The U.S. Environmental Protection Agency installed a green roof this past March on its Denver headquarters. Photos: Green Grid Green Roofs
The American Red Cross of Greater Chicago building has a modular green roof. The 2800-square-foot roof was installed in the summer of 2004.

A study conducted by the University of Wisconsin—which installed an extensive, modular green roof system in 2003—found that it helped reduce stormwater runoff by as much as 75%. Studies conducted by Weston Solutions Inc., based in West Chester, Pa., showed that cumulative water retention of a green roof during a simulated two-hour rainstorm produced the following results:

  • 1 inch of rainfall: 72.2% of the water retained
  • 2 inches: 57.3% retained
  • 3 inches: 43.2% retained
  • 4 inches: 33.7% retained

Both studies also report that the installation of a green roof helps reduce peak velocity runoff. This means that what was not absorbed by the green roof was released more slowly into local sewer systems so that it would not overload waste-water treatment facilities; this helped reduce overflow into local waterways.

Get the bugs out

Stormwater runoff contaminants can be reduced.

Contaminated stormwater can have adverse impacts on people, plants, animals, and the environment. As storm-water moves across rooftops, streets, parking lots, yards, and driveways, it picks up debris, chemicals, dirt, sediments, animal waste, and other contaminants.

To evaluate how effective a green roof system is at reducing stormwater runoff contaminants, the University of Wisconsin installed a 6500-square-foot modular green roof atop a campus building. Sandra McLellan, a scientist at the university, spearheaded the project.

“We found that by the time the untreated stormwater enters a city's sewer system, it can have as much as 20,000 E. coli per 100 mils, more than 20 times what is considered safe,” says McLellan. “Reducing the amount of runoff by installing a green roof can considerably reduce this—and prevent both E. coli and other pollutants and debris—from entering the surrounding waterways.”

— Robert Kravitz, a writer for the professional cleaning and buildings industries, is based in Chicago.