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Gutter and Downspout Sizing Guide

Gutter and Downspout Sizing Guide

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The right gutter system goes a long way in effectively collecting rainwater, followed by the perfect-sized downspouts that ensure that the water is flushed away from your house. But again, not everyone knows how to pick suitable gutter and downspout sizes for their homes.

You’re in luck because this guide is here to help you determine the gutter size you need as well as how to match it to downspout sizes to create a watertight drainage system.

Let’s dive in!

What Size Gutters Do I Need?

Before knowing the minimum gutter size that you can install on your roof, let’s walk you through common gutter styles and sizes first.

Standard Gutter Sizes

The two standard gutter sizes you’ll find on the market are K-Style and half-round gutters.

As the name suggests, K-Style gutters have profiles with flat bottoms and front sides that loosely look like the letter K. These gutter types are widely available as residential gutter profiles, and their sizes are either 5 or 6 inches across the top part of the profile.

In the case of half-round gutters, they resemble cylindrical pipes that’ have been cut in half lengthwise. These gutters are definitely more good-looking than K-Style alternatives, especially in home settings.

As for their standard size, half-round gutters come in 5- or 6-inch varieties between the front and rear edges.

You may come across 7- and 8-inch gutters, but those usually go with larger houses or commercial and industrial settings.

Calculating the Right Gutter Size for Your Home

So, which size gutter should you go for? There are several steps you’ll have to go through to ensure your investment is well-placed.

Step 1: Calculate the Square Footage and Pitch of Your Roof

Start by measuring the area of each section of your roof. If two sections come together in a valley, make sure to add the sections together.

Next, you’ll have to use a bubble level to get the pitch of your roof, and this value, along with your roof’s square footage, will help you calculate the right gutter size. Follow these steps to calculate your roof’s pitch:

  1. Put a corner of the level on your roof and hold it in a leveled position, then create a mark on the tool 12 inches from the spot where the roof and the level touch.
  2. Measure the distance between the actual spot on the roof underneath the mark to the mark on the tool.
  3. The result will represent the pitch of your roof, shown as a numeral in 12 (3:12, 4:12, etc.)
  4. Match the pitch to a pitch-factor number for each section of your roof, using the following chart.
PitchPitch-factor number
3:12 or less1.0
12:12 or more1.30

Finally, you’ll have to multiply the square foot measurement of every section of your roof by the pitch-factor number as determined by the pitch.

The result you’ll get is called the adjusted roof area number, which you should write down to use later.

Step 2: Find Your Region’s 5-Minute Maximum Rainfall Intensity Amount

Because it’s important to factor in the average rainfall intensity amount of your geographical area, you’ll need to do some digging. Most of the time, this value will be easy to get your hands on by visiting the NOAA website.

Otherwise, you can always browse your county’s records to find that value.

Once you figure it out, it’ll be time to multiply the five-minute maximum rain intensity number by the adjusted roof area number that you calculated earlier.

Step 3: Determine the Minimum Gutter Size for Your Roof

Lastly, look at the chart below to see where your measurements get you when it comes to gutter styles and sizes.

Maximum adjusted roof area (in square feet)Gutter shape and size
2,5005-inch half-round
3,8406-inch half-around
5,5205-inch K-Style
7,9606-inch K-Style

Are Bigger Gutters Better?

Whenever you’re in doubt, your best shot is to go for bigger gutters over smaller ones because they’ll be able to keep up with increased rainfall any time of the day.

Still, we don’t recommend you install too big gutters because they’ll look huge when attached to your house. Not to mention, overly large gutters can be more challenging to install, and they’ll also cost you more money.

What Size Gutters to Use for a Metal Roof?

It’s best to go for larger gutters for roofs with an overhang, which is mostly the case in metal or tile roofs. Here, 6-inch K-Style gutters should be most suitable.

On the other hand, roofs with no overhangs, like shingle or slate roofs, can work with 5-inch gutters.

What Size Gutter to Use for a Shed?

Sheds are small structures that require what many people like to call a mini guttering system.

In this scenario, look for gutters that have a 3-inch profile.

What Size Gutters Are Best?

Arguably, the best gutter size for your house is the one that fits its roof’s pitch and square footage as we’ve previously discussed.

Still, many people choose to go for a 6-inch gutter size anyway because it’s less likely to clog and promises more effective drainage. So, the short answer to your question is 6-inch K-Style gutters.

What Is the Standard Downspout Size?

For each gutter profile size and style, there’s a standard downspout size that’s made to fit with it like two pieces of a jigsaw puzzle.

When it comes to 5-inch and 6-inch K-Style gutters, the standard downspout sizes available are 2 x 3 inches, 3 x 4 inches, and 4 x 5 inches.

These downspouts can be either shaped like a K or have a rectangular profile. However, determining the right downspout form and size to match each gutter style may require an expert.

For example, it’s common knowledge that 5-inch K-Style gutters go with 2 x 3-inch downspouts, and 6-inch K-Style gutters pair with 3 x 4-inch downspouts. Still, it might not always be this straightforward to you as a homeowner, which is why we advise you to seek professional help.

On the contrary, 5-inch and 6-inch half-round gutters go with downspouts that range from 3, 4, to 5 inches in diameter. For a half-round gutter, a circular downspout is the way to go.

What Size Are Oversized Downspouts?

Oversized or extra-large downspouts are those that you pair with larger gutters. In other words, we often refer to 3 x 4-inch downspouts as oversized downspouts.

To further specify their dimensions, oversized downspouts are 3 inches deep and 4 inches wide. Thanks to their larger size, their drainage performance is more pronounced than their smaller counterparts because they can move water two times faster!

What Are Some Guidelines to Follow When Choosing the Right Downspout Size for a Gutter?

Besides the calculations we’ve already gone through, here are a few tips to keep in mind before fitting a downspout with a gutter if you’re determined to keep it a DIY project!

  • Ensure that the downspout size is consistent all the way throughout its entire length.
  • The outlet capacity of both the downspout and gutter should match.
  • Don’t space downspouts at intervals of more than 30 feet.
  • The downspout size shouldn’t be larger than the bottom of the gutter.

Final Thoughts

Learning about the right downspout sizes for your guttering system may sound like rocket science, but it’s pretty simple.

Once you understand how to calculate your roof’s pitch and the square footage of each section, you’ll be halfway through picking the perfect gutter and downspout.

To summarize, you’ll likely end up installing a 5- or 6-inch gutter, whether it’s K-Style or half-round. As for the downspouts, you can go with 2 x 3 inches, 3 x 4 inches, and 4 x 5 inches for K-Style gutters or 3, 4, or 5 inches in diameter for half-round versions.


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