Why downspouts often matter more than gutters
A gutter is a channel; water only leaves it through the outlets. If an outlet is too small, or one downspout has to drain a long run under a big roof, water backs up in the gutter and spills over the front in heavy rain, no matter how wide the gutter is. Upgrading or adding downspouts is often the cheapest fix for overflowing gutters. See gutters overflowing in heavy rain.
The two common sizes
Most homes use rectangular aluminum downspouts in two sizes:
- 2x3-inch (about 6 square inches of opening): the traditional size, usually paired with 5-inch gutters.
- 3x4-inch (about 12 square inches): twice the opening, usually paired with 6-inch gutters.
Published rules of thumb, such as those used in rainwater harvesting guidance from Texas A&M AgriLife, put the capacity of a 2x3-inch downspout at roughly 600 square feet of roof and a 3x4-inch downspout at roughly 1,200 square feet. The exact figure depends on local rainfall intensity, but the takeaway is clear: a 3x4-inch downspout carries about twice as much water and clogs less easily, because leaves and needles pass through more readily.
Round downspouts, used with half-round gutters, come in 3-inch and 4-inch diameters. See half-round vs. K-style gutters.
How many downspouts you need
Two rules work together:
- Roof area: divide the roof area draining to a gutter run by the capacity of the downspout size. A 1,800-square-foot roof plane drained by 2x3-inch downspouts at about 600 square feet each needs three; with 3x4-inch downspouts, two.
- Gutter length: a common rule of thumb is one downspout for every 30 to 40 feet of gutter, because water can only travel so far along a gutter before it overflows. On long runs, installers often slope the gutter from the middle toward a downspout at each end.
Use whichever rule gives more downspouts. Add one near any roof valley, where two roof planes pour water into one corner.
Professional sizing uses the same logic in more detail: SMACNA’s sizing calculator combines local rainfall intensity, drainage area and gutter slope to size gutters and downspouts. See 5-inch vs. 6-inch gutters.
Where to put them on the house
- At the ends of runs and near valleys, where water collects.
- At corners where the ground drains well and there is room for an extension.
- Away from entries, walkways and driveways, where water and ice cause problems.
- Not behind decks and patios where water cannot be carried away.
- Where they will be least visible, if appearance matters, as long as drainage comes first.
- Where barrels or rain gardens will go, if you plan to harvest rainwater.
Where the water should go
The bottom of the downspout matters as much as the top. The International Residential Code requires roof water to discharge at least 5 feet from foundation walls in areas with expansive or collapsible soils (section R801.3), and that is a sensible minimum anywhere. Options include extensions onto splash blocks, flip-up or roll-out extensions where paths cross, underground lines to a daylight outlet or dry well, rain barrels with safe overflow, and rain gardens. See downspouts and your foundation.
In older cities, some downspouts still connect to sewers. Many cities now encourage or require disconnection. See disconnecting downspouts.
A worked example
Take a simple two-story house with a gable roof. Each side of the roof is about 1,100 square feet and drains into one 45-foot gutter run. With 2x3-inch downspouts at roughly 600 square feet each, each run needs two downspouts by area, and the 45-foot length also calls for two. One downspout at each end, with the gutter sloped from the middle toward both, handles typical storms.
Now picture the same house in a heavy-rain area, with a dormer and a valley dumping extra water near one corner. Switching to 3x4-inch downspouts doubles the capacity at each outlet, and the corner by the valley gets the larger outlet directly under it. For a modest added cost, the system can handle the kind of intense rainfall that overwhelms smaller outlets.
On a hip roof, water comes off all four sides, so every side needs its own plan. Wraparound porches and additions often have their own small roofs that need a downspout too, rather than draining onto the main roof or into a neighboring gutter.
Upper roofs draining onto lower roofs
On two-story homes, it is common for a downspout from the upper roof to empty onto a lower roof, which then drains into its own gutter. That is acceptable when done well: a splash guard or diverter spreads the water so it does not wear out the shingles in one spot, and the lower gutter is sized for the combined flow. Better still, where the layout allows, run the upper downspout down the wall directly to the ground. Concentrated water on a lower roof is a frequent cause of shingle wear and leaks.
Elbows, straps and details
- Elbows: every bend slows water and catches debris. Use as few as possible, and larger elbows with larger downspouts.
- Straps: fasten downspouts to the wall at every joint and at least at the top and bottom, more in windy and hurricane areas.
- Outlet strainers: simple strainers at the gutter outlet keep leaves out of the downspout; guards do this more completely.
- Cleanouts: where downspouts connect to underground lines, a cleanout or accessible connection makes clogs easy to clear.
Signs your downspouts are the problem
- Gutters overflow near one end even when clean
- Water gushes out at a downspout joint or elbow
- Water backs up where the downspout enters the ground
- Dents or crushed elbows at the bottom
- Puddles, erosion or wet basement walls next to the outlet
What it costs
| Project | Published range |
|---|---|
| Downspouts per linear foot, installed, national (HomeGuide) | about $2 – $4 |
| Upgrade from 5-inch to 6-inch gutters (HomeGuide) | about $1 – $2 more per foot |
| Seamless aluminum gutters per linear foot, installed (HomeGuide) | $6 – $12 |
Published figures for orientation only, not quotes.
Adding a downspout or upsizing existing ones is one of the least expensive gutter improvements. In heavy-rain areas such as Houston, New Orleans and South Florida, 3x4-inch downspouts are worth the small extra cost on nearly every home.