Roof Drainage Calculator
Estimate peak roof runoff, downspout quantity, flow per outlet and storm-water volume from roof area, rainfall intensity and runoff coefficient.
Default project preview
53.3 gal/min
Minimum downspouts
2
Flow per downspout
26.65 gal/min
Storm rainfall depth
1.5 in
Storm runoff volume
1,599 gal
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Imperial flow uses roof area × rainfall intensity × 0.0103896 × runoff coefficient in gallons per minute. Metric flow uses area × intensity ÷ 3600 in litres per second.
FAQ
Roof Drainage Calculator questions
Should sloped roof area or horizontal area be used?+
Rainfall is normally collected over the horizontal projected area because rain depth is measured vertically.
Why include a runoff coefficient?+
It accounts for small losses such as wetting and minor retention before water reaches the drainage system.
Does this size gutters?+
It estimates peak flow and downspout count. Gutter profile, slope, outlets and local design rules still need verification.