The Size of Drainpipes to Use for Rain Gutter Downspouts

Pitch Factor

• When determining the size of the drainpipe for a downspout, you need to know the maximum potential amount of rainfall your downspouts must be able to handle. According to the This Old House website, the easiest way to do this is to determine the pitch factor and average rainfall intensity and multiply those two numbers by the surface area of your roof. Measure the pitch by placing an air bubble level horizontal to your roof, then measure the distance between the roof and the air bubble level 12 inches from the roof. If the distance is 5 inches, then your pitch is 5 in 12. Use the pitch to determine the pitch factor. The pitch factor for a pitch of 3 in 12 or lower is 1, 4 in 12 up to 6 in 12 is a pitch factor of 1.05, 6 in 12 to 8 in 12 is a pitch factor of 1.1, 9 in 12 to 11 in 12 is 1.2 and a pitch of 12 in 12 is a pitch factor of 1.3.

Figuring Gutter Size

• Measure the total surface area of your roof, then multiply that by the pitch factor. You then need to multiply that by the maximum potential storm intensity for your area, which can be found in a chart located in the Resources section of this article. If the total surface area of your roof is 2,000 square feet and you have a pitch factor of 1.2, then you have 2,400 square feet. If you live in Jacksonville, Florida, then your maximum rain intensity is 7.9, which gives you an adjusted roof area of 18,960 square feet. According to This Old House, half-round gutters are used for adjusted surface areas of 5,519 square feet and less, while K-style gutters are to be used for any surface area over 5,520 square feet.

Size of Downspouts

• According to the Gutter Supply website, your downspout drainpipe should be no less than 7 square inches at the opening. Downspouts can be as long as necessary to reach from the gutters to the ground, but the drainpipe should be aerated every 40 feet to prevent a vacuum from forming inside the downspout. You should use the same size downspout for the entire gutter system.

Downspout Frequency

• The wider the opening on your downspout, the more efficiently it will move water. The Gutter Supply website recommends installing one downspout every 50 linear feet of guttering. Each downspout should be fitted with an angled piece at the end and an extension of at least 4 feet in length, to move the water safely away from the home's foundation.