Green roofs are a living vegetated roofing alternative to traditional impervious roofing materials. They have environmental, economic and social benefits. If widely adopted, green roofs could dampen the urban heat island effect by reflecting the sun's radiation, by providing shade, or by evaporative cooling. It is estimated that ambient air temperature in the USA would be 2°C lower in urban areas if 8% of buildings had green roofs. As plants photosynthesise they absorb carbon dioxide and produce oxygen improving air quality around the site of a green roof. Green roofs attract bugs and insects that pollinate the plants. A further benefit is their ability to absorb rainwater, thereby reducing water flow into sewers and drainage systems. The rate of storm water flow reduction varies according to the depth and the type of substrate specified, and also the temperature, amount and timing of the rainfall. For example, if the roof is already sodden through extended rainfall, the attenuation rate is diminished, however if the roof is dry attenuation rates as high as 90% are recorded. Some cities have mandated green roofs on this basis as it is deemed a better option than digging up roads to enlarge sewer pipes, avoids inconvenience to residents and costs less.
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