Here’s Your Ultimate Gutter And Downspout Sizes Guide

Your roof is responsible for shedding rainfall and keeping your home’s upper levels dry. Your gutters and downspouts deliver the water collected from the roof and distribute it away from the house to keep your basement, crawlspace, foundation or slab from flooding. Water pooling in your basement after rainfall is unpleasant at best. The bigger concern is the damage that water can do to a foundation, even if that foundation is just a concrete slab.

A gutter and downspout system must be at least large enough to handle the heaviest rainfall that your geographic location experiences to provide adequate protection for your foundation. The gutters collect the water, and the downspouts allow the water to drain away effectively.

Two gutter types are common for residential use, plus a few others that are less common. The name of the gutter type describes its shape or profile.

K-Style Gutters

K-style gutters are the most common residential gutter type. They’re recognizable by their profiles that have a flat bottom and a front side that resembles a scrolled letter K. Common gutter sizes for K-style gutters are five and six inches wide across the top of the profile.

Half-Round Gutters

Half-round gutters are just as their name implies. Their profile matches a half-circle resembling a cylinder cut in half lengthwise. Standard sizes are five and six inches between the front and rear edges. Half-round gutters are arguably more attractive than other gutter styles, but have less water-draining capacity.

Other Gutter Types

Several other gutter styles exist. Most of them are variations on the K-style and half-round versions, but not all. You may find box or rectangular gutters on large houses or homes striving for an industrial appearance. Fascia gutters have K-style profiles but are unique in that the back side of the profile acts as both a fascia and a gutter simultaneously. They are typically custom-created for each house.

The basic rule for gutter sizing is when in doubt, bigger is better. Creating a gutter system too large for a home would be difficult. However, bigger also means more expensive. Here’s how to find the minimum size gutter you’ll need.

Calculating Gutter Size

Gutter sizes are determined by the size and pitch of your roof. Find the pitch using a bubble level. Place one corner of the level on the roof and hold it level. On the tool, make a mark 12 inches from where the roof and level touch. Measure from a spot on the roof directly below the mark to the mark. Your measurement number is the pitch of your roof stated as a numeral in 12, such as 4:12 or 6:12. You’ll match your roof’s pitch to a multiplier or pitch-factor number in the steps below.

1. Measure to determine the square footage of each section of your roof. If two sections meet at a valley, add the sections together.
2. Determine the pitch of your roof.
3. Use the chart below to determine your multiplier number for each roof section.
   

   ROOF PITCH	MULTIPLIER
   Up to 3:12	1
   4:12 to 5:12	1.05
   6:12 to 8:12	1.1
   9:12 to 11:12	1.2
   12:12 or steeper	1.3

   

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4. Multiply the square foot measurement by the pitch multiplier number. Repeat for each section of your roof. Your answer is called the adjusted roof area number.
5. Now, you’ll need to do a little research. Go to the NOAA website and search for your area’s five-minute maximum rainfall intensity amount, or check with your county for records.
6. For each roof section, multiply the adjusted roof area number by the five-minute maximum rain intensity number.
7. Look up your total in the chart below to determine your minimum gutter size.
   

   MAXIMUM ADJUSTED ROOF AREA (SQ. FT.)	GUTTER SHAPE	Gutter Size
   2,500	Half-round	5-inch
   3,840	Half-round	6-inch
   5,520	K-style	5-inch
   7,960	K-style	6-inch

   

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Just as crucial as the size of the gutters are the downspout sizes and quantity. The size is determined mainly by the size of your gutters. 5-inch K-style gutters typically use 2-by3-inch or 3×4-inch rectangular downspouts, while 6-inch ones use 4×5-inch spouts. Three, four or five-inch diameter round gutters accompany half-round gutters.

Calculating Downspout Quantity

To calculate the number of downspouts necessary for each roof section, refer to your square foot measurements and compare them to the following chart. Remember that these are maximum capacity numbers, and you can’t have too many downspouts. These are just guidelines. Steeper roofs drain water faster and benefit most from larger and more frequent downspouts. When in doubt, add another.

DOWNSPOUT SIZE	MAXIMUM SQ. FEET OF ROOF SECTION PER DOWNSPOUT
2-by-3-inch rectangular	400
3-by-4-inch rectangular	650
4-by-5-inch rectangular	1000
3-inch round	450
4-inch round	700
5-inch round	1,200

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As far as directing water from your roof is concerned, what the gutters and downspouts are made of matters little outside of appearance and durability. These are the most readily available materials.

• Vinyl gutters are inexpensive and resist corrosion.
• Aluminum is a lightweight gutter material that won’t rust.
• Steel gutters are used for custom installations, cost more and require maintenance to prevent rust.
• Copper gutters are costly, but won’t rust or corrode, and will develop a desirable patina over time.
• Zinc gutters are long-lasting, strong and will develop a patina.

Vinyl and aluminum gutters are available in sections at home centers and are excellent for DIY installation. Professional installation is necessary for seamless, steel and copper gutters.