WHAT IS THE WATER RUNOFF COEFFICIENT OF EPDM?
There is no exact figure as conditions such as rain water evaporation off a hot EPDM surface are difficult to test. The formula for calculating this flow is called the Maming Formula. The American Society of Plumbing Engineers Data Book suggests that initially there will be some friction along the drainage path but it will quickly go to zero. On a low slope fully adhered EPDM Roof with slope of 1/4″ per foot one could justify a coefficient of .95 to .99. If calculating drainage flow for design or legal matters it is recommended that you contact a plumbing engineer, registered in the state in which the building in question is located.