Stormwater retention and water use by extensive green roofs

Impervious surfaces dominate urban areas and generate considerably more stormwater runoff than natural areas of the same size. Green roofs are one practice that can aid in mitigating stormwater runoff. To determine the degree that green roofs aid stormwater mitigation, simulated roof platforms were utilized to compare stormwater retention capabilities of various roof surfaces, roof slopes, and media depths. On average, the vegetated green roofs composed of Sedum spp. retained 34% more stormwater per rain event than the standard roofs. In the roof slope and media depth study, the greatest retention (87%) occurred on platforms set at a 2% slope with a 4 cm media depth. Results from both studies indicate that green roofs also delay the start of runoff for several minutes and spread it out over a longer period of time. A third study determined water use trends and minimum irrigation requirements for green roof vegetation consisting of seven Sedum spp. After watering, substrate moisture was often reduced to zero in as little as 24 hours. Sedum in deeper substrates resulted in greater growth if provided with sufficient water, but these plants also experienced an increased evapotranspiration rate. Over the 88 day study, water was required at least once every seven days to promote growth in the shallowest green roof substrates and every 28 days with 6 cm of media.