Especially in urban areas, over 75% of a rain event can become surface runoff depositing pollutants into waterways. Many older cities have combined stormwater and sanitary sewers where, in the event of a significant rainfall, the sewage treatment plant becomes unable to treat all of the excess stormwater flowing into it. Bypass valves are opened, and raw sewage can be directed to the receiving waters. Green roofs help alleviate this problem through retention of rainfall and detention of roof water runoff. Testing shows:

* GreenGrid® Intensive (8-inch) system retained 93% of a 1-inch rainfall that occurred in a 15-minute interval.

* GreenGrid® Extensive (4-inch) system retained 72% of the same type of rainfall.

Green roofs can greatly reduce the risk of flooding, sewer overflows, and subsequent discharges. Once the green roof is saturated, the water slowly percolates through the GreenGrid® growth media, becoming roof runoff. This occurs several hours after peak flows from impervious surfaces have subsided and provides additional time for municipal sewer systems to handle other uncontrolled runoff. For the building owner, a GreenGrid® green roof can often mean that costly stormwater detention and retention systems can be substantially reduced in size or even rendered unnecessary.