The downpour that matters
A gutter is not sized for a year of rain but for its hardest five minutes. SMACNA's Architectural Sheet Metal Manual sizes gutters and downspouts from the five-minute rain expected once in 10 or once in 100 years, and sends designers to NOAA's Precipitation Frequency Data Server for the local value. The calculator above takes that value, the ten-year figure, for the center of the city you pick. Across metro Atlanta it runs from about 7.7 to 8.2 inches an hour.
What the calculator does
- Flow. The International Plumbing Code converts rain to flow with its Equation 11-1: gallons a minute = rainfall in inches an hour × roof area in square feet × 0.0104.
- Downspouts. The Copper Development Association's design handbook: one square inch of downspout opening drains 1,200 square feet of roof at one inch of rain an hour, and half that at two inches. The calculator divides by the city's design rate.
- Yearly water. The city's normal yearly rain × 0.62 gallons per square foot per inch.
Use the roof area as seen from above, for the part of the roof that drains into the one gutter run. A valley that brings two roof planes into one corner adds both planes to that corner's gutter.
Reading the result
If the calculator says a section needs two 2-by-3 downspouts and it has one, that corner will spill in a hard storm however clean it is. The usual fixes, in order of cost: add a downspout, go to 3-by-4 downspouts, then go to a larger gutter. A contractor checks the pitch and the roof itself before choosing; the calculator gives you the right questions to ask.