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What Size Gutters Do You Need?

Gutter replacement
Roof pitch matters when you’re considering the size of your future gutters.

Is a gutter just a gutter? While a certain shape and size may come to mind when you think about gutters, these water-bearing troughs can be many different shapes and sizes. Gutters are more diverse than you think, and knowing what types are out there can help you determine what gutter product would be a good fit for your home.

Gutter Replacement: Gutters come In Different Sizes

When you are completing a gutter replacement, the first point to note is that gutters come in different sizes. They can be 4, 5, or 6 inches. Downspouts also come in different sizes, such as 2×3 inches and 3×4 inches in size.

According to This Old House, while 5-inch K style gutters and 6-inch half rounds are usually suitable for homes, “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.” 

How can you determine what size of gutter you need? It’s a question of practical math. Here’s what you need to know to determine your gutter needs.

Consider Your Drainage Area

How do you do the math to figure out how large your gutters should be? 

You’ll also need to look at how much water is going onto your roof and flowing into your gutters. A larger roof means that more water will move into the gutters. Don’t forget to count all of those little parts of your roof if you don’t have a simple roofline. 

To find the drainage area of your roof, multiply the length and width of all the parts on one side of your roof and add them together to get the drainage area. This is simple if you have a straightforward roof style, but it can get more complex when your roof has many different parts. 

Gutter Replacement: Calculate Your Roof Pitch

If your roof is steep, you’ll need a larger gutter. That’s because your roof is like a waterslide, and the water slips down quickly into your gutter when there is a steep pitch. It can easily overwhelm your gutter capacity.

You need to multiply that drainage area by the roof pitch and the rainfall intensity. To find the roof pitch, hold a level against the roof until it is level, then look at the middle of the level and measure upwards until you get to the roof. This number will tell you your roof pitch. If the number is 3 inches or less, you don’t need to add any additional capacity for your roof pitch, because your roof is very flat. If it’s 12 inches or more, you need to multiply your roof’s square footage by 1.3.

Here are the numbers you need to adjust your drainage area number.

  • 4 inches: multiply drainage area by 1.05
  • 6 inches: multiply drainage area by 1.1
  • 9 inches: multiply drainage area by 1.2
  • 12 inches or more: multiply drainage area by 1.3

You can also work with a professional who knows the climate in your area and who understands what size of gutter you need to work with the roof that you have. Download our free gutter guide for more helpful information.

Consider how intense your rainfall is when you are thinking about the size of your gutters.

Tan colored home with grey shingled roof and white gutters.
New gutters should be appropriately-sized for your home.

Figure Out Your Rainfall Intensity

Look at the rainfall in your area, particularly the intensity of the rainfall events. When the rain falls, does it come slowly over many hours? Does it all pour down within a few minutes? To your gutters and your garden, these two events feel very different. That rush of water can be difficult for your gutters to accommodate.  You can determine the maximum rainfall intensity by looking at data from the US Weather Bureau

Gutter Installation: Consider Your Gutter Shape

In addition to understanding how much water is coming off of your roof and how quickly, you’ll need to adjust for the shape of gutter you have at your home. Different gutter shapes can accommodate different amounts of water at one time. 

K-style gutters look more like crown molding and can accommodate approximately double the amount of water as half-round gutters. According to This Old House, the following measurements apply to gutter capacity for K-style and half-round gutters: 

K-Style     Adjusted Square Footage of Roof 

5-inch      5,520 square feet

6-inch      7,960 square feet

Half-round  Adjusted Square Footage of Roof 

5-inch          2,500 square feet

6-inch          3,840 square feet

When you are considering what kind of gutters to get, keep in mind that different gutter shapes have a significant impact on the capacity of the gutter.

Are you looking for a gutter replacement? At Harry Helmet, gutter installation and gutter covers are our specialities. If you have questions about gutter replacement and installation, connect with us today.

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I recently had the Gutter Helmet by Harry Helmet installed on my house. It not only looks amazing, but the guys arrived on time, were professional and efficiently got the job done. I am very satisfied with Gutter Helmet's attention to detail and fulfilling their work as promised. I will definitely tell my neighbors about the Gutter Helmet.
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I have had Gutter Helmets for 20 years. In all that time I've never had a problem until recently a bird's nest found its way into one of my gutters. The installer who came to replace the product was the same installer from 20 years earlier. Now that's impressive. Everyone at Gutter Helmet inclusive of the office staff were professional and a pleasure to work with.
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Gutter Helmet by Harry Helmet just replaced our roof and we could not be more satisfied. The crew showed up early, worked all day and stayed late, were professional and caring regarding the project and the surrounding impact it had on our property and home. They completed the roof quickly and it looks simply amazing!
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VERY happy with our entire experience adding Gutter Helmet by Harry Helmet to our house. Installing this was a great value for us. The gutters are working perfectly in heavy downpours and with year-long leaves and debris coming down from our river birches. Terrific customer service and a first class product.
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We needed our roof repaired due to wind damage. After several quotes, Gutter Helmet was the only company that went into our attic to look and see if more damage had occurred. If it wasn't for them taking the extra time to look at everything, we would have had rotting wood under our new roof. They were very professional and gave a fair quote.
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We just had Gutter Helmet installed on our gutters and couldn't be happier. They provided us with useful information about Gutter Helmet and cleaned up everything, leaving no mess behind. We definitely recommend Gutter Helmet to anyone considering this product.
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