Gutters and downspouts must be appropriately sized for the amount of rainwater they will transport. If not, water will flow over the sides during periods of intense and heavy rainfall.
If you are planning to install a new gutter system on your own, you will need to determine what size to get first. And if you notice post-rain flooding around your house, you might want to look into whether or not the gutters and downspouts are big enough.
Gutter and Downspout Sizing Hack
Want the easiest way to find the right gutter and downspout size? Use the Downspout and Gutter Sizing Calculator from the Sheet Metal and Air Conditioning Contractors’ National Association.
Want to delve into how to get there yourself? Read on.
What Gutter and Downspout Sizes Are Available?
Most residential gutters and downspouts are either K-Style or half-round style.
K-Style Gutters and Downspouts
K-Style gutters are rectangular-shaped, with a flat bottom, flat back, and curving ridged ogee-shaped front. These gutters are the most popular residential style and can move the most amount of water.
These gutters are available in 5”-wide to 8”-wide sizes, with 5” and 6” being the most popular.
- 5”-wide gutters fit a 5,520 square feet roof and can hold 1.2 gallons of water per foot
- 6”-wide gutters fit a 7,960 square feet roof and can hold 2.0 gallons of water per foot
Half Round Gutters and Downspouts
Half-round gutters have a simple semi-circle shape. These gutters don’t transport as much water as K-Style gutters and may need to be sized up in comparison.
Half-round gutters aren’t found as often in contemporary housing styles and are considered a historically appropriate choice for older homes.
These gutters can be found in 4” to 8” sizes. However, it can be a little more difficult for DIYers to find larger half-round gutter sizes since they aren’t as popular.
- 5” wide half-round gutters fit a 2,500 square foot roof and can hold less than 1 gallon of water per foot
- 6” wide half-round gutters fit a 3,840 square foot roof and can hold 1.2 gallons of water per foot
Downspouts
Downspouts are available in K-style, round, or rectangular shapes. Residential downspout sizes range from 2 to 6 inches wide, varying by style.
- K-Style and rectangular downspouts are available in 2”x3” or 3”x4” sizes.
- Round downspouts are available in 3”, 4”, or 5” sizes.
Matching Gutters and Downspouts
K-style gutters work with K-style or rectangular downspouts.
- 5” wide K-style gutters fit with 2”x3” downspouts.
- 6” wide K-style gutters fit with 3”x4” downspouts.
Half-round gutters work with round downspouts.
- 5” wide half-round gutters fit with 3” round downspouts.
- 6” wide half-round gutters fit with 4” round downspouts.
Adding Water Capacity with Downspouts
Downspouts increase a gutter system’s water capacity. So incorporate downspouts into your planning. It’s possible to use a standard gutter size and add extra downspouts to accommodate more rainfall.
Rectangular downspout water capacity:
- One 2”x3” downspout = 600 square feet
- One 3”x4” downspout = 1,200 square feet
Round downspout water capacity:
- One 3” downspout = 706 square feet
- One 4” downspout = 1,255 square feet
Find the Right Gutter Size to Install
To find the right gutter size, you first need to calculate the square footage of the area to be drained. This requires finding the square footage of each roof slope or surface plane. If your roof has intersections, hips, or jutting sections, those will need to be calculated separately.
Then adjust this square footage by the roof’s pitch factor and your area’s rainfall intensity.
Find the Roof Pitch Multiplier Factor
Steeper roofs collect more windblown rain. To account for this, you will need to find your roof’s pitch factor. Here’s how to do that.
- Get a 12”-long level, ruler, and extension ladder.
- Place the ladder against the roof, making sure it’s level, stable, and secure.
- Safely climb onto the roof and up to the top.
- Place the level against the roof’s top ridge and level it out.
- While keeping the level in place, hold the ruler perpendicular to (or straight down from) the end of the level.
- Make sure that the level is still touching the ridge of the roof and that the ruler is against the level.
- Write down the inches between the roof and the end of the level, where it’s 12” away from the ridge.
- Write this measurement as X/12 with x = the vertical distance down from the level to the roof. (If it was 5 inches down, the measurement would be 5/12.)
This measurement is the roof’s pitch. You can then find the roof pitch multiplier factor by checking this chart.
Roof Pitch to Roof Pitch Multiplier Factor
- 1/12: 1.0035
- 2/12: 1.0138
- 3/12: 1.0308
- 4/12: 1.0541
- 5/12: 1.0833
- 6/12: 1.1180
- 7/12: 1.1577
- 8/12: 1.2019
- 9/12: 1.2500
- 10/12: 1.3017
- 11/12: 1.3566
- 12/12: 1.4142
Find Your Local Maximum Rainfall Intensity
The maximum rainfall intensity is the most rainfall in inches that your area can get within a 5-minute timeframe. Higher rainfall intensities require larger gutters and/or more downspouts.
You can find probable maximum precipitation data from the National Weather Service.
Adjust the Square Footage by the Roof Pitch Multiplier Factor and Maximum Rainfall Intensity
Your final step is to multiply the roof’s draining square footage by the pitch multiplier factor and maximum rainfall intensity.
Get an Expert Roofing Company to Install Your Gutters
At Canopy Roof and Restoration, we know exactly what sizes and styles of gutters and downspouts are best for Charlotte and the Carolinas weather. You can rely on us to put in gutters and downspouts that hold up to the heaviest Charlotte rainfall, that’s the Canopy roofing way.
Schedule an inspection to get started on a gutter estimate for your gutter installation.
