It depends on the roof area draining into that run. One 2x3 downspout handles about 600 square feet of roof, so a larger area needs a second downspout or a 3x4.
Get My Free Estimate →Most houses need one standard 2x3 downspout for about every 600 square feet of roof, or one larger 3x4 downspout for about every 1,200 square feet. Any gutter run of 60 feet or longer needs at least two outlets, and steep roofs call for more capacity.
Those figures come from a common rule of thumb published by This Old House, about 1 square inch of downspout for every 100 square feet of roof. When clean gutters still overflow in heavy rain, too little downspout capacity is a likely cause.
Buckshot installs seamless gutters and downspouts sized to the roof area each downspout drains. This guide shows how to count what your house needs and where each downspout should go.
How downspout capacity is figured
A downspout's capacity depends on its opening. The rule of thumb is about 1 square inch of downspout opening for every 100 square feet of roof that drains into it.
| Downspout size | Opening | Roof area it handles |
|---|---|---|
| 2x3 inch | 6 square inches | About 600 square feet |
| 3x4 inch | 12 square inches | About 1,200 square feet |
A 3x4 downspout carries about twice the water of a 2x3. It also clogs less, which matters in a Wisconsin fall when wet leaves come down the gutter.
How to count the downspouts your house needs
Work out the roof area that drains into each gutter run, then divide it by what one downspout handles. Do it run by run, because each gutter only carries the water from the roof above it.
- Measure each roof section. Multiply the length of the eave by the slope length from eave to ridge, or use the area on your roofing estimate.
- Adjust for pitch. Slope increases the water flowing into the gutters, so multiply the area by the pitch factor in the next section.
- Divide by downspout capacity. Divide by about 600 for 2x3 downspouts or 1,200 for 3x4, and round up.
- Check the run length. Any run of 60 feet or longer needs at least two outlets, whatever the area says.
For example, a 1,000-square-foot roof section at an 8/12 pitch works out to about 1,100 square feet after the pitch factor. That needs two 2x3 downspouts or one 3x4 downspout.
Adjusting for roof pitch
This Old House adjusts the roof area for slope, because a steeper roof increases the water flowing into the gutters.
| Roof pitch | Multiply roof area by |
|---|---|
| Flat to 3/12 | 1.0 |
| 4/12 to 5/12 | 1.05 |
| 6/12 to 8/12 | 1.1 |
| 9/12 to 11/12 | 1.2 |
| 12/12 or steeper | 1.3 |
Our guide to roof pitch explains how to read the pitch of your roof from the ground.
Where downspouts belong
Placement matters as much as the count. A downspout in the wrong spot leaves part of the gutter holding water while another part overflows.
- At the ends of long runs: a run with a downspout at each end is pitched toward both, which splits the water between them.
- Below valleys: a roof valley funnels water from two slopes into one spot, so the gutter below it needs an outlet close by, as our guide to roof valleys explains.
- Away from walks and drives: water released over a walkway or driveway freezes there in winter.
- Where the water can drain away: each downspout needs a clear path at least 4 to 6 feet from the foundation, as our guide to downspout drainage explains.
If a door, patio or deck sits where a downspout should go, upsizing to a 3x4 downspout at the next available corner is often the better fix.
Gutters overflowing in heavy rain? Our gutter and drainage inspection is free, and we size each downspout to the roof area it drains.
Free on-site inspection · Family-owned since 1979Signs your house needs more or bigger downspouts
Watch your gutters in a heavy rain, after they have been cleaned. These signs point to too little downspout capacity rather than a clog.
- Water spilling over the front of the gutter far from the nearest downspout
- Overflow at the gutter below a valley
- A downspout that clogs every fall
- Soil washing out or splash marks at one corner of the house
- Water standing in the gutter after the rain stops
Standing water can also point to a sagging run or failing joints, as our guide to gutters leaking at the seams explains. Our gutter repair service fixes both.
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We'll inspect it, document everything, and tell you honestly what you're looking at, even if the answer is patch it for now. No pressure.
More downspouts or bigger downspouts?
Both fixes add capacity, and the right one depends on the house. Bigger 3x4 downspouts double the capacity of each outlet without cutting new holes, and they pair well with 6-inch gutters, as our comparison of 5-inch vs 6-inch gutters explains.
Adding a downspout means cutting a new outlet into the gutter and giving the water somewhere to go. That works well on long runs and below valleys, where one outlet cannot keep up.
If the water has nowhere to go at the new spot, a buried line can carry it away, as our guide to underground downspout drains explains.
Planning downspouts with new gutters
New gutters are the best time to get the count and placement right. Seamless gutters are formed on site to the length of each run, so outlets can be placed exactly where the roof sends the water.
Our gutter crews size and place downspouts with new gutters. On gutters that are still sound, our downspouts service adds new or replacement downspouts.
Get your gutters and downspouts checked
Buckshot Exteriors is family-owned, has served homeowners since 1979, and has offices in Madison, Wisconsin and Centerville, Iowa. We install seamless gutters and downspouts sized to your roof, and our on-site inspection is free.
Call (608) 909-9109 to schedule your free inspection, or send us a message and we will set a time. If you would rather check first, count your downspouts against the steps above.
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Wisconsin storm season and insurance non-renewal letters do not wait. Get a free, no-obligation inspection and an honest assessment of exactly what your roof needs, even if the answer is to wait.