Gutter and Downspout Sizing
The gutter size and number of downspouts a roof needs: the adjusted (design) roof area from the plan area, the roof pitch, and the local rainfall intensity, then the gutter and downspout count.
Formula and source
Adjusted area = plan area x pitch factor x (rainfall / 5 in/hr); downspouts = ceil((adjusted / 100) / downspout sq in); gutter 5 in K-style up to ~5,520 sq ft else 6 in.
SMACNA / standard residential gutter method (by name).
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