John Henderson Construction, LLC Westerly, RI

Rhode Island:

(401) 615-2224


(860) 415-4055

What You Need to Know About Gutter Sizing

by Nov 19, 2019

The most common residential gutter sizes are five-inch K-styles and six-inch half-rounds. They should be able to withstand rainfall in most parts of the U.S. However, homes with big, steep roofs or homes that are situated in climates prone to heavy rains may need wider gutters and more downspouts to prevent rainwater overflow.

One of the area’s most trusted gutter and cedar roof companies shares what you need to know about gutter sizing. 

Adjusting for Pitch & Rainfall

You’ll have to calculate the square footage of the gutter’s drainage area to determine what size of gutters you need. For a gable-end roof, you’d need to make calculations for each slope. In hip and intersecting roofs that have different facets, you’ll need to add up the area of each surface within a drainage area to obtain the total square footage. 

Once you have the total square footage for each gutter, you’ll need to adjust for the roof pitch and maximum rainfall intensity. Steeper roofs can collect more windblown rain. Measuring the roof pitch is a task you’ll want to leave for professional roofers

The U.S. Weather Bureau records the maximum rainfall that could happen within five minutes. The higher the number, the larger your gutter has to be. Once you know your roof pitch, you can find the pitch factor 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

Sizing Gutters

You’ll find out the adjusted square footage by multiplying the drainage area by the roof-pitch factor. Go for the biggest gutter size if your roof’s various drainage areas require different gutter sizes. See the table below to help you determine which is the best gutter size for your home. 


5-inch   — 5,520 square feet

6-inch   — 7,960 square feet


5-inch    —    2,500 square feet

6-inch    —    3,840 square feet

For instance, your home has a drainage area of 1,000 square feet and you have an 8:12 pitch factor (1.1). Multiply the two and you’ll get an effective area of 1,100 square feet. You then multiply the product by the maximum rainfall intensity to get the adjust square footage. If the maximum rainfall intensity in your area is 6.8, it yields an adjusted square footage of 7,480. It means six-inch K-style gutters are best for your home. 

John Henderson Construction, LLC is one of the area’s most trusted gutter and roof coatings installation companies. Our team specializes in installing high-quality aluminum gutters. Call us at (401) 615-2224 or (860) 415-4055. You can also fill out our contact form to request an estimate. 

Latest Posts

Get an estimate