HYDRAULIC RUNOFF ESTIMATOR
Roof Drainage Calculator
Calculate peak storm runoff flow rate in gallons per minute (GPM) using the SMACNA / IPC rational method.
Roof Drainage Calculator
Calculate peak storm runoff flow rate in gallons per minute (GPM) using the SMACNA/IPC rational method.
sq ft
outlets
SMACNA / IPC Runoff Formula
$$Q = frac{C times I times A}{96.23} quad (text{in GPM})$$
Where $C = 1.0$ (impervious roof), $I = text{Rainfall Intensity (in/hr)}$, $A = text{Catchment Area (sq ft)}$.
Estimating hydraulic roof drainage demand ensures gutters, scuppers, and downspouts prevent overflow during intense localized storm events.