Green Roof Mitigation of Stormwater and Energy Usage

Abstract

With growing urbanization comes increased impervious areas and decreased openland for stormwater BMPs. Urbanization also increases stormwater runoff volumes and peak flowrates while decreasing infiltration and evapotranspiration. Green roofs, sometimes called vegetatedroofs 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 stormwaterand decrease the heating and cooling load within structures.<br><br>A research effort at Penn State has concentrated on quantifying the stormwater retention anddetention capabilities of a specific green-roof system, and quantifying the energy used to maintain anindoor temperature of 21C. The green-roof system studied consists of a conventional flat roofcovering, a 13-mm thick Enka drainage layer, 89mm of porous medium, 25mm of Porous ExpandedPolypropylene (PEPP), and Sedum spurium planted 76mm on center. The study was conducted intwo parts. The water retention capability of each roof layer was characterized in the laboratory forroof slopes of 0:12, 0.5:12, and 1:12. At each slope and for each layer water was applied to saturatethe various layers and then the roof system was allowed to drain by gravity. Laboratory data showsmaximum detention to be 30mm and maximum retention to be 34mm.<br><br>The field study was conducted at the Russell E. Larson Agricultural Research Center at RockSprings, Pennsylvania. Rainfall, runoff, temperature, and energy data were collected, whichconsisted 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 atthe field site were correlated to the laboratory data. Field data showed an average 40% reduction inrunoff from the green-roofed buildings. Temperature differences between green and control roofs atthe roof surface ranged from 12oC in the winter to over 40oC in the summer. Differences in energyusage were observed in the summer months at a daily level, but not at the monthly level. No energyusage differences were observed in the winter months.