Gutters & ventilation
Gutter sizing by roof area and rainfall
Gutters are sized for the worst few minutes of a storm on one face of the roof. Three numbers decide it, and one of them is local.
What a gutter has to carry
A gutter overflows in the heaviest few minutes of a storm, not in a day of drizzle. The sheet-metal industry’s method, set out in the SMACNA Architectural Sheet Metal Manual and reproduced by gutter makers, sizes a gutter for three things:
- The roof area draining into that run, measured in plan, because rain falls vertically.
- The pitch of that roof face, because wind drives rain onto steep roofs that a flat projection would miss.
- The rainfall intensity at your location, in inches per hour, for a short, intense storm, typically the 5-minute, 10-year figure from NOAA Atlas 14.
Design area
| Pitch | Allowance |
|---|---|
| Flat to 3/12 | 1.00 |
| 4/12 to 5/12 | 1.05 |
| 6/12 to 8/12 | 1.10 |
| 9/12 to 11/12 | 1.20 |
| 12/12 and steeper | 1.30 |
On a gable house each gutter carries the plan area of the roof face above it: half the footprint, overhangs included. On a 52 by 34 foot footprint that is 884 square feet per side; at 6/12, 972 square feet of design area.
Gutter capacity
Level K-style gutters carry about 5,520 square feet of roof per inch of hourly rainfall (5-inch) and about 7,960 (6-inch); half-round gutters about 2,500 and 3,840. Divide by your local intensity to get the capacity at your rainfall.
| Rainfall intensity | 5 in K-style | 6 in K-style |
|---|---|---|
| 3 in/hr | 1,840 sq ft | 2,653 sq ft |
| 5 in/hr | 1,104 sq ft | 1,592 sq ft |
| 6 in/hr | 920 sq ft | 1,327 sq ft |
| 8 in/hr | 690 sq ft | 995 sq ft |
The figures look small next to the 5,520 because 5-minute intensities are high: a storm that drops an inch in ten minutes is falling at 6 inches per hour. The gutter and downspout sizing table has more intensities.
A 1,000 square foot roof face
Plan area 1,000 sq ft at 6/12, intensity 6 in/hr, 5-inch K-style gutter on a 40-foot run.
- Design area: 1,000 × 1.10 = 1,100 sq ft
- Gutter capacity: 5,520 ÷ 6 = 920 sq ft: too small, 120 percent of capacity
- Fixes: a 6-inch gutter (1,327 sq ft), or a downspout at each end so each half carries half
The downspout calculator does this check and counts the downspouts.
Downspouts
The gutter trade’s rule of thumb is about 100 square feet of roof per square inch of downspout for a typical design storm: a 2 × 3 inch downspout (6 square inches) drains about 600 square feet, a 3 × 4 (12 square inches) about 1,200. Lower it where storms are very intense. Separately, no run should go more than about 30 to 40 feet without a downspout, however small the roof. The count is the larger of the two rules, and downspouts: how many and where covers placing them.
5-inch or 6-inch: the practical choice
Most houses in North America get 5-inch K-style gutters because they are the default in supply yards and seamless machines. The arithmetic above shows when that is not enough: a large roof face, a steep pitch, intense local rain, or a valley pouring into one spot. A 6-inch gutter carries about 45 percent more water for a modest increase in price, takes 3 × 4 downspouts without an adapter, and clogs less. On a new house or a full replacement in a region with heavy summer storms, the 6-inch gutter is usually the better buy even where the 5-inch passes on paper. Where only one face of the house fails the check, a 6-inch run on that face and 5-inch elsewhere is common; the profiles connect with a reducer.
Half-round gutters, chosen for older and historic houses, carry much less than K-style of the same width because of their shape; size them up, or add downspouts.
Finding your rainfall intensity
NOAA’s Atlas 14 precipitation frequency server gives, for any US location, the rainfall depth for each duration and return period. Take the 5-minute, 10-year depth and multiply by 12 for inches per hour. Values vary several-fold across the country: lowest along the Pacific coast, highest along the Gulf and in the Southeast. Plumbing codes use a 60-minute, 100-year figure for roof drains, which is a different design storm; do not mix the two.
From size to parts
Once the size is right, the rest is counting: sections or seamless runs, corners, end caps and outlets from the gutter length calculator, hangers from the hangers calculator at the spacing your snow load needs, and the slope toward each outlet from the gutter slope calculator. Measuring the eaves is part of measuring a roof from the ground.
Valleys and flat roofs
A valley concentrates the water from two planes onto a short stretch of gutter. Where one discharges, a 6-inch gutter, a downspout close by, or a splash diverter on the roof keeps the gutter from overshooting. Flat roofs drain to scuppers and gutters sized the same way, with no pitch allowance; flat roof membranes covers the roof itself.
How much water that is
One inch of rain on one square foot of roof is 0.623 gallons. The 1,000 square foot roof face above sheds about 620 gallons in an inch of rain, which is why gutters overflow and why a rain barrel fills in minutes. The rain barrel calculator does that arithmetic.