Sized for a few minutes, not a year
What overwhelms a gutter is not a wet year but a hard five minutes. The sheet-metal industry's manual, from SMACNA, sizes gutters and downspouts from the five-minute rain expected once in 10 or 100 years and sends designers to NOAA for the local value. Across the country that value differs several times over: under 4 inches an hour in much of the Pacific Southwest, around 8 in parts of the Southeast (NOAA Atlas 14). Your town's page gives its figure, and the calculator turns it into flow and downspouts.
Seamless or sectional
Seamless aluminum gutters are rolled on site from a coil to the exact length of each wall, so the only joints are at corners and ends. Sectional gutters are joined every few feet. Fewer joints means fewer places for sealant to fail.
5-inch or 6-inch, and the downspouts
A 6-inch gutter carries more water than a 5-inch, but the downspouts are often the limit. The plumbing code converts rain to flow (gallons a minute = inches an hour × square feet × 0.0104); the Copper Development Association's handbook says one square inch of downspout drains 1,200 square feet of roof at an inch an hour. Adding a downspout, or going from 2-by-3 to 3-by-4 inches, doubles what a corner can shed.
What the written quote should list
- Material and thickness; size and profile
- Linear feet, corners, end caps, outlets
- Number and size of downspouts, and where they discharge
- Hanger type and spacing
- Fascia repair, priced per foot or as an allowance
- Guards, if included
- Removal and hauling of the old gutters
- Warranty on materials and on the work
Before signing, check what your state requires of the contractor: hiring a gutter contractor.