Gutter Design: Drawing Layouts & Calculating Dimensions

Determining the proper gutter design for your home and the required size and number of the downspouts is best left to a professional gutter installer.

A professional gutter installer should be familiar with the industry-standard SMACNA Architectural Sheet Metal Manual 6th edition, which details the expected rainfall for each area of the country.

With the information in the SMACNA, we can make calculations based on the square footage of your home, the pitch of your roof, and the average rainfall in your area.

“Rules of thumb” for gutter size and the number of downspouts needed are often based on the length of the roof edge that is being drained, this gutter design approach is flawed because it fails to consider the roof pitch and the total roof area to be drained.

For almost all residential homes, five-inch gutters work fine. Gutters aren’t made to hold water anyway, their purpose is to direct the water to the downspouts.

The number of downspouts is much more important than the size of the gutter. Steeply pitched roofs usually require extra downspouts

Six-inch gutters are for most commercial buildings and five-inch are for most residential homes. About the only time, I would probably install a six-inch gutter on a residential home is if the roof shingles stick out so far that a five-inch gutter won’t catch the water.

For the reasons listed above, I will demonstrate on this page how to calculate the minimum number of downspouts required for a residential home using a five-inch gutter.

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Calculating downspout size

OK, let’s begin by determining the area of your roof in the horizontal plane.

Since the roof area increases as pitch increases, steep roofs will collect more water than flat roofs.

So the next step is to figure a roof’s gutter design area by multiplying the roof area by the appropriate factor from the pitch table (below).

The roof pitch is the distance a roof rises vertically over a 12-inch horizontal distance. So a roof that rises 8 inches vertically for every 12 inches horizontally has an 8/12 pitch.

Downspouts can handle about 100 square feet of watershed area per square inch. Round the watershed area up to the next even 100 then divide by 100 to determine the square inches of downspouts required.

For example, a 1,550 square foot watershed area needs at least 16 square inches of the downspout.

A 2 inch by 3-inch downspout has a 6 square inch cross-section, while a 3 inch by 4-inch downspout has a 12 square inch cross-section.

In this example, you need three 2 by 3-inch downspouts (a total of 18 square inches cross-section) or two 3 by 4-inch downspouts (24 square inches total cross-section) to equal the minimum 16 square inches required.

Gutter splash guards should always be installed on all inside corners, or anywhere two roof lines meet to form a valley. This will prevent the rainwater from overshooting your gutter.

The valley acts to concentrate a higher volume of water (from both roof sections) in one channel leading down to the intersection of two gutters.

I often see early roof shingle wear, staining, or even roof leaks on lower roof slopes in areas where a downspout from an upper or secondary roof slope drains onto the lower roof slope.

You never want a downspout to drain water directly onto a lower level roof, because this will cause water and ice damage to the shingles and roof deck. It must always drain into a lower roof’s gutter.

Gutter And Downspout Colors

Another important aspect of the gutter design is the color of your gutters and downspouts. Your gutters should blend well with the colors of your home.

Most people tend to match their gutter colors to the fascia trim. Just remember that lighter colors will show dirt and black stripes known as tiger stripes more so than darker colors.

Painting your aluminum gutters and downspouts is another option. Before painting, the gutters must be clean. Make sure to use an oil-based metal primer and a compatible oil-based paint.

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