Green Roofs are Gaining Acceptance in U.S. Cities, Says Survey

April 19, 2006

Photo of an urban rooftop with large patches of vegetation around its periphery.

Chicago, Illinois, is one of the leading U.S. cities for green roofs. In 2001, the roof of Chicago City Hall was retrofitted with a 22,000-square-foot rooftop garden.
Credit: Katrin Scholz-Barth

The area of U.S. roofs covered by vegetation has increased more than 80 percent in the past year, according to Green Roofs for Healthy Cities (GRHC), a trade association. So-called green roofs are rooftop gardens that reduce storm water runoff; insulate against heat and sound; increase energy savings; and improve air quality. They also reduce the urban heat island effect, which is caused by dark urban roofs, pavement, and other infrastructure absorbing the sun's heat. The GRHC recently completed its first survey of its members to gauge the growth of green roofs, finding that in 2005, green roofs covered at least 2.5 million square feet of roof space in North America, up from 1.3 million square feet in 2004. Cities that incorporate the largest area of green roofs in 2005 include Chicago, Illinois; Washington, D.C.; and Suitland, Maryland. Toronto may catch up to these other cities quickly, since the city council recently passed a policy that requires green roofs to be incorporated into city buildings and provides financial incentives for green roofs. GRHC is currently gearing up for its annual conference, to be held May 11th and 12th in Boston, Massachusetts. See the GRHC press release (PDF 196 KB), survey report (PDF 220 KB), and conference Web page, and the City of Toronto's Green Roof Strategy. Download Adobe Reader.

Green roofs are not the only way cities are working to combat the urban heat island effect, which causes urban and suburban temperatures to be 2 to 10 degrees Fahrenheit hotter than nearby rural areas. So-called "cool roofs," which absorb less heat than standard roofs, can reduce the urban heat island effect and lower the cooling needs for buildings. While cool roofs are typically thought of as white, recent research has developed darker roofs that absorb less energy than traditional dark roofs. On April 12th, the California Energy Commission (CEC) awarded more than $1.2 million to DOE's Lawrence Berkeley National Laboratory for a three-year project to develop, deploy, and validate cool roof technologies. See the CEC press release.

The U.S. Environmental Protection Agency (EPA) is also doing its part through its Urban Heat Island Initiative, which has instituted pilot projects and strategies for combating the urban heat island effect. According to the EPA, urban heat islands increase peak energy demand, air conditioning costs, air pollution levels, and heat-related illness and mortality. As part of the initiative, the EPA has launched a new online database that tracks state and local initiatives to reduce heat islands. See the database and the EPA Heat Island Web site.