How many downspouts
By Gutter Report Editorial. Last reviewed August 2026. How we research this.
In brief
One downspout every 20 to 30 feet of gutter, and one extra wherever a roof valley dumps into a short stretch. The published rules are three different numbers: Amerimax says one every 20 feet, the Copper Development Association caps one at 50 feet of gutter, and This Old House says runs of 60 feet or longer need at least two outlets. They are answering slightly different questions, so the safe course is the tightest of them. This is also the cheapest capacity you can buy. Fine Homebuilding states that going from one downspout to two doubles the roof area a section of gutter can drain, against about 40 percent for stepping a 5 inch gutter up to 6 inch. If your gutters overflow, this page is the fix far more often than a new gutter is.
1.0Three rules, and which to use
| Source | Rule | What it is really about |
|---|---|---|
| Amerimax | One downspout per 20 ft of gutter | Consumer guidance, the most conservative |
| Copper Development Association | Each downspout drains a maximum of 50 ft of gutter | Architectural specification for copper |
| Berger, via Gutter Supply | 50 ft is a practical maximum served by one downspout | Thermal expansion, not flow |
| This Old House | Runs of 60 ft or longer need at least two outlets | Residential practice |
Notice that these are not competing answers to one question. Amerimax is thinking about flow on an ordinary house. Berger's 50 feet is about metal expanding and contracting along a run, which is a different constraint entirely. This Old House is giving a floor, not a target.
Use one per 20 to 30 feet. If a contractor quotes you two downspouts on a 60 foot elevation, that meets the loosest published rule and none of the others.
2.0What one downspout actually drains
| Downspout | Roof area served | Stated cross section |
|---|---|---|
| SQ FT | SQ IN | |
| 2 x 3 in. rectangular | about 600 | 6.00 |
| 3 in. round | about 706 | 7.07 |
| 3 x 4 in. rectangular | up to 1,200 | 12.00 |
| 4 in. round | up to 1,255 | 12.57 |
This Old House and Modernize publish those four roof area figures identically. The rule under them is one square inch of downspout for every 100 square feet of roof. Texas A&M AgriLife Extension, This Old House and Fine Homebuilding all state it.
That rule of thumb hides a design assumption. One square inch per 100 square feet corresponds to a 12 inch per hour design intensity, which is more severe than almost anywhere in the United States. It is conservative on purpose, and that is why it survives as a rule of thumb.
One caution before you rely on the table. Those areas use the geometric cross section. A SMACNA method calculator uses smaller effective areas: 4.8 square inches for a 2 by 3 against 6.0 geometric, and 10.31 for a 3 by 4 against 12.0. Sizing off the geometric figure overstates what a downspout carries by roughly 14 to 20 percent.
3.0Size the downspout to the gutter
A bigger gutter feeding the same outlet just relocates the problem. Amerimax states the pairing: 5 inch gutter systems can use 2 by 3 or 3 by 4 inch downspouts, and 6 inch systems use 3 by 4 only.
There is also a minimum that the residential market quietly ignores. The Copper Development Association specifies a downspout cross section of at least 7 square inches, held constant the whole way down, except for small areas such as porches and canopies. A 2 by 3 downspout is 6.00 square inches. The most common downspout in America does not meet the architectural minimum, which is worth knowing when yours backs up.
| Downspout, CDA Table 10.7A | Cross section |
|---|---|
| SQ IN | |
| 2 x 3 in. plain rectangular | 6.00 |
| 2-3/8 x 3-1/4 in. corrugated rectangular | 7.73 |
| 3 x 4 in. plain rectangular | 12.00 |
| 3 in. corrugated round | 5.91 |
| 4 in. corrugated round | 11.01 |
| 5 in. plain round | 19.63 |
| 6 in. plain round | 28.27 |
4.0Where to put them
- At the ends of runs. The gutter pitches toward the outlet, so an end outlet uses the whole run's fall.
- Two outlets on a long run, pitched both ways. This Old House notes that when you add a second outlet, the gutter should be pitched so water runs toward both.
- Below a valley. This is the case nobody plans for. Two roof planes meeting send a concentrated stream into one short stretch, which overflows while the rest of the house is fine. Upsizing the whole house for one valley is the expensive answer. An outlet near it is the cheap one.
- Away from doors and walkways. A downspout discharging over a path makes ice.
- Where the water can leave. The residential code, in expansive or collapsible soil areas, requires roof drainage discharged to the ground not less than 5 feet from foundation walls, or into an approved drainage system. That is a good rule everywhere.
Adding an outlet against every other way to buy capacity
Four things change how much roof a gutter run can handle. Here they are ranked by what they buy, which is not how they are usually ranked by price.
| Change | Capacity gained | Source |
|---|---|---|
| One downspout to two | Doubles it | Fine Homebuilding |
| Pitch the run at 1/8 in. per foot | Multiplies by 1.4 | Fine Homebuilding |
| 5 in. gutter to 6 in. | About 40 percent | Englert |
| 2 x 3 downspout to 3 x 4 | About double the outlet area | CDA Table 10.7A |
The pitch line has a catch. Berger states that a gutter pitched at 1/16 inch per foot or less is treated as a level gutter, and earns no capacity credit at all. So a token slope buys nothing. See gutter slope.
Berger also notes the reverse trade: a gutter can be run dead level for appearance, but then additional downspout is necessary. That is a real choice on a long front elevation where a visible fall would look wrong.
5.0What it costs
Three publishers price downspouts three different ways, which is why quotes are hard to compare.
| Source | Figure |
|---|---|
| HomeGuide | $30 to $75 installed, each |
| Fixr | $7.43 to $13.51 per linear foot |
| Angi and HomeAdvisor | $5 to $8 per foot vinyl and aluminum, $9 to $12 steel, $17 to $20 copper |
Against a full replacement at $2,200 to $5,000, adding one outlet is the cheapest fix on this site. Full detail is on the downspout installation cost page.
6.0You may not need another downspout either
Before adding an outlet, rule out the free causes. A clogged run overflows at any downspout count. A low spot holds water and spills at that spot, whatever is at the ends. A drip edge problem sends water behind the gutter, where no outlet can help it.
Clear the run, check the pitch with a string line, and watch it in real rain if you can. If water still crests the front edge along a whole run rather than at one point, that is a capacity problem and this page is the answer. If it spills at one spot, that spot is the problem.
We can say that plainly because we do not sell gutters and we do not install them. If the run itself is failing, work through repair or replace first, and size the new system on gutter sizes.
Get an outlet added by a licensed contractor
Ask how many outlets the roof has now, and how many it needs. Tell us your ZIP code and we will pass your details to one licensed contractor who works there.
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- What happens after I send this?
- Your details go to one licensed gutter contractor who works in your ZIP code. They contact you to arrange a quote. We are not a contractor and we do not do the work.
Common questions
How many downspouts do I need?
The published rules disagree. Amerimax recommends one downspout every 20 feet of gutter. The Copper Development Association caps one downspout at 50 feet of gutter. This Old House says runs of 60 feet or longer need at least two outlets. Use the conservative rule, one per 20 to 30 feet, and add one anywhere a valley concentrates water into a short stretch.
Does adding a downspout help more than a bigger gutter?
Yes, and by a wide margin. Fine Homebuilding states that going from one downspout to two doubles the roof area a section of gutter can drain. Moving from a 5 inch gutter to a 6 inch buys about 40 percent, per Englert. So one outlet is worth more than a size increase, and it costs a fraction as much.
How much roof does one downspout drain?
This Old House and Modernize publish the same four figures. A 2 by 3 inch rectangular downspout serves about 600 square feet. A 3 by 4 serves up to 1,200. A 3 inch round serves about 706, and a 4 inch round up to 1,255. The rule under all of them is one square inch of downspout for every 100 square feet of roof.
What size downspout goes with what gutter?
Amerimax states it plainly. Five inch gutter systems can use 2 by 3 or 3 by 4 inch downspouts. Six inch systems use 3 by 4 only. Fit a 6 inch gutter to 2 by 3 downspouts and you move the bottleneck rather than remove it. The outlet becomes the smallest part of the system.
Is a 2 by 3 downspout big enough?
For a small roof, yes, and the architectural standard would say no. A 2 by 3 has 6.0 square inches of cross section. The Copper Development Association specifies a minimum of 7 square inches, held constant the whole way down, except for small areas such as porches and canopies. That rule would disqualify the most common residential downspout in America.
Where should downspouts go?
At the ends of runs where possible, at the low point of the pitch, and near anything that concentrates water. A roof valley dumps a stream into one short stretch of gutter, so that stretch overflows while the rest of the house is fine. Discharge matters too: getting the water 5 feet from the foundation is the rule the residential code uses in expansive soil areas.
- Rev
- RevAugust 2026
- Source
- SourceAmerimax, CDA, Berger, This Old House, Modernize, Fine Homebuilding, Englert, IRC via UpCodes (read 2026-08-21)