April 20, 2007—A green roof has the potential to lower stormwater runoff from 65 to 94%, according to a two-year study.
In addition, green roofs allow building owners to reduce the size of stormwater detention tanks needed for peak rainfall. That can offset the cost of a green roof by 30 to 60%, according to Magnusson Klemencic Associates, the Seattle-based engineer that led the $125,000 green-roof evaluation project. The study quantified the effectiveness of green-roof stormwater management, based on rainfall and weather conditions, from July 2005 through this January.
At best, “only 6% of the rain became runoff,” said Drew A. Gangnes, MKA’s director of civil engineering, who led the study. “Thats a huge finding.” The engineer had projected 50% mitigation based on earlier modeling.
MKA also found the “magic number” for soil thickness to be 4 inches, not 6 inches as suspected. The thinner soil bed can dry out more quickly, readying itself to absorb the next rainfall, says Gangnes.
Green roofs often get value-engineered out of a project because even the simplest installations, off limits to the public, costs $7 to $10 per square foot. Gangnes wants more cities to offer developers’ incentives.
The seeds of the MKA study were planted about five years ago by clients asking whether a green roof could replace a detention tank. To answer the question, MKA developed a tool that models water evaporation, percolation and storage in the soil matrix. Research and a tour of green roofs in Sweden convinced Gangnes that green roofs were a promising stormwater management tool. But there were no measurements to prove it. There were green roofs, but they were not being monitored, says Gangnes.
