Gutters & ventilation
Downspouts: how many, where, and how far from the foundation
A gutter is only as good as the place its water ends up. Here is how to place downspouts so the water leaves the house and stays away.
Two rules decide the number
Every gutter run needs enough downspouts for two separate reasons, and the larger answer wins.
Capacity. Each downspout can drain only so much roof. The gutter trade’s rule of thumb is about 100 square feet of roof per square inch of downspout: roughly 600 square feet for a 2 × 3 inch downspout and 1,200 for a 3 × 4, less where storms are intense.
Distance. Water should not have to travel too far along a gutter: a common limit is 30 to 40 feet of gutter per downspout. Long runs fill up at the far end even when the total capacity looks adequate.
Two runs, two different reasons
A 40-foot run under a 1,000 square foot roof face at 6/12 (1,100 square feet of design area) with 2 × 3 downspouts:
- By capacity: 1,100 ÷ 600 = 1.8, so 2; by distance: 1
- 2 downspouts: capacity decided it
A 60-foot run under 700 square feet with 3 × 4 downspouts:
- By capacity: 770 ÷ 1,200 = 0.6, so 1; by distance: 60 ÷ 40 = 1.5, so 2
- 2 downspouts: distance decided it
Both from the downspout calculator, which also checks the gutter’s own capacity, as gutter sizing by roof area explains.
Where to put them
- At the ends of runs, and at both ends of long ones. Two downspouts at the ends with the high point of the gutter in the middle halve the distance water travels and the drop the gutter needs.
- At the low point of the ground. Water discharged where the yard already falls away keeps moving; water discharged into a hollow beside the house soaks in.
- Away from walkways, driveways and entrances, where discharged water freezes in winter.
- Near valleys. A valley pours concentrated water onto a short length of gutter; a downspout close to it stops the gutter overflowing there.
- On the corners, not the middle of a wall, where they are less visible and where corner trim gives the straps something solid to bite.
Slope the gutter to them
Gutters fall toward their downspouts at about 1/16 to 1/8 inch per foot. A 40-foot run sloped one way at 1/16 inch drops 2½ inches, enough that the low end may sit below where water leaves the roof; split into two 20-foot slopes from the middle it drops 1¼ inches each way. The gutter slope calculator gives the marks for the chalk line, and the outlet positions come from the gutter length calculator.
How far to take the water
The point of the whole system is to keep water away from the foundation. A downspout that empties at the base of the wall delivers the roof’s water to the footings, where it saturates the soil, pushes on basement walls and finds every crack.
| Discharge | Reaches | Use it where |
|---|---|---|
| Splash block only | about 2 ft | The ground already falls steeply away |
| Surface extension or flexible pipe | 4 to 10 ft | Most houses; the minimum is 4 to 6 ft |
| Buried pipe to daylight or a pop-up emitter | 10 ft or more | Basements, expansive clay, tidy yards |
| Rain barrel with overflow | the barrel, then its overflow | Garden use; the overflow still needs an extension |
The usual guidance is at least 4 to 6 feet from the foundation, 10 feet on expansive clay or beside a basement. The ground itself should fall away from the house, about 6 inches over the first 10 feet (IRC R401.3). Never connect downspouts to the foundation drain around the footings: that pipe exists to remove groundwater, and roof water overwhelms it.
Four downspouts on a ranch house
Four downspouts, 10-foot eaves, 5-foot surface extensions.
- Pipe: 40 ft of vertical pipe (4 pieces) plus 4 extensions
- 12 elbows (two at the gutter, one at the ground, each); 8 straps; 4 splash blocks
From the downspout extensions calculator.
Size and shape of the downspout
Downspouts come rectangular (2 × 3 and 3 × 4 inches are the common sizes) or round (3 and 4 inches). The 3 × 4 carries twice the water of a 2 × 3 and is much harder to clog with leaves, which makes it the better choice under trees even when the arithmetic says the smaller size is enough. The outlet in the gutter must match: a large downspout on a small outlet carries only what the outlet passes. Where a downspout drops onto a lower roof rather than the ground, it needs a splash diverter or an extension that carries the water to that roof’s gutter, not a spout that pours onto the shingles.
Rain barrels on downspouts
A barrel under a downspout catches the first few minutes of a storm: on a 500 square foot roof face a 55-gallon barrel fills in about a fifth of an inch of rain. Everything after that overflows, so the barrel’s overflow needs the same extension as the downspout would have. The rain barrel calculator shows how quickly it fills.
Signs the downspouts are wrong
Water marks or erosion below the gutter mean overflow: too few or too small downspouts, a blocked outlet, or no slope. Damp basement walls on one side of the house, or soil washed out beside a downspout, mean the water is not going far enough. Peeling paint and rot at the corner of a wall below a roof edge usually mean a missing kick-out flashing rather than a gutter problem, which the flashing guide explains. And a gutter pulling away from the fascia under the weight of standing water is one of the small repairs worth doing before it becomes a fascia replacement, a judgement the repair or replace guide covers.