What Size Gutters Do I Need?

To figure out what size gutters you need, first you’ll need to calculate the square footage of the gutter’s drainage area. For a simple gable-end roof, you would only need to make two calculations, one for each slope. Hip roofs and intersecting roofs have multiple facets, and for those you’ll need to add up the area (length x width) of each surface within a drainage area to get the total square footage.

How Do You Calculate Gutter Size?

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.

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 information 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.

Extra Capacity

What if the runoff is off the chart for standard gutters? You have three options:

Rectangular

2 by 3 inches = 600 square feet 3 by 4 inches = 1,200 square feet

Round

3 inches = 706 square feet 4 inches = 1,255 square feet

Information courtesy of the Sheet Metal and Air Conditioning Contractors National Association (SMACNA), a standards-setting organization based in Chantilly, Va.

How to Install Gutters

How to Size Gutters and Downspouts - 83