Recently, changes have been made to the current model building and plumbing codes that may affect roof drains and roof drainage system design for low-slope roof systems. You should be aware of how these changes can affect your new construction and reroofing work.
The design of roof drainage systems, including roof drains, scuppers and gutters, traditionally has been addressed in the applicable plumbing code and generally is considered to be the responsibility of plumbing designers. For example, in the International Code Council®'s (ICC's) International Plumbing Code® (IPC), sizing of roof drains, scuppers and gutters—and any related conductors and leaders (drain piping, downspouts)—are addressed in Chapter 11-Storm Drainage.
Through the 2012 edition of the IPC, the sizing of roof drainage systems primarily was based on designing vertical conductors and leaders to accommodate a 100-year hourly rainfall rate or another rainfall rate determined from local weather data approved by the code official or other authority having jurisdiction. The IPC includes maps indicating the 100-year hourly rainfall rates expressed in inches.