Green roofs are starting to sprout in American cities (Energy Bulletin)

by Bruce Stutz

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Long a proven technology in Europe, green roofs are becoming increasingly common in U.S. cities, with major initiatives in Chicago, Portland, and Washington, D.C. While initially more expensive than standard coverings, green roofs offer some major environmental — and economic — benefits.

The biggest green roof in New York City and one of the largest in the country, the Morgan facility’s verdant covering was completed in December 2008 and has thrived since. As the inscription above the landmark James Farley Post Office might have it, the roof has been affected by “neither snow nor rain nor heat nor gloom of night,” and has flourished through freezes and thaws, through summer rooftop temperatures that reach 150 degrees, and through weeks of drought and torrential summer storms, despite never being watered, weeded, or fertilized.

The 2.5-acre park on the Morgan Processing and Distribution facility is the biggest green roof in New York.

The vegetation is a densely planted assemblage of some 25 hardy, low-growing species that have thrived in their few inches of planting material. The plants’ size and modest requirements, however, belie their substantial biological capacities and environmental benefits. Since the roof has been installed, the building’s storm water runoff into the New York municipal water system has been reduced by as much as 75 percent in summer and 40 percent in winter. The U.S. Postal Service estimates that the plants’ ability to cool the roof in summer and insulate it in winter will reduce the building’s energy costs by $30,000 a year.

The sprouting of a large, living roof in midtown Manhattan is a sign that this universally lauded green practice, which has spread rapidly across Europe, is now gaining a serious foothold in the U.S. Although initially more expensive than standard asphalt or shingle roofs, green roofs offer major environmental and economic advantages, from slashing storm water runoff and energy costs, to cooling overheated cities and cleaning their air.

Chicago, which now has more green roofs than any other U.S. city, last year added 600,000 square feet of green roofs and has some 600 projects that will bring its total to 7 million square feet. Washington, D.C. added 190,000 square feet in 2009 and has set a goal of 20 percent green roof coverage by 2020.

In Portland, Oregon, the city provides incentive grants of $5 per square foot for what they call “eco-roofs.” There’s no limit on the size of the roof,

In Europe, Stuttgart and Copenhagen have begun to mandate green roofs on most new construction.

but Tom Liptan, a storm water specialist with the city, says that most of the grants have so far gone to homeowners or to buildings in the commercial district. “It was a cost/benefit evaluation,” says Liptan. “The issue here was storm water. We were trying to find a way to reduce the burden on the city. If we trap it on the roofs, we don’t have to build bigger pipes to carry it or cisterns to store it for treatment.”

Liptan figures that if half the roofs in the city were green, Portland would reduce its storm water burden by some 3 billion gallons, a quarter of the rainfall that hits the city’s roofs annually.

This year Toronto became the first city in the Western Hemisphere to mandate green roofs. New buildings with a total floor area of more than 21,527 square feet will, depending on their size, have to cover from 20 to 60 percent of their roofs with vegetation. A 2005 study calculated that if 75 percent of the flat roofs in the city were greened, Toronto could reap $37 million a year in savings on storm water management, energy bills, and costs related to urban heat island effects.

In Europe, green roofs have been a proven technology for nearly 30 years, as a low flight over Stuttgart, Germany — courtesy of Google Earth — will show. Up Eberhardstrasse approaching Marktstrasse, along Friedrichstrasse to Hauptbahnhopf, the central train station, and north of the city center along Oswald-Hesse-Strasse, there’s often more green to be seen on the rooftops than on the ground (including the vast 27-acre green rooftop of the Daimler company). While most cities in the U.S. measure their green roof area in thousands of square feet, Stuttgart can measure its in millions. Some 20 to 25 percent of the city’s flat roofs are green and, due to a combination of government incentives, tax abatements, and regulations, so are 10 percent of the roofs throughout Germany. Cities such as Stuttgart and Copenhagen have begun to mandate green roofs on most new construction.

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