Gutter Size, Rainfall Capacity, and Downspout Count From Your Roof Footprint

Roof length, depth, and pitch into the drainage area, gallons per minute at your worst storm, the gutter size that carries it, the downspout count and size, the slope the run needs, and what to do when the rain outruns the gutter.

Gutter sizing is a drainage-area arithmetic with a slope allowance. A pitched roof sheds more water at the eave than its flat footprint suggests, because the upper roof surface drains down onto the lower one: the standard allowance adds one-half of the roof rise to the eave length it drains, which is the plan area times (1 plus pitch divided by 24) on a gable roof. The published gutter capacities are tabulated at a 1/16 in per ft slope, so the capacity number and the slope number are the same table. Leaf guards reduce capacity by about 20 percent.

—Effective drainage area on the pitched roof
—Gallons per minute at design rainfall
—Gutter capacity against the inflow
—Downspout count and size, at 40 ft spacing
—Slope the run needs and the hanger line
—Upsizing and overflow plan
How the math works
  • Drainage area: effective area = plan area x (1 + pitch / 24) on a gable roof, which adds one-half of the roof rise to the eave it drains; the rise is pitch x depth / 24 in feet. The ASSE INA-1 and IFS gutter-sizing method.
  • Inflow: gpm = drainage area x rainfall intensity / 96. The 96 is 12 in per ft x 60 min per hr / 7.48 gal per cubic foot. Design rainfall is the NOAA Atlas 14 5-minute, 5-year intensity for the site.
  • Gutter capacity (ASSE INA-1, IFS table at 1/16 in per ft slope): 5 in K-style 283 gpm, 6 in K-style 474 gpm, 5 in half-round 210 gpm, 6 in half-round 350 gpm, 7 in round 500 gpm, 8 in round 680 gpm.
  • Downspout capacity: 2 x 3 in 116 gpm, 3 x 3 in 176 gpm, 3 x 4 in 244 gpm, 4 x 4 in 362 gpm, with downspouts at no more than 40 ft of gutter spacing.
  • Slope: 1/16 in per ft is the basis the capacity tables are published at; 1/8 in per ft on runs over 40 ft and on a run near capacity, with hangers at 24 in and 12 in spacing in snow and ice country.
  • Guards: about 20 percent of the gutter capacity, applied to the table number, which is the number to check before a guard goes on a run that is already near capacity.
Gutter and downspout planning tool on the ASSE INA-1 and IFS published gutter and downspout capacity tables at the 1/16 in per ft slope basis, with the standard roof-slope drainage-area allowance and NOAA Atlas 14 intensity-duration-frequency practice. The exact IDF point for a site, the 100-year and 25-year storm lines a municipal storm system is designed on, a roof with an interior drain, a scupper and overflow scupper on a low-slope roof, a roof drain tied into a storm system sized by a mechanical engineer, a standing-seam roof with a sheet-line discharge, a mansard, a dormer discharging onto a lower roof, a valley that concentrates flow, a gutter on a rake edge, an ice-dam and snow-line install with the ice load as the design load, a historic fascia, a copper and zinc gutter with its own capacity and thermal movement, a buried drain line, a dry well, a rain garden, a cistern with an overflow to the storm sewer, a rainwater harvesting tank with a first-flush diverter, a gutter that feeds a solar panel array, and a gutter on a roof with a wildfire ember and a Leaf-Gutter code line are calls for the local intensity data, the drawings, the county storm requirements, and the licensed installer. Capacity tables assume clean, freely draining gutters, and a guard, a pine needle, a shingle grit, and a squirrel are the field conditions that take 40 percent out of a table number.

What size gutter do I need for a 40 ft run?

5 in K-style carries 283 gpm and covers a 40 ft eave at 2 in per hour on a typical pitched roof, and 6 in K-style at 474 gpm is the upsizing line for the same run at 3 in per hour, in hurricane country, on a steep pitch, and on a run with two upper roof surfaces dumping onto it. The number is the drainage area times the local rainfall intensity divided by 96, and the gutter is picked off the capacity table.

How many downspouts for a 40 ft gutter run?

Two, on the 40 ft maximum spacing, and more if the inflow needs them: a 2 x 3 in downspout carries 116 gpm, so a run collecting 230 gpm needs two, and 350 gpm needs three of them or two 3 x 4 in drops. The gutter gets the water to the drop; the drop is the bottleneck, and the standard production house has one 2 x 3 in drop on a 40 ft run, which is the overflow people blame on the gutter.

Should gutters slope to the downspout?

Yes, at 1/16 in per ft, which is the slope the published gutter capacities are measured at, and 1/8 in per ft on runs over 40 ft and on a run that is near capacity. A 40 ft run needs about 2-1/2 in of fall from the high end to the drop, set at the fascia line with the high end at the far end. A flat seamless gutter on a long run holds water in the middle, spills standing water at the fascia in a gust, and freezes into one ice slab in a cold winter.

Do gutter guards reduce capacity?

About 20 percent on the published tables, which is the number to check before a guard goes on a run that is already near capacity. The trade is real: a guard on a run under pine and oak is the difference between a gutter that drains in the storm and a gutter that is a dam with a leaf mat on it. Put the guard on the upsized gutter when the inflow is close to the table capacity, and take the derate out of the capacity number, not out of the inflow number.

What rainfall intensity do I size a gutter on?

The NOAA Atlas 14 5-minute intensity at the 5-year return period for your county, which is 2 in per hour across much of the East and mid-continent, 3 to 4 in per hour on the Gulf, Florida, and the hurricane coast, and 1 to 1.5 in per hour in the desert and intermountain West. The 5-year line is the gutter-sizing convention because a gutter sized on a 100-year storm is a gutter you are buying as a foundation insurance policy.

Related tools

Gutters and downspoutsFootage, linear feet per drop, and elbows Roof snow loadThe ice and snow line on the same fascia Shingles and underlaymentThe roof that is shedding this water Foundation drainage slopeThe 5 ft slope-away the drop feeds Driveway paving costThe apron the drop is dumping on Fence material listThe line the splash hits