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Living Roofs Reduce Stormwater Runoff

by Karin Heineman, Inside Science TV

A good rainfall is vital for plants, trees and grass. But rain falling on roofs, concrete and roads poses a problem for the environment. This is because the runoff can carry pollutants directly into lakes, streams and rivers.

One solution to reduce this stormwater runoff is what is known as a green roof—a roof covered in living, growing plants. Architect Elizabeth Grant at Virginia Polytechnic Institute and State University in Blacksburg, Va. is testing how effective the roofs are at controlling urban runoff.

“Instead of having a plain roof that just has water coursing off of it all the time, you put the plants on there to hold the water for a period of time to slow down the flow of water off of it,” said Grant.

Plants help evaporate the water, releasing it back into the atmosphere instead of down drains and sewers. A study compared runoff rates from roofing platforms with different depths of plants ranging from  2 ½ to 6 inches. Researchers found the deeper the platform, the more stormwater runoff it held. 

“We’re looking at about 50 percent of the runoff in the summer months being withheld versus with a control roof which is just a normal flat roof, something like 5 or 6 percent,” said Grant.

Green roofs also reduce energy costs by cooling buildings in the summer.

Grant explained, “…if you don’t have a lot of capacity to have a lot of ability to put extra weight on your roof, you can go with a shallower system and still get a lot of the benefit.”  

Researchers suggest that perennial plants that can tolerate dry, hot conditions that don’t grow very high are best to use for low maintenance green living. It is estimated that the cost of installing a green roof starts at about $10 per square foot for a simple roofing system.

Inside Science is supported by the American Institute of Physics, a nonprofit publisher of scientific journals, and a group of underwriters that include multiple science, technology, engineering, and mathematics organizations.

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