Canadian Consulting Engineer

Green roofs in Seattle show 65% reductions in run-off

Engineers in Seattle have completed an 18-month evaluation of green roofs to see how well they actually perform.

April 20, 2007  Canadian Consulting Engineer

Engineers in Seattle have completed an 18-month evaluation of green roofs to see how well they actually perform.
The final report by Magnusson Kelmencic Associates is the result of over 1.5 million measurements of data gathered from five green roof test plots around Seattle’s downtown area.
The results showed that the green roofs reduced the rainfall runoff by between 65% to 94% — much more than had been modeled.
In an interview in the company’s newsletter, Drew A. Gangnes, P.E. described the environmental advantages of reducing the volume of run-off in combined sewer systems: “Every inch of rain (or gallon of runoff) eliminated by a green roof is runoff that does not enter the municipal storm drainage collection system. In Seattle, (as in many U.S. cities), much of this system is a combined sewer, which sends rainfall combined with sewage to wastewater treatment plants. Therefore, every gallon of rainfall eliminated at the building site by a green roof is a gallon that does not require treatment with chemicals, processing, etc.”
Despite absorbing so much rain run-off, the green roofs were not able to reduce it enough to eliminate the need for detention tanks. But even in the wet climate of Seattle, the roofs were able to rebound between events and absorb the rainfall. For Seattle’s wet climate, they found that four to six inches was the best thickness for the vegetation.
The engineers also found that the drainage layer was very important, and that while cellular drainage mats are often used, aggregate drainage layers can perform better and may be cheaper. The test plot planting media they used were sedums and sempervivums.
The weather during the 18-month data collection period July 2005 to January 2007 recorded 55.4 inches of cumulative rainfall, which was around average for Seattle. The period included several extreme dry and wet periods.
The Magnusson Klemencic report is dated March 2007, and posted on the company website at



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