How many downspouts does your roof need?
Enter the roof footprint, the design rainfall and the downspout size. Get the number of downspouts and the flow they carry.
The footprint, not the sloped surface. For one downspout, enter the part of the roof that drains to it.
Look up your site in NOAA Atlas 14 (hdsc.nws.noaa.gov/pfds), or ask your building department.
Why the rainfall rate matters more than the roof size
A roof does not drain once per storm; it drains at the speed of the heaviest burst. A downspout that is fine in a steady rain of an inch per hour can overflow in a cloudburst of four. That is why the design rate is an hourly intensity for a rare storm, not a yearly total, and why the same roof can need two downspouts in one city and four in another.
How many downspouts do I need?
Divide the roof's footprint by the area one downspout can drain at your design rainfall rate, and round up. A 2 × 3 inch downspout drains 2,880 ÷ rate square feet, so at 4 inches per hour it handles 720 ft²; a 2,400 ft² roof needs 4 of them, or 2 of the 3 × 4 inch size.
What rainfall rate should I use?
The plumbing code sizes leaders for a design storm from local data, reported as the rate for a 100-year, 1-hour rain. You can look it up for your address in NOAA Atlas 14 (the Precipitation Frequency Data Server, hdsc.nws.noaa.gov/pfds), or ask your building department which rate it adopts. A lower rate, such as the 2.8 inches per hour in a Connecticut training example for New Haven, gives fewer downspouts than a Gulf Coast rate would.
How much water does a roof shed per minute?
Every square foot of roof sheds 0.0104 gallons per minute for each inch per hour of rain: 144 cubic inches per inch of rain on 1 ft², a gallon is 231 cubic inches, 60 minutes in an hour. Put the other way, 1 gpm comes from about 96 ft² of roof at 1 inch per hour. A 2,400 ft² roof at 4 inches per hour sheds 99.7 gpm.
Do I use the sloped roof area?
No. Use the horizontal projection, the footprint seen from above, because rain falls vertically. The IPC table is in horizontally projected roof area. A steeper pitch does not collect more rain.
Is a rectangular downspout as good as a round one?
The IPC table says its sizes apply to any shape that fully encloses a circle of the diameter listed. A 2 × 3 inch rectangle encloses a 2 inch circle and is rated as 2 inches, a 3 × 4 inch one as 3 inches. Round pipes of the same number are not rated higher in that table.
Does the gutter limit the answer too?
Yes. This page sizes the vertical pipes. A gutter that is too small or too flat overflows before the downspout is full, and a clogged outlet makes the roof run off the front edge. Use the gutter calculator for lengths and outlets, and keep gutters and screens clear.
Where should the water go afterwards?
Away from the foundation: onto splash blocks, a buried drain, a rain garden or a rain barrel. The rainwater harvesting calculator gives the volume per storm and the rain garden size calculator sizes a basin for it.
The method: area, rain rate, table
The International Plumbing Code table 1106.2 gives, for each leader size, the horizontally projected roof area it can serve at a rainfall of 1 inch per hour. For other rates the area is divided by the rate: at 2 in/h a 4 inch leader serves half of 18,400 ft². The numbers here are as reproduced in Michigan's plumbing rules (R 408.30713) and checked against a Connecticut training presentation on the IPC; we could not open ICC's own text, so check the table in your adopted edition. The count is the roof area divided by the capacity of one pipe, rounded up, and the table lists every size so you can compare.
Worked example
A 2,400 ft² roof footprint in a 4 in/h storm sheds 2,400 × 4 × 0.0104 = 99.7 gpm. A 2 × 3 inch downspout covers 2,880 ÷ 4 = 720 ft², so 4 are needed, each at 83.3 percent of its rating. With 3 × 4 inch downspouts (8,800 ÷ 4 = 2,200 ft²) two do the job. A Connecticut training example runs 12,500 ft² at 2.8 in/h: one 4 inch leader serves 18,400 ÷ 2.8 = 6,571 ft², so two are needed (2).
Limits of this method
The table is for the plumbing code's storm drainage design, which assumes the whole roof runs off, with no allowance for lag. Local rules for houses may differ from the plumbing code, and we did not find a leader sizing table in the International Residential Code. We did not use the SMACNA gutter tables, which are paywalled. The result is a planning figure; for a large or flat roof, a roof with a valley that collects several slopes, or a code-required design, ask the plumbing inspector or a licensed designer. The 100-year, 1-hour rule is reported by secondary sources and was not checked against the ICC text.
Common mistakes
Using the sloped roof area instead of the footprint. Using the average yearly rainfall instead of an hourly rate: 40 inches a year is nothing like 4 inches in an hour. Counting a downspout that is partly blocked by a bend or a screen as full capacity. Sending several downspouts into one small drain pipe.
Sources: International Plumbing Code, table 1106.2, Size of Vertical Conductors and Leaders, as reproduced in Michigan Administrative Code R 408.30713 and in a Connecticut Department of Consumer Protection continuing-education presentation on storm drainage (ICC’s own text was not opened); NOAA Atlas 14, Precipitation Frequency Data Server. The gallon, 231 cubic inches, is from NIST Handbook 44. Related: gutter calculator, rainwater harvesting calculator, rain garden size calculator, roof calculator. How we check our calculators.