into the Hudson and East Rivers because the city's overstressed wastewater treatment plants can't handle it.

The problems of stormwater runoff are found in virtually every major urban area around the world. Among the solutions, city planners and others realize, is to build more and larger wastewater treatment plants that can better handle the runoff and remove pollutants before they enter waterways. But these facilities can be extremely costly. An alternative is to encourage or require that new commercial and residential developments include enough trees, plants, and parks around them to absorb the runoff.

To address this issue in cities and large urban areas, many scientists and planners are recommending that more buildings be outfitted with green roofs. These vegetation-covered roofs act as sponges, absorbing huge amounts of stormwater, and what is not absorbed is filtered by the roof and released gradually so that it doesn't overburden local sewer systems.

Stopping Runoff

Many city officials, planners, and environmentalists view green roofs as an attractive way to reduce the runoff problem. Recent studies also have found that green roofs help lower energy costs because of their insulating qualities and also help reduce noise. Green roofs minimize the urban heat island effect that makes cities warmer in summer than surrounding forested areas, provide a natural habitat for birds and insects, and absorb carbon dioxide and other pollutants while releasing oxygen into the atmosphere.

The type of green roof most often considered today is referred to as an extensive green roof system. Hearty, climate-resistant plants and vegetation, such as sedum, are planted into approximately 4 inches of soil.

Some extensive green roofs are built directly on the roof, although a newer technology—modular green roofs—recently has been developed where the plants and soil are pre-planted into modules made of recycled plastic that are placed directly on an existing roof. This system was developed to reduce the overall cost of green-roof installation.