How Much Roof Drains Into One Downspout? Pittsburgh Math
Most gutter overflow in Pittsburgh comes down to one number that nobody calculated when the gutters went up: how many square feet of roof each downspout is expected to drain. Here's how to work it out for your own house, using the rain rates we actually get, and what to do when the number comes out wrong.
The rain rate that matters
Annual rainfall is irrelevant to gutter sizing; what matters is the worst hour. The industry standard for sizing (the sheet-metal contractors' association tables, which is what a good installer is using whether they say so or not) uses the intensity of a five-minute burst in a storm that recurs every ten years. For Pittsburgh that's roughly 6 inches an hour for five minutes — a July thunderstorm at its peak. Averaged over a full hour, the storms we see a dozen times a summer run about 1 inch an hour, with 2 inches an hour a few times a decade.
Since gutters are sized for the burst, not the average, the practical capacities look like this:
| Component | Roof area it drains at Pittsburgh design intensity |
|---|---|
| 5-inch K-style gutter, level | about 5,500 sq ft (the gutter is rarely the limit) |
| 6-inch K-style gutter | about 7,900 sq ft |
| 2x3 downspout | about 600 sq ft |
| 3x4 downspout | about 1,200 sq ft |
| 3-inch round downspout | about 700 sq ft |
| 4-inch round downspout | about 1,250 sq ft |
Notice the gutter almost never runs out of capacity in the trough. The downspout is the bottleneck.
Working out your roof area
You need the horizontal area of each roof plane that drains to a given gutter run, then which downspout that run drains to. Measure the wall length under the run and the horizontal depth from the eave to the ridge (from the ground, pace it out or use the property survey). Multiply. Don't add anything for pitch — the rain falls on the horizontal footprint; a steeper roof just delivers it faster, which is why steep slate roofs and roofs with valleys get a margin added.
A worked example
A two-story colonial in Upper St. Clair. Main roof is a simple gable, 44 feet long, ridge in the middle, 16 feet horizontal from eave to ridge on each side. Rear plane: 44 × 16 = 704 square feet, draining to the rear gutter. There's a one-story family-room addition off the back with a shed roof, 20 × 12 = 240 square feet, that also drains into the rear gutter at one end. Total onto the rear run: 944 square feet. The builder put one 2x3 downspout at the left end.
One 2x3 handles 600 square feet. The rear run is carrying 944. In a July storm the right end of that run overflows onto the patio, and the homeowner has had it cleaned three times looking for a clog that isn't there.
Three fixes, cheapest first:
- Upsize the existing downspout to 3x4. Now the run drains 1,200 square feet. Done, for $120–$220. On most houses this is the answer.
- Add a second 2x3 at the right end. Two 2x3s handle 1,200 between them and the run is split, so water travels half as far. $180–$320.
- Both — two 3x4s — if the roof is slate or steep and sheds in a surge, or if there's a valley dumping into one spot.
What isn't the answer is 6-inch gutter on its own. The 5-inch trough on that run could carry 5,500 square feet if the downspout could take it away. Six-inch earns its place when a single run collects more than about 1,000 square feet and the roof is steep enough that the surge overwhelms the trough before the downspout, or where snow sliding off slate needs the extra depth. We use it on a lot of big roofs in Fox Chapel and on slate in Mt. Lebanon; on a typical Bethel Park colonial the 3x4 downspout does the job.
The rules of thumb that fall out of this
- One downspout per 35–40 feet of gutter, minimum, regardless of area.
- 3x4 downspouts as standard on any two-story house. The upgrade costs $30–$50 per drop on a new install.
- Wherever a roof valley or an addition dumps into a run, put a downspout near that point, not at the far end.
- Steep slate or metal: add 25% to the area to allow for the surge, then size.
Where this shows up
The most common undersized run we see is exactly the example above: a rear gutter on a 1970s–2000s two-story with an addition feeding it and one 2x3 at the end. The second most common is a long front run on a 1950s ranch with a single downspout in the middle of 50 feet. Both are repair-priced fixes. If you're replacing gutters anyway, the sizing is built into the installation quote at no extra charge beyond the downspout upgrade. If you're seeing overflow now and want to know which of the three causes it is, why gutters overflow sorts it, and a gutter repair visit can measure the roof and the run in ten minutes.
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