Flat roof scupper question – 2018 IRC R903.4.1 – GreenBuildingAdvisor

Hey all.

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