Gutter and downspout size calculator

How it is worked out

One inch of rain an hour on one square foot is 144 cubic inches an hour, or 0.0104 gallons a minute. A 40 ft gutter on a roof 15 ft deep drains 600 sq ft. At 3 in/h that is 18.7 gpm. At 1/8 in per ft, the 4 in semicircular gutter in Table 1106.6 carries 39 gpm, and the 2 in round downspout in Table 1106.3 carries 30 gpm, so both are enough on paper; most houses use 5 in gutters for leaves and margin.

What the code says

• Gutters are sized on the flow from the roof, which must not exceed the capacity in IPC Table 1106.6 (Section 1106.6).
• Downspouts (vertical leaders) are sized on the flow from the gutters, which must not exceed Table 1106.3 (Section 1106.3).
• Walls that divert rain onto the roof add half their area to the drainage area (Section 1106.4).

The code uses the area of the roof in plan, measured level, not along the slope: rain falls on the footprint of the roof, whatever its pitch.

K-style gutters

The IPC table lists semicircular and rectangular gutters, not K-style. K-style gutters hold more than a semicircle of the same width, so sizing them by the semicircular row of their width, 5 in K-style as a 5 in semicircle, is on the safe side. Your building department decides what it accepts.

Rainfall

Storm drainage is sized for the 100-year, 1-hour rainfall. IPC Appendix B lists it for 217 cities, from 0.6 in/h in Juneau and 1.4 in/h in Seattle to 3.0 in/h in Chicago, 4.8 in/h in New Orleans and 6.2 in/h in Hilo. Pick the nearest city above, or use NOAA Atlas 14 or your building department for your exact address.

The parts list assumes hangers every 24 in, a common manufacturer spacing; use closer spacing where snow and ice load the gutters. For the roof itself, see the roofing calculator.