The last thing you want to happen after you buy gutters is to look up one day and see water cascading over the top during a rainstorm. That can occur with undersized gutters. Oversized gutters? They can be eyesores and collect a lot of debris. So it’s essential to understand what gutter size is right for your home.
Why Gutter Size Matters
Gutters catch and direct rainwater away from your home’s foundation, preventing water damage. But they do their job well only if they are properly sized.
Undersized gutters clog frequently and cause water to overflow, damaging your roof, fascia, landscaping, and home’s foundation. Oversized gutters don’t overflow but are significantly more expensive and can spoil your home’s curb appeal.
Five-inch gutters are the most common because they perfectly suit mid-sized homes. If you have a larger home or live in an area with a lot of heavy rainfall, go for 6-inch gutters. Gutters also come in 7 or 8 inches but are usually used on commercial buildings (more on this later).
One foot of a 5-inch K-style gutter can manage 1.2 gallons of water before overflowing, while a 6-inch gutter can handle 2 gallons. Half-round gutters hold slightly less water.
So the key to determining what size gutters you need is knowing how much water will flow off your roof in the heaviest rainstorms.
Let’s get into it.
How To Find the Right Gutter Size
If you already have a perfect-fitting gutter and want to replace it with the same type, you can measure it and opt for a similar-sized gutter system.
If you are looking for a new gutter system, it’s critical to understand the volume of water that will come off your roof in heavy rainstorms, as well as the rate it will flow. It’s not that difficult. You simply need to calculate the area of your roof, the pitch of your roof and learn the maximum rainfall intensity you can expect in your area.
You will multiply those calculations to arrive at the “adjusted square footage” of your roof. That single number will then determine the size of gutters you need.
Here are the steps to get to that adjusted square footage number:
Step 1: Calculate the Roof Area
- For a gable roof, multiply the length and width of two slopes to find the total square footage.
- For hip, intersecting, gambrel, and mansard roofs, multiply the length and width of each flat area of your roof. Add up those values to find the total square footage.
Step 2: Find the Roof Pitch Factor
Roof pitch refers to the steepness of your roof. It tells you how many inches your roof rises for every 12 inches. For example, if your roof rises 4 inches for every foot, the roof pitch is 4:12.
Calculating the roof pitch is necessary to determine the roof pitch factor. Here are the steps to calculate roof pitch.
- You’ll need a sturdy ladder, a 2-foot level, and a tape measure.
- Measure the level 12 inches in from the end and mark it.
- Hold the level flat, with one end touching the roof surface.
- From the 12-inch mark, measure the distance from the bottom of the level to the roof surface. The measurement value you get is called rise. For example, if there are 4 inches between the bottom of the level and the roof surface, your roof rises – or is pitched – 4 inches per foot.
Here’s a table to determine the pitch factor from the roof pitch.
| Roof Pitch | Roof Pitch Factor |
| 12:12 or higher | 1.3 |
| 9:12 to 11:12 | 1.2 |
| 6:12 to 8:12 | 1.1 |
| 4:12 to 5:12 | 1.05 |
| 0:12 to 3:12 | 1 |
Check out this video for a visual representation of how to determine pitch.
Step 3: Find the Maximum Rainfall Intensity
The National Weather Service records the maximum rainfall intensity in various regions. It’s the measure of maximum rain (in inches) that could occur over a 5-minute sample period. That’s the number you want for this calculation.
If your area receives high rainfall, we recommend you opt for larger gutters, as they can handle the flow and intensity of water much better.
Step 4: Find Adjusted Square Footage
Now multiply the raw area of your roof by the roof pitch factor by the maximum rainfall intensity.
For example, say you live in Fairbanks, Alaska, which has a maximum rainfall intensity of 2.1 inches per hour. If the roof area of your home is 1,500 square feet and the roof pitch is 7:12, the roof pitch factor is 1.1.
So you would multiply 1,500 by 1.1 by 2.1.
1,500×1.1×2.1 (drainage area × roof pitch factor × maximum rainfall intensity) = 3,465 adjusted square feet.
Step 5: Choose the Right Size Gutter
Among various types of gutters, K-style and half-round gutters are the most popular across the United States.
K-Style Gutters
K-style gutters are affordable, durable, and easy to install. They are also relatively deep, so they hold a good deal of water, and their design blends well with modern roofs. The sides of K-style gutters resemble the letter ‘K,’ thus the reason for their name.
- If your roof area is under 5,520 adjusted square feet, choose 5-inch K-style gutters.
- Go for 6-inch gutters if your roof area is between 5,530 and 8,000 adjusted square feet.
Half-Round Gutters
Half-round gutters look similar to round pipes cut into half. Many people find them more visually appealing than K-style gutters, and they are easier to clean. But they aren’t as deep as K-style gutters and don’t hold as much water, so you may need larger ones on a standard house.
- Go for 5-inch half-round gutters if your roof area is under 2,500 adjusted square feet.
- If your roof area is between 2,500 and 3,850 adjusted square feet, go for 6-inch gutters.
When To Consider 7- or 8-Inch Gutters
It is rare that a home will need 7- or 8-inch gutters. But if you have a tile roof with the tile set high above the roofline, water streaming down the roof can shoot out over the top of gutters. In this case, larger (wider) gutters might be a better option.
If your roof is too large (over 8,000 adjusted square feet), opt for 7- or 8-inch gutters to prevent water from overflowing.
Note: Seven- or 8-inch gutters are expensive and are usually not readily available. You may have to place a custom order through a professional installer.
How to Size Downspouts
Rain gutters without downspouts are useless. Downspouts carry water from your gutters to the ground below and away from the house. Experts recommend you place a downspout for every 30 feet of gutters.
Your downspout size should have the capacity to handle water runoff. If you have 7- or 8-inch gutters, adding extra downspouts can enhance your gutter’s drainage capacity.
Here’s how to size downspouts.
- K-style gutters: Go for a 2×3-inch rectangular downspout if you have a 5-inch gutter. If you have a 6-inch gutter, go for a 3×4-inch rectangular downspout. Most homeowners opt for 5-inch seamless gutters. Homeowners with larger roofs may need 6-inch gutters.
- Half-round gutters: Round downspouts go well with half-round gutters. They are available in 3-, 4-, 5-, and 6-inch diameters and 6-inch gutters are generally the best option.
FAQs
Most homeowners opt for 5-inch seamless gutters. Homeowners with larger roofs may need 6-inch gutters.
One foot of a 5-inch K-style gutter can manage 1.2 gallons of water, while a 6-inch gutter can handle 2 gallons. Half-round gutters hold slightly less water.
Gutter guards prevent debris buildup and pests from building their home in your gutter system, thus reducing the need for gutter cleaning.
The Final Word
Gutters divert rainwater away from your home’s foundation, preventing water damage. Measure your roof and get the right gutter size for the best performance. Don’t neglect downspouts, as they are necessary to direct water to the drainage system.
DIY gutter installation isn’t easy, especially if you’re doing so for the first time. We recommend having it professionally installed to avoid errors.
Get in touch with MyGutterGnome gutter experts if you need help with anything related to gutters.
You’ve got gutter and gutter guard questions. We’ve got answers.
Main Image Credit: ronstik / Canva Pro / License
