The authors of Roof Drainage are IIBEC member Stephen L. Patterson, RRC, PE, of Roof Technical Services, Inc. and Dr. Madan Mehta, PhD, PE, with the University of Texas at Arlington.
“We now know that the flow through a vertical leader and a roof drain are unrelated. Unlike the flow through a vertical leader, which is simply related to the pipe diameter, the flow through a roof drain is a function of several factors, such as the head of water over the drain’s rim, rim diameter, drain bowl depth, drain outlet diameter, and the strainer. As a result, the familiar plumbing code tables giving maximum drainable roof areas for roof drains became inapplicable and have been deleted from the codes. This left a large gap for the design disciplines to address.”