Green roofs, also known as vegetated roof covers, eco-roofs or nature roofs, are multi-beneficial structural components that help to mitigate the effects of urbanization on water quality by filtering, absorbing or detaining rainfall. They are constructed of a lightweight soil media, underlain by a drainage layer, and a high quality impermeable membrane that protects the building structure. The soil is planted with a specialized mix of plants that can thrive in the harsh, dry, high temperature conditions of the roof and tolerate short periods of inundation from storm events. Green roofs provide storm water management benefits by utilizing the biological, physical, and chemical processes found in the plant and soil complex to prevent airborne pollutants from entering the storm drain system and reducing the runoff volume and peak discharge rate by holding back and slowing down the water that would otherwise flow quickly into the storm drain system. The quantity of rainfall retained or detained by a green roof can vary, but for small rainfall events little or no runoff will occur and the majority of the precipitation will return to the atmosphere through evapotranspiration. For storms of greater intensity and duration, a vegetated roof can significantly delay and reduce the runoff peak flow that would otherwise occur using conventional roof design. In addition to their storm water benefits, green roofs also reduce the urban "heat island" effect, reduce CO2 levels, reduce summer air conditioning costs and winter heat demand, and help preserve habitat and biodiversity by providing an oasis of life in an otherwise sterile urban environment.