A Guide to Sizing Gutters and Downspouts

A Guide to Sizing Gutters and Downspouts

August 7, 2019

You probably have an ever-growing list of home improvement projects. They are all worthy projects to have your agenda, but replacing your home’s rain gutters is one of the most important tasks of them all. Move it to the top of the list, especially if right now is a non-rainy season in your area.

The first step is to make sure your rain mitigation system will work flawlessly through the heaviest of winter storms. This requires proper implementation of both gutters and downspouts. Are your gutters and downspouts the right size for your house? Below is a guide to sizing gutters for your Oklahoma City home.

Common sizes and calculations

What are the most common residential gutter sizes? You’re going to run into five-inch K-style gutters and six-inch half-round gutters. Both are able to handle the type of rainfall most houses in the United States get during storm season. However, houses with big, steep roofs or in areas prone to heavy downpours will need wider gutters and additional downspouts. This setup will do better at keeping rainwater from overflowing.

To figure out what size gutters you need, all you have to do is calculate the square footage of the gutter’s drainage area. If you have a gable-end roof, you only need two calculations: one for each slope. Intersecting and hip roofs have multiples facets, so you will need to add up the area of each roof surface within the drainage area to get the total square footage.

Adjusting for roof pitch and rainfall

Now that you have the total square footage of drainage for each gutter, it’s time to adjust for pitch and rainfall:

  • Roof pitch factor: A steeper-than-normal roof pitch can cause more rain to blow into and collect in the gutters. Take a two-foot level and a tape measure and measure the pitch. Hold one end of the level against the roof. Once level, measure the distance between the roof and underside of the level at the midpoint. This gives you a 12-inch run. For example, a five-inch gap is a five-in-12 pitch, and an eight-inch gap is an eight-in-12 pitch.
  • Rainfall: The maximum amount of rainfall that could occur in a five-minute period, in inches per hour, in your region (according to the U.S. Weather Bureau) is also used to determine ideal gutter size. The more intense the rainfall, the bigger a gutter has to be to function properly and not break.

Sizing the gutters and downspouts

To find out the adjusted square footage, take the drainage area and multiply it by the roof pitch factor and rainfall intensity. Here’s the size you need:

  • K-style: Five-inch (5,520 square feet) or six-inch (7,960 square feet)
  • Half-round: Five-inch (2,500 square feet) or six-inch (3,840 square feet).

Finally, don’t forget to add the downspouts! A downspout’s capacity must match or exceed the expected rain runoff. For example, adding a 2” x 3” rectangular downspout can boost your gutter’s capacity by 600 square feet of drainage area.

At L & S Seamless Guttering, our rain gutter installers are experts at sizing gutters in Oklahoma City. Call us today to schedule a consultation.

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