Flat roof scupper question – 2018 IRC R903.4.1
Zone 2 (Phoenix, AZ)
Roof overhang, 16″
Just got told by a County rep that she wants,
SHEET #A3.0: Call out the size of each roof drain and scupper. Design shall be based on 6” rainfall in 1 hour. Provide emergency overflow drain(s). Overflow drains having the same size as the roof drains shall be installed with the inlet flow line located 2 inches above the low point of the roof, or overflow scuppers having three times the size of the roof drains and having a minimum opening height of 4 inches shall be installed in the adjacent parapet walls with the inlet flow located 2 inches above the low point of the roof served. Typical throughout. (2018 IRC R903.4 and R903.4.1)
The draftsman shows a 4” parapet on the plans. With the length of the cricket, the parapet wall would have to be ~8” or taller…is my math crazy? I’m considering removing the cricket all together and making the parapet 2” tall thus eliminating the need for an “overflow drain.”
If I do make the parapet 2”, is there guidance or code that suggests how far apart scuppers should be? The total length of the roof pictured is ~60ft with scuppers on each end.
Also, how the heck do I figure how big the drain must be for 6” of rainfall in 1 hour?
TIA, Paul
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Replies
Scuppers and flat roofs are always problematic, best to avoid it.
You are on the right path, reduce the parapet height and the roof won't be able to hold water.
Depending on which way you slope the iso, you only need to do this on one side if you want to keep the existing height as part of the design.