Where the downpour figure comes from
Each town page on this site gives the five-minute rain expected about once in ten years at the town's center, from NOAA Atlas 14. That is the kind of figure the sheet-metal industry's manual (SMACNA) sizes gutters and downspouts from. The link from a town page fills it in above; any other rate can be typed in. NOAA Atlas 14 does not cover Washington and Oregon; there, ask the local building department for its design rate.
The three calculations
- Flow off the roof section, from the International Plumbing Code's Equation 11-1: gallons a minute = rainfall rate in inches an hour × roof area in square feet × 0.0104.
- Downspouts, from the Copper Development Association's handbook: one square inch of downspout drains 1,200 square feet of roof at one inch an hour, proportionally less at harder rates. A 2-by-3-inch downspout has 6 square inches; a 3-by-4, 12.
- Yearly water, if you enter the normal yearly rain from your town's page: inches × square feet × 0.62 gallons.
Measuring the roof section
Use the area as seen from above, for the part of the roof that drains to one gutter run, not the sloped surface. Where a valley joins two roof planes into one corner, both planes count toward that corner.
Reading the answer
If a section needs two downspouts and has one, it will spill in a hard storm however clean the gutter is. The usual fixes run from least to most costly: another downspout, larger downspouts, then a larger gutter. The contractor confirms with the roof and the pitch in front of them.