Flat Roof Drainage Calculator
Size drains and scuppers for flat roofs based on roof area, rainfall intensity, and slope.
Inputs
sq ft
in/hr
in/ft
Results
Drains Needed
2
Scuppers Needed
5
Design Flow Rate207.84 GPM
Slope AdequacyAdequate
How to Use This Calculator
- Enter the roof area (sqft) and design rainfall rate (in/hr) for your climate zone.
- Set the flow capacity per primary drain (GPM) and per overflow scupper (GPM).
- Review required drain count, scupper count, and total flow capacity in GPM.
- Verify drain layout provides drainage within maximum spacing requirements for your roof type.
- Ensure overflow scuppers are positioned at the correct elevation to trigger before ponding reaches structural capacity.
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