In urban areas, impermeable surfaces (streets, sidewalks, rooftops etc.) dominate the landscape, preventing rainwater from following a natural cycle of absorption. Instead, as much as 75% of the rainwater runs into sewer systems carrying contaminates from the air and roadways (salt, oil, mercury, etc.). This water flows untreated into lakes and streams. In some cases, it may cause overflows of sanitary sewer systems, causing the dumping of untreated sewage (combined sewer overflow).
In contrast, LiveRoof green roof surfaces can absorb a substantial amount of rainfall reduce runoff, lessen the risk of combined sewer overflows and flooding, and to some degree filter the water as it percolates through the LiveRoof soil. Even when a rainfall is such that it can not all be absorbed by your LiveRoof, the excess rain that runs through the growing medium is filtered and delayed until after peak flows, which allows additional time for sanitary sewer systems to handle uncontrolled runoff.