Living roofs: gardens and meadows are sprouting on rooftops as city planners and pioneering homeowners give urban life a spiritual lift, a respite from summer heat and a little fresh air - Community

Natural Health, April, 2004 by Nancy Stedman

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,ooo square feet will be completed this year, versus 20 in 2003. Adds British observer Noel 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 (see illustration, page 94). 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.

stormy weather

In several cities, the impetus for green roofs has come from federal mandates for improving storm-water management. When rain falls in the woods, most of it is absorbed by the soil. But in densely built cities, about three-quarters of the rainfall fails to sink into the ground or evaporate. Instead, it runs into city sewer systems, which often overflow during intense storms. "One out of two times that it rains in New York City, raw sewage flows into city waterways like the East River," says Colin Cheney, director of the Green Roofs Initiative for Earth Pledge, a nonprofit group that facilitates the development of green roofs in New York.

The soil on green roofs can slow the runoff from buildings. The water evaporates into the atmosphere or is taken up by the vegetation. "In summer, depending on the plants, growing medium and climate, green roofs retain 70 percent to 80 percent of the precipitation that falls on them; in winter, they retain between 25 percent to 40 percent," says Peck.

To encourage these benefits, incentives are sometimes offered. The developer of Manhattan's Chelsea Residence will receive a check for $55,000 from the New York State Energy Research and Development Authority for using green roofs, thermally efficient insulation and energy-efficient windows, according to Richard Vitto, the project's architect.

Incentives are necessary since, in the short run, green roofs are more expensive to construct than standard-issue roofing. "You have to convince policymakers that green roofs deliver tangible benefits to the public that justify the investment" says Peck.

Those who experience green roofs are easier to convince--any new patch of greenery gets raves from urban dwellers enveloped by concrete and steel. The two green roofs on the Chelsea Residence, including a vegetable plot maintained by the tenants, are intended to make up for the complete lad of open space in the area.