City employees in Atlanta enjoy their lunches on the newly landscaped roof of City Hall, probably unaware that the colorful collection of low-growing succulents reduces storm-water runoff and keeps summer temperatures in check. Atop City Hall in Toronto, downtown workers watch butterflies and birds loiter among native plants like bottlebrush grass, eastern columbines, grey-headed coneflowers and New Jersey tea bushes. And in New York, more than 200 formerly homeless tenants will soon be able to enjoy the greenery on the roofs of the brick Chelsea Residence, a former YMCA hostel.
The phenomenon of green roofs--structures completely covered with a soil medium that supports drought-tolerant plants--was almost unheard of five years ago. "We're in an exponential growth phase, which is easy since we started out small," says Charlie Miller, owner of Roofscapes Inc. in Philadelphia. Miller estimates that 100 projects larger than 1,000 square feet will be completed this year, versus 20 in 2003. Adds British observer Noël Kingsbury, co-author of Planting Green Roofs and Living Walls, "The speed with which North America has taken up green roofs in the last few years has been surprising."
Modern green roofs had their first growth spurt in Germany in the '60s, and have since become commonplace there. In Stuttgart, an estimated 20 percent of flat roof space has been "greened," which involves putting layers of material on a roof. Typically, these include a water- and root-barrier membrane, a drainage layer for diverting excess rain, several inches of lightweight growing medium, and the plants themselves. Shallow soil (about 5 inches or less) supports low-maintenance sedums and grasses, while many perennials, shrubs and small trees can grow in deeper mixtures of 3 or 4 feet.
greening american cities
During a visit to Germany, Mayor Richard M. Daley of Chicago became inspired to create green roofs in the Windy City, which led to the nation's first high- profile roof garden on City Hall. Completed in 2001, the project is part of a campaign that includes planting thousands of trees.
It's not just a question of aesthetics. In the summer of 1995, an estimated 700 to 1,200 people died during a heat wave in Chicago--a lethal side effect of the tendency for cities to become several degrees hotter than neighboring areas. This "urban heat island effect" occurs because pavements and ovenlike black-tar roofs absorb and reflect back heat. But green roofs cool temperatures through a process known as transpiration, in which plants release moisture into the air. "Air temperatures on top of the green roofs in City Hall can be 60 degrees lower than on nearby roofs," says Barry Burton, assistant to Mayor Daley. "We can safely say that City Hall isn't contributing to the urban heat island."
The more buildings that participate, the more a city can benefit. A computer simulation done by Canada's environmental department concluded that the temperature of Toronto would fall by 2 to 3 degrees if just 6 percent of its nonresidential roofs were greened. Significantly, this cooling effect would reduce smog episodes by 5 percent to 10 percent, according to Steven Peck, executive director of the Toronto-based Green Roofs for Healthy Cities.