Rain Gutter Sizing

Calculate Correct Gutter & Downspout Size

Five-inch K-style gutters or 6-inch half-rounds, the most common residential sizes, are able to handle the rainfall on most houses in most parts of the country. But houses with big, steep roofs or those located in climates prone to heavy downpours may need wider gutters and extra downspouts to keep rainwater from overflowing.

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.

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.

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.

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

1. Get 7- or 8-inch gutters. They’ll cost more and probably require a custom order through a professional installer.

2. Increase the pitch of the gutter. The standard is about ¼ inch per 10 feet. Increasing the pitch increases a gutter’s handling capacity, but the gutter may look askew over a long run.

3. Add downspouts. The above recommendations assume that you have properly sized downspouts every 40 feet. As with gutters, a downspout’s capacity must match or exceed the expected runoff. Use the chart below to figure out how many extra downspouts you need. Adding a 2 by 3 rectangular downspout, for instance, boosts your gutter’s capacity by 600 square feet of drainage area.

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.

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