A prototype green roof was constructed and monitored in Pittsburgh, Pennsylvania, to demonstrate a way to reduce storm water runoff and combined sewer overflow (CSO) events. The large impervious surface area created by urban development in Pittsburgh creates a wet weather flow that the existing combined waste waster and storm water sewer system cannot contend with. Green or vegetated roofs can reduce the amount of storm water that reaches the sewer conveyance system by replacing an otherwise impervious roof with porous soil that retains rainwater and plants that evapotranspire resulting in reduced storm water flows to the sewer system. The prototype green roof reduced the runoff volume by up to 70% compared to a conventionally ballasted roof covering a control portion of the same building. A reduction of at least 20% was found for rainfalls of 1.5 cm (0.60 inches) or less. The green roof also reduced the flow rate of runoff throughout storms. Peak values from the green roof were between 5% and 70% lower than control roof flow rates. For some small storms, the time of peak flow rate was delayed by up to several hours. In general, the green roof delayed the start of runoff and extended the time period of low residual flows that existed at the end of a storm. Water quality tests indicate that in most cases for the storms observed, a first-flush phenomenon was not evident in green roof runoff samples. Levels of phosphorus and chemical oxygen demand (COD) were elevated by the green roof. Both the control and green roofs produced runoff that demonstrated neutralization of slightly acidic rainfall.