Perhaps, the single largest benefit that greenroofs can provide is a reduction in the amount of stormwater runoff. On most structures in the D.C. metropolitan area 70% of the rain that falls on the greenroof will stay there and if used in conjunction with water retention systems it is possible to hold even more. Plus, greenroofs can easily meet both water quality and quantity credits in the District and if a roof is at least 50% vegetated can be used as a BMP device. On some projects greenroofs have accounted for 100% of the sites stormwater management storage, thereby significantly reducing additional site development costs related to installing additional filtration devices or storage pipes.
