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By Gutter Supply Chicago · 1816 S Trumbull Ave, Chicago, IL
How many downspouts a house needs depends on roof area, gutter length and downspout size. Too few downspouts is one of the most common reasons gutters overflow. This guide gives contractors simple rules of thumb.
A common rule of thumb is one downspout for every 30 to 40 feet of gutter, and at least one at each end of long runs. As a rough guide, a 2x3 inch downspout drains about 600 square feet of roof and a 3x4 inch downspout about 1,200 square feet. Add downspouts where valleys send extra water to one spot.
| Downspout size | Roughly drains | Usually paired with |
|---|---|---|
| 2x3 inch | About 600 sq ft of roof | 5-inch K-style gutters |
| 3x4 inch | About 1,200 sq ft of roof | 6-inch K-style gutters |
| 3 inch round | Similar to 2x3 | 5-inch half round gutters |
| 4 inch round | Similar to 3x4 | 6-inch half round gutters |
Measure each gutter run. On runs longer than about 40 feet, place a downspout at each end or one in the middle and pitch the gutter toward both sides. Shorter runs usually need one downspout at the low end.
Large or steep roofs send more water to the gutter than the length alone suggests. Add up the roof area that drains into each run and compare it with the downspout capacity in the table above. If one downspout is not enough, add another or move up to a larger size.
Where two roof slopes meet, the valley concentrates water into a small section of gutter. A downspout near the valley prevents overflow at that corner.
Every downspout needs an outlet, two or three elbows, straps and enough length to reach the ground and turn away from the foundation. Use our gutter materials checklist to count them. Gutter Supply Chicago supplies aluminum downspouts in several colors; call (312) 823-7355 to confirm sizes.
For roofing and drainage best practices, see the National Roofing Contractors Association.
A common rule is one downspout for every 30 to 40 feet of gutter.
Rarely. Extra downspouts drain water faster, but each one needs a safe place to discharge away from the foundation.
2x3 inch downspouts usually go with 5-inch gutters and 3x4 inch downspouts with 6-inch gutters.
Gutter Supply Chicago supplies aluminum downspouts and elbows with same-day quotes and job-site delivery. Call (312) 823-7355.
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