Gutter size vs. capacity — how much roof a 5-inch vs 6-inch gutter drains
At 1 in/hr and ⅛″/ft, a 5″ gutter drains 3,520 sq ft of roof and a 6″ drains 5,440 (≈1.5×). Divide by your local rainfall intensity — big roofs need 6″–7″ or more downspouts.
What this diagram shows
Two K-style gutter cross-sections compared. At 1 inch per hour of rainfall and ⅛-inch-per-foot slope, a 5-inch gutter drains 3,520 square feet of roof and a 6-inch gutter drains 5,440 square feet — about 1.5 times as much, per 2021 UPC Table 1103.3 for semicircular gutters. No code lists K-style gutters, so a K-style is credited as a half-round of the same width. Divide by the local rainfall intensity; big roofs step up to 6 or 7 inch gutters or add more downspouts.
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