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Green roofs reduce urban runoff
Proponents of green roofing have been finding lots of ways that green roofs can help the environment. The plant life absorbs sunlight and can decrease the temperature in urban areas. Green roofs have also been proven to reduce pollution in larger cities. Now research is revealing yet another environmental benefit to green roofs. Elizabeth Grant from Virginia Polytechnic Institute is performing experiments to test how green roofs can reduce urban runoff.
What is urban runoff?
Rainfall is crucial for the survival of the planet; it helps the environment in numberless ways. However, rainfall can also have some negative effects on the environment. That’s because rain that falls in urban areas will land on roofs, concrete, and roads. That rainfall eventually makes its way into rivers, lakes, and oceans carrying pollutants from the city with it. The polluted rainfall takes its toll on the environment. Controlling urban runoff is a very important task that environmentalists are working towards.
With a typical roof, rainfall runs right off it, into the rain gutters, down the downspouts, and into drains and sewers where it will eventually make its way to rivers, lakes, and oceans. Green roofs on the other hand, have been proven to slow down the flow of water off the roof. That’s because much of the rainfall is absorbed into the soil and plants. The plants then help to evaporate the water sending it back into the atmosphere. Ultimately, less pollutants end up in natural bodies of water.
In Elizabeth Grant’s experiment, she wanted to see how the depth of the plant life affected the runoff rate. She used green roofs that ranged from 2 ½ to 6 inches deep. Not surprisingly, the experiment showed that the deeper the roof platform, the less runoff there was. Compared to regular roofs which only retain about 5 to 6 percent of the runoff, green roofs that are 6 inches deep retain about 50% of the runoff.
Choosing the best green roof
Green roofs aren’t for everybody. They don’t do well in certain climates for one thing. Some structures can’t handle the extra weight either. If a structure can’t hold too much extra weight, home or business owners may have to go with a shallower green roof. In hotter climates, perennial plants do best because they can survive the hot, dry weather. Finally, cost needs to be considered. A simple roofing system can start at about 10dollars per square foot.
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