5 inch vs 7 inch gutters

As you can see in the picture above, that one extra inch makes quite a difference. In fact, 6-inch gutters can hold about 50% more water than 5-inch gutters! When choosing the size of your gutter, we need to take into consideration a couple of factors:

• Size of Your Home / Roof Planes
• Roof’s Pitch
• Amount of Rainfall in Your Area
• Look/Style

Size of Your Home/Roof Planes

The optimal size of the gutter you need will depend on the amount of water the gutter needs to handle. The bigger the square footage of the roof, the more water goes into the gutter. 

For example, if your roof’s plane is 600-800 square feet, you should be fine with having 5-inch gutters. The plane refers to one complete area, which includes four edges, typically one side of a gable or hip. On the other hand, if your roof’s plane is 1,400+ square feet, we recommend putting on 6-inch gutters as this results in double the water amount going into your gutters.

If you recently replaced your roof and your roofing contractor extended your shingles more than 1.5 inches past the roof edge, water will most likely overshoot the gutter during a heavier rainstorm. In this case, we also recommend a 6” gutter as a longer overhang equals less water in the gutter.

Roof’s Pitch

Pitch – aka, how steep your roof is – is another key factor to consider when choosing your gutter size. The speed at which water gets directed into your gutter increases with the pitch. 

A higher pitch results in a higher speed of water flow. Gutters need to be appropriately selected to accommodate water accumulation and get it delivered to the downspouts and away from the structure of your home. It’s possible that even if you have a roof plane in that 600-800 square feet range we mentioned above, but the pitch is very steep, you may be better off with 6-inch gutters.

Rainfall in your area

At the end of the day, it comes down to how much rainfall your area gets. If you live in a dry environment that does not get much rainfall, 5” gutters could be good for you. In the Chicagoland area, we recommend 6” gutters for a lot of homes.

Look/Style

When choosing the gutter, you may want to take the size of your fascia boards into consideration as well. The wider the fascia, the smaller the gutter will look. So, if your fascia is 1”x6” you are most likely going to install 5-inch gutters. If your fascia is 1”x8” or 1”x10” you are going to need 6-inch gutters to keep everything looking proportional.

Summary

6-inch gutters can handle almost 50% more water than 5-inch gutters. They are installed with bigger 3”x4” downspouts for better water management. 6-inch gutters are less likely to get clogged up because they can handle more water flow, and they come with a bigger outlet. If you have a small home with a roof that isn’t very steep, 5” gutters will suffice. Otherwise, go for 6” gutters. Whichever size you decide to go for, make sure to check the materials and components your contractor uses; you can learn more about what makes a great seamless gutter system here. 

Use this handy guide to make sure your rain collection is up to the task during even the heaviest storm.

By Thomas Baker

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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.

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:

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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