Gutter size calculator
The gutter size, downspouts and parts for a roof, from its area, pitch and local rainfall.
Gutter size
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How it works
adjusted area = roof footprint x pitch factor x rainfall in inches per hour gutter size = the smallest gutter whose capacity is at least the adjusted area downspouts = the most of: one per run, one per 40 ft, roof area / area each drains
Size the gutter for the biggest stretch of roof that drains into a single run, using its flat footprint. The pitch factor allows for wind driving rain onto a steep roof: 1.0 up to 3/12, 1.05 to 5/12, 1.1 to 8/12, 1.2 to 11/12 and 1.3 above that. For the rainfall, use the heaviest short burst your area sees, in inches per hour; NOAA's Atlas 14 rainfall maps give it for any US address.
Gutter capacity
| Gutter | Roof area at 1 in per hour | At 4 in per hour |
|---|---|---|
| 5 inch K-style | 5,520 sq ft | 1,380 sq ft |
| 6 inch K-style | 7,960 sq ft | 1,990 sq ft |
A 2 x 3 inch downspout drains about 600 square feet of roof and a 3 x 4 about 1,200, by the common rule of 1 square inch of downspout for every 100 square feet. Keep downspouts no more than about 40 feet apart along a run. Each one usually takes three elbows.
Questions
- Do I need 5 or 6 inch gutters?
- Most houses are fine with 5 inch K-style. Large or steep roofs, long runs and places with heavy downpours often need 6 inch. The calculator compares the roof area, pitch and rainfall against what each size carries.
- How many downspouts do I need?
- At least one per run and one for about every 40 feet of gutter, and more where a big roof area drains to one run. A 2 x 3 inch downspout handles about 600 square feet of roof.
- How far apart should gutter hangers be?
- Every 24 inches is a safe spacing, and the usual one where snow and ice load the gutters. Some makers allow up to 36 inches in mild climates.
- Seamless or sectional?
- Seamless gutter is formed on site to the length of each side, so it has joints only at the corners. Sectional comes in 10 foot pieces joined with connectors.
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