Sizing the Gutters
Multiply the drainage area by the roof-pitch factor and rainfall intensity to find out the adjusted square footage. Then use the chart below to see what size gutter you need. (If a roof's various drainage areas call for different size gutters, go for the biggest one.)

K-Style
5-inch      5,520 square feet
6-inch      7,960 square feet

Half-round
5-inch          2,500 square feet
6-inch          3,840 square feet

For example: A house in Chicago has a roof whose actual drainage area is 1,000 square feet. The 6-in-12 pitch factor (1.1) multiplied by 1,000 yields an effective area of 1,100 square feet. Multiplying that number by the local maximum rainfall intensity (6.8 inches per hour) yields an adjusted square footage of 7,480 square feet. Therefore, this roof should be equipped with 6-inch K-style gutters.

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