Gutter IQ
The short answer
Allow about 1 square inch of downspout cross section per 100 square feet of roof draining to that run. That is roughly 600 square feet per 2 x 3 and 1,200 square feet per 3 x 4, and it is the one figure that appears identically in a US Department of Energy source, a university extension guide and mainstream trade press.
Then check two ceilings: no run longer than about 50 feet served by a single drop, and every separate run gets at least one regardless of length.
On this page
The method that holds up
- Area per run, not whole roof. Take the horizontally projected roof area that drains into that specific stretch of gutter. This is the step that catches valleys: a valley concentrates a large tributary area into a few feet of gutter, and whole roof math misses it entirely.
- Add diverting wall area. Where a vertical wall sheds onto the roof, the base plumbing code adds half of it to the projected area. Some jurisdictions amend this to a quarter.
- Apply the pitch factor. 1.0 flat through 3 in 12, 1.05 for 4 to 5, 1.1 for 6 to 8, 1.2 for 9 to 11, 1.3 for 12 in 12 and up.
- Divide by your local rainfall intensity. Published figures are at 1 inch per hour. Yours is not 1.
- Check the ceilings. 50 feet maximum per drop, minimum one drop per run.
Downspout sizes and what they serve
| Downspout | Cross section | Roof area at 1 in per hr | Relative to 2 x 3 |
|---|---|---|---|
| 2 x 3 rectangular | 6.00 sq in | 600 sq ft | 1.0x |
| 3 in round | 7.07 sq in | 707 sq ft | 1.18x |
| 3 x 4 rectangular | 12.00 sq in | 1,200 sq ft | 2.0x |
| 4 in round | 12.57 sq in | 1,257 sq ft | 2.10x |
| 4 x 5 rectangular | 20.00 sq in | 2,000 sq ft | 3.33x |
Your rainfall rate is the input that matters
Every published capacity number is quoted at 1 inch per hour. Design rainfall in the United States ranges from well under 1 inch per hour in parts of the west to well over 4 in the southeast, so this division changes the answer by a factor of several.
Worked example. A 3 x 4 downspout serves 1,200 square feet at 1 inch per hour. At a 4 inch per hour design rate that is 300 square feet. A 2,400 square foot roof draining evenly therefore needs two 3 x 4 downspouts in a mild region and eight in a heavy one. Same house, same gutter, entirely different drop count.
Get your number from NOAA’s Precipitation Frequency Data Server, then check it against the rainfall table in the code edition your jurisdiction has adopted.
The rules of thumb, and how much they disagree
We went looking for the source behind "one downspout every X feet" and found that there is not one. Here is the actual spread:
| Source | Type | Spacing given |
|---|---|---|
| SMACNA Architectural Sheet Metal Manual | Trade association standard | 50 ft maximum |
| US Dept of Energy, Building America Solution Center | Government | 20 to 50 ft |
| This Old House | Edited trade press | 30 to 40 ft, 20 to 30 in heavy rain |
| Manufacturer blogs | Marketing | 20 ft, or 30 to 40 ft |
Twenty feet to fifty feet is a two and a half fold spread, and nobody publishes a derivation. That is because length is a proxy for the thing that actually matters, which is capacity, and capacity depends on gutter profile, slope and rainfall rate rather than on length alone.
The area rule is far more consistent. One square inch of downspout per 100 square feet of roof appears identically in the Department of Energy guidance, Texas A&M's extension material and This Old House. Use that, and treat the spacing figures as a ceiling check rather than as the method.
Spacing, slope and the ceilings
- 50 ft maximum per drop. The one figure with a standards body behind it. It is doing two jobs at once, flow capacity and thermal expansion, which is why it holds even when the drainage math would allow more.
- One drop minimum per run. A ten foot run over a porch still needs somewhere for the water to go.
- Two drops on any run over about 60 ft, and when a run has a drop at each end you size the gutter for half the length and half the area, a point the National Bureau of Standards made in 1934 and which still holds.
- Slope 1/16 in per foot, about five eighths of an inch over ten feet. SMACNA, the Department of Energy and Texas A&M all agree on this. This Old House gives a quarter inch per ten feet, which is two and a half times shallower and is the outlier. Note that 1/16 in per foot is also the shallowest column in the code capacity table, so it is a floor.
- Level is allowed for appearance, with the explicit tradeoff that a level gutter needs additional downspouts.
- Hangers at 36 in on centre maximum, tightened to about 18 in where snow and ice sit for long periods.
- Discharge at least 5 ft from the foundation, with grade sloping away at half an inch per foot for at least 10 ft.
What the code actually says
Worth being precise here, because it is widely misquoted.
The residential code has no downspout sizing table. The IRC requires drainage and specifies materials, and delegates sizing to the plumbing code. Do not cite the IRC for capacity numbers.
The plumbing code sizes leaders in gallons per minute. From the 2015 edition onward the table gives capacity in gpm rather than roof area. The conversion is that 96.15 square feet of roof at 1 inch per hour equals 1 gpm, or equivalently gpm equals 0.0104 times area times rainfall in inches per hour.
| Leader size | Capacity |
|---|---|
| 2 in round, 2 x 2, 1 1/2 x 2 1/2 | 30 gpm |
| 2 1/2 in round, 2 1/2 x 2 1/2 | 54 gpm |
| 3 in round, 2 x 4, 2 1/2 x 3 | 92 gpm |
| 4 in round, 3 x 4 1/4, 3 1/2 x 4 | 192 gpm |
| 5 in round, 4 x 5, 4 1/2 x 4 1/2 | 360 gpm |
| 6 in round, 5 x 6 | 563 gpm |
One further caveat, and it is the most important one on this page. All of those figures describe a vertical leader running full under head, as at a roof drain where water ponds. A residential gutter drop is limited instead by the inch or two of head available in the trough, so none of these numbers describe what actually constrains a 2 x 3 hanging off a K-style gutter. We looked for a modern quantified test of real gutter outlet capacity and could not find one. The National Bureau of Standards observed the problem in 1934, writing that the usual cause of overflowing gutters is insufficient capacity of leader connections and that outlets are very commonly too small, and the practical advice from that work still stands: make the outlet opening as nearly the full width of the gutter as you can.
Common questions
How many downspouts do I need?
The defensible method is by area, not by length: allow about 1 square inch of downspout cross section for every 100 square feet of roof draining to that run. That gives roughly 600 square feet per 2 x 3 downspout and 1,200 per 3 x 4. The length based rules you will see quoted, one drop every 20 to 50 feet, are a proxy for the same thing and the sources genuinely disagree about the number.
How far apart should downspouts be?
Published guidance ranges from 20 feet to 50 feet and there is no consensus. The number with a standards body behind it is SMACNA’s 50 feet, described as a practical maximum length of gutter served by one downspout, and it exists as much for thermal expansion as for flow. The US Department of Energy building science guidance says 20 to 50 feet. Trade press commonly says 30 to 40. Treat 50 feet as a ceiling rather than a target.
Does a 3x4 downspout really carry twice as much as a 2x3?
By cross section, exactly twice: 12.00 square inches against 6.00. By the published leader capacity tables it is closer to three times, because vertical leader capacity scales with the dimension rather than the area. So the common claim understates it rather than overstating it. One caveat worth knowing: those tables describe a leader running full under head, as at a roof drain. A residential gutter drop is limited by the inch or two of head available in the trough, and we could not find any modern quantified test of real gutter outlet capacity.
What is the rainfall rate I should be sizing for?
Your local design rainfall intensity, in inches per hour, not a national average. Published capacity tables are stated at 1 inch per hour, so you divide by your local rate. Look it up on NOAA’s Precipitation Frequency Data Server, and check it against the figure in the code edition your jurisdiction has adopted, because code rainfall data has been essentially unchanged since 1995.
Should I count the walls above the roof?
Yes, where a vertical wall diverts water onto the roof. The base International Plumbing Code adds one half of that wall area to the projected roof area. Note that adopting jurisdictions amend this: New York City and Chicago both use one quarter rather than one half. Check what your jurisdiction actually adopted.
What slope should gutters have?
One sixteenth of an inch per foot is the figure that SMACNA, the Department of Energy and university extension guidance all give, which works out to about five eighths of an inch over a ten foot run. It is also the shallowest slope column in the code capacity table, so treat it as a floor rather than a target. Gutters can be run dead level for appearance, with the tradeoff that they need more downspouts.
Does roof pitch change how many downspouts I need?
Indirectly. Multiply the horizontally projected roof area by a pitch factor before you size: 1.0 for flat through 3 in 12, 1.05 for 4 to 5, 1.1 for 6 to 8, 1.2 for 9 to 11, and 1.3 for 12 in 12 and steeper. Steeper roofs do not collect more rain per square foot of plan area, but the water arrives faster and with more momentum. The largest factor is 1.3, so pitch alone rarely changes the answer.
Sources
Every figure on this page traces to one of these. Where two of them disagree, the page says so rather than averaging them.
- US Department of Energy, Building America Solution Center, gutters and downspouts (US government)
- Berger Building Products, Proper Gutter and Downspout Sizing (reprints SMACNA tables and rules) (manufacturer, reprinting a trade association standard)
- SMACNA downspout and gutter sizing calculator (trade association)
- New York City Plumbing Code 1106.3, vertical leader sizing (adopts the International Plumbing Code table) (code document)
- Minnesota Rule 4715.2710, roof area served per leader size (state law)
- NOAA Precipitation Frequency Data Server (US government)
- IIBEC, Rainfall Intensity Changes Over Time: Have the Codes Kept Pace? (professional institute)
- Texas A&M AgriLife, rainwater conveyance sizing guidance (university extension)
- Beij, Flow in Roof Gutters, National Bureau of Standards RP644 (1934) (US government research)
- This Old House, how to size gutters and downspouts (edited trade press)