5 or 6 inch gutters? Gutter and downspout size calculator

How we work it out

We follow the SMACNA method as Berger Building Products reprints it. A steep roof catches more wind-driven rain, so the plan area gets a pitch factor: 1.00 up to 3 in 12, 1.05 at 4 to 5, 1.10 at 6 to 8, 1.20 at 9 to 11 and 1.30 at 12 in 12.

design area = roof footprint × pitch factor

Downspouts. SMACNA's city table works out to about 1,200 sq ft of roof per square inch of downspout at 1 in/hr of rain. A 2 × 3 in downspout is 6 sq in, a 3 × 4 in is 12.

sq ft per downspout = downspout sq in × 1,200 ÷ rainfall in/hr

We also keep each downspout to 50 ft of gutter, Berger's practical maximum.

Gutters. Each section between downspouts carries its share of the roof. A 5 in half-round gutter drains 2,500 sq ft at 1 in/hr when level (or pitched 1/16 in per ft or less), 3,520 at 1/8 in per ft and 5,000 at 1/4; a 6 in drains 3,840, 5,440 and 7,680. Divide by your rainfall rate. Chicago's plumbing code table gives the same numbers at 4 in/hr. A K-style gutter holds at least a half round of its width, so we size it as one.

Rainfall rate. From NOAA Atlas 14 at your county's center. Plumbing codes use the 100-year, 1-hour rate. SMACNA's tables use 5-minute bursts, about three times as intense; use that if you never want them to overflow. NOAA has no Atlas 14 estimates for Oregon or Washington yet.