Green roofs help control pollution

Landscape group hopes its garden influences builders

Go Garden


WASHINGTON -- In its infancy the nation's capital boasted wide boulevards thick with trees that mollified its Spanish summers and English winters.

For transportation, its modest population relied on horses, carriages and walking. Two centuries later, Washington's air quality has plummeted and much of the greenery has vanished. This month, the American Society of Landscape Architects came forward with a solution: a roof that grows grass.

In a revitalized area of the city's downtown, ASLA has built a new roof over its headquarters to grow grass and other plants that help cities with limited green space improve air and water quality.

A few weeks ago, ASLA invited visitors to stroll around its roof, which will be opened periodically to the public.

"Green roofs really have terrific potential for helping cities with their air- and water-quality problems," said Nancy Somerville, ASLA's executive vice president and chief executive officer.

Cities, with their concrete and asphalt, retain heat and raise temperatures, creating an intense "heat island." In addition to being uncomfortable for residents, the heat traps smog and thereby exacerbates pollution. Green plants on a roof can reduce this heat effect, in addition to filtering polluted air and exhaling oxygen.

Green rooftops also retain rainwater, preventing sewerage systems from becoming overwhelmed during heavy rainfalls. And they improve water quality by filtering the storm water that does run off.

The Washington project covers 3,300 square feet, and ASLA officials predict the roof will influence builders nationwide as well as in the capital.

Just inside the ASLA headquarters, visitors confront a photo portrait of Frederick Law Olmsted (1822-1903), a leading pioneer in landscape design. He helped create such notable landmarks as New York's Central Park, Brooklyn's Prospect Park, the Chicago suburb of Riverside and the design for the 1893 World's Fair in Chicago.

The demonstration project occurred in the heart of Washington's Chinatown, with its stylized lantern streetlights. Clear skies allowed visitors to see the faded metal tower of a nearby Victorian department store, now undergoing conversion to condominiums, the tile dome of a restored synagogue and the gold tentacles of the stylized dragon on the Chinese arch on the street below.

Christopher Counts, a landscape architect with the Maryland firm of Michael Van Valkenburgh Associates Inc., said, "Sixty percent of landscape architecture is done on structures that could use green roof technology.

"In Europe, the green roof garden has an established history," he added, mentioning the green roof that Le Corbousier, the French architect, installed on top of his Paris apartment in the early 1920s. Germany has been using roof gardens for decades.

"The making of a traditional roof garden is not really a lot different than building one outside the traditional roof garden," Counts said.

Washington's new roof garden took shape within three months inside inverted metal gratings with soil depths of 6 inches and 4 inches overhanging the front entrance and 4 inches over the back entrance. Mixing stone with grass and other plantings such as cactus and trumpet vine to prevent runoff of water, the designers installed a wooden deck, gravel service walkway and steel railings for visitors. The stair tower on the other side of the roof has soil depths of 12 inches.

In the District of Columbia, the new green roof was first supported by the Chesapeake Bay Foundation to improve the water quality of the city's Anacostia and Potomac rivers, which flow into the bay.

Other projects on the foundation's agenda will go mainly to the developers of the new 68,000-square-foot U.S. Department of Transportation headquarters, the 5,000-square-foot green roof of the Department of Parks and Recreation Center slated for completion late this year or early next year and the 10,000-square-foot green roof of the Anacostia Economic Development Corp., scheduled for planting next year.

Glen Elsasser is a freelance writer for the Chicago Tribune.


Public tours of the green roof are available by appointment on Tuesdays and Thursdays by calling the American Society of Landscape Architects at 202-898-2444.

You can also see the green roof by visiting the ASLA Webcam at

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