As an Amazon Associate I earn from qualifying purchases.
Rain falls vertically, so the water your roof collects depends on the flat footprint it covers rather than the sloped surface. One inch of rain over one square foot is 0.6233 gallons, which means a thousand square foot roof yields roughly 623 gallons per inch of rainfall before losses. This calculator returns the gross and net runoff for any storm you enter, the volume in cubic feet, the weight of that water in pounds, and the peak flow rate in gallons per minute using the rational method — coefficient times rainfall intensity times area in acres. Peak flow is the number that actually breaks drainage systems, because an outlet that cannot pass the rate causes the gutter to overshoot at the eave regardless of how large the gutter is. It also returns the minimum downspout count by capacity for both 2x3 and 3x4 outlets, how many rain barrels the storm would fill, and an annual rainwater harvest figure.
Roof drainage is about moving the water that lands on the roof away from the building fast enough that it never backs up. Two numbers drive it: the area draining to each outlet, and the rainfall intensity where you are.
| Eave run | Gutter with 10% waste | Downspouts | Downspout run |
|---|---|---|---|
| 80 ft | 88 ft | 3 | 30 ft |
| 120 ft | 132 ft | 4 | 40 ft |
| 160 ft | 176 ft | 5 | 50 ft |
| 200 ft | 220 ft | 6 | 60 ft |
| 240 ft | 264 ft | 7 | 70 ft |
One downspout per 30 to 40 feet of gutter run, minimum two per gutter. On a two-storey house the downspout run doubles, which is why the material list grows faster than the eave length.
One per 30 to 40 feet of gutter run, with a minimum of two per gutter. On a two-storey house each downspout also runs twice as far, so the material list grows faster than the eave length.
At least 4 to 6 feet from the foundation, and further on clay soils. Splash blocks are the minimum; extensions or a buried line that actually daylights somewhere are better.
A minimum of a quarter inch per foot. Water still standing 48 hours after rain is ponding, and ponding is the single biggest predictor of early membrane failure.
Yes. Every primary drain needs a scupper or a secondary drain set slightly higher, so a blockage cannot pond the roof to the point of structural failure.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.