Extra Capacity
What if the runoff is off the chart for standard gutters? You have three options:

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.

Rectangular
2 by 3 inches = 600 square feet
3 by 4 inches = 1,200 square feet

Round
3 inches = 706 square feet
4 inches = 1,255 square feet

Information courtesy of the Sheet Metal and Air Conditioning Contractors National Association (SMACNA), a standards-setting organization based in Chantilly, Va.
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