An inch of rainfall doesn’t sound like much. But when it falls on an average-size roof, it adds up to a 1,900-gallon torrent sluicing off the eaves. That’s an awful lot of water that can cause an awful lot of damage if your gutters aren’t up to the task of controlling it. Yet we barely give gutters a second thought until they’re clogged and overflowing, or ripped from their moorings by ice and snow.
Maybe a simple cleaning is all your gutters need, or maybe they need to be replaced altogether.
If you’re starting fresh, there is a veritable deluge of shapes, sizes, and materials to choose from. Aside from pricey, maintenance-heavy wood troughs and short-lived vinyl ones, the best option for most of us is metal—elegant copper, understated zinc, rugged steel, or affordable aluminum. Metal gutters are durable and need relatively little care.
How to Size Gutters and Downspouts
To figure out what size gutters you need, first you’ll need to calculate the square footage of the gutter’s drainage area. For a simple gable-end roof, you would only need to make two calculations, one for each slope. Hip roofs and intersecting roofs have multiple facets, and for those you’ll need to add up the area (length x width) of each surface within a drainage area to get the total square footage.
Adjusting for Pitch and Rainfall
The steeper a roof’s pitch, the more windblown rain it can collect. You can measure pitch with a 2-foot level and a tape measure: Hold one end of the level against the roof, level it, and then measure the distance between the roof and the underside of the level at its midpoint, which gives you a 12-inch run. A 5-inch gap, for instance, is a 5-in-12 pitch. Once you know pitch, you can find your roof-pitch factor in the table below.
12 in 12 or higher 1.3
9 in 12 to 11 in 12 1.2
6 in 12 to 8 in 12 1.1
4 in 12 to 5 in 12 1.05
Flat to 3 in 12 1
Sizing the Gutters
5-inch 5,520 square feet
6-inch 7,960 square feet
5-inch 2,500 square feet
6-inch 3,840 square feet
1. Get 7- or 8-inch gutters. They’ll cost more and probably require a custom order through a professional installer.
2. Increase the pitch of the gutter. The standard is about ¼ inch per 10 feet. Increasing the pitch increases a gutter’s handling capacity, but the gutter may look askew over a long run.
3. Add downspouts. The above recommendations assume that you have properly sized downspouts every 40 feet. As with gutters, a downspout’s capacity must match or exceed the expected runoff. Use the chart below to figure out how many extra downspouts you need. Adding a 2 by 3 rectangular downspout, for instance, boosts your gutter’s capacity by 600 square feet of drainage area.
2 by 3 inches = 600 square feet
3 by 4 inches = 1,200 square feet
3 inches = 706 square feet
4 inches = 1,255 square feet