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With growing urbanization comes increased impervious areas and decreased open land for stormwater BMPs. Urbanization also increases stormwater runoff volumes and peak flow rates while decreasing infiltration and evapotranspiration. Green roofs, sometimes called vegetated roofs or eco-roofs, consist of layers of specially designed and selected materials combined with shallow-rooted living plants to form a biological system that has the potential to mitigate stormwater and decrease the heating and cooling load within structures.

A research effort at Penn State has concentrated on quantifying the stormwater retention and detention capabilities of a specific green-roof system, and quantifying the energy used to maintain an indoor temperature of 21°C. The green-roof system studied consists of a conventional flat roof covering, a 13-mm thick Enka drainage layer, 89mm of porous medium, 25mm of Porous Expanded Polypropylene (PEPP), and Sedum spurium planted 76mm on center. The study was conducted in two parts. The water retention capability of each roof layer was characterized in the laboratory for roof slopes of 0:12, 0.5:12, and 1:12. At each slope and for each layer water was applied to saturate the various layers and then the roof system was allowed to drain by gravity. Laboratory data shows maximum detention to be 30mm and maximum retention to be 34mm.

The field study was conducted at the Russell E. Larson Agricultural Research Center at Rock Springs, Pennsylvania. Rainfall, runoff, temperature, and energy data were collected, which consisted of three replications of the experimental green roof system, at 1:12 slope, and a control. Data were collected intermittently between June 2002 and March 2003. The runoff data collected at the field site were correlated to the laboratory data. Field data showed an average 40% reduction in runoff from the green-roofed buildings. Temperature differences between green and control roofs at the roof surface ranged from 12oC in the winter to over 40oC in the summer. Differences in energy usage were observed in the summer months at a daily level, but not at the monthly level. No energy usage differences were observed in the winter months.