Flow first
Size starts with flow: roof area times rainfall intensity, for the area that drains to each outlet.
In U.S. units, Q (gal/min) = 0.0103896 x A (ft2) x I (in/h): exact arithmetic on the gallon and the foot.
Sources
- Design Center roof drainage flow screening method - Architectural Flashing Supply
- U.S. liquid gallon = 231 in3 (statutory definition) - U.S. customary units