Adjusting for Pitch and Rainfall
Once you know the total square footage of drainage for each gutter, you'll need to adjust for the following two factors:

1. Roof-pitch factor
The steeper a roof's pitch, the more windblown rain it can collect. You can measure pitch with a 2-foot level and a tape measure: Hold one end of the level against the roof, level it, and then measure the distance between the roof and the underside of the level at its midpoint, which gives you a 12-inch run. A 5-inch gap, for instance, is a 5-in-12 pitch. Once you know pitch, you can find your roof-pitch factor in the table below.

Roof pitch     /     Roof-pitch factor
12 in 12 or higher        1.3
9 in 12 to 11 in 12        1.2
6 in 12 to 8 in 12          1.1
4 in 12 to 5 in 12          1.05
Flat to 3 in 12                1

2. Maximum rainfall intensity
The U.S. Weather Bureau records the maximum rainfall that could possibly happen in a 5-minute period, in inches per hour, for various regions. The higher the amount, the bigger a gutter has to be to keep from being overwhelmed in a storm burst. Download this handy table to find out the number for your area.
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