Stormwater mitigation and surface temperature reduction by green roofs

Green roofs on small 1.8 by 2.4 m buildings consisting of a conventional flat roof covering, a root barrier, a 12 mm thick Enka drainage layer, 89 mm of growth medium, 25 mm of porous expanded polypropylene (PEPP), and Sedum spurium planted 76 mm on center were evaluated to determine their potential to reduce stormwater impacts and roof surface temperatures. Hydrology data were collected from three replicate buildings with experimental green roofs, and roof surface temperatures were collected from both green and conventional roofs, each with a 1:12 slope. The green roof media had an average porosity of 55 m(3) m(-3) and afield capacity of 34 m(3) m(-3). Rain and roof runoff data collected from seven rains during October and November 2002 showed that the green roofs delayed the start of runoff an average of 5.7 h. The green roofs retained an average of 45% (range 19% to 98%) of the rain from the seven storms evaluated and delayed the peak runoff by 2 h. Roof temperature data collected between April 2002 and February 2003 showed that the green roof maximum surface temperatures averaged 6 degrees C higher in the winter and more than 19 degrees C lower in the summer. The differences between the average maximum diurnal temperature change at the roof surface averaged 19 degrees C during the cooling season (August and September) and 8 degrees C during the heating season (October to February).