What size gutters do I need?
Short answer. Most homes need 5-inch K-style gutters, but 6-inch gutters are the better choice when a large roof area drains to one run, the roof is steep, valleys empty into the gutter, or 5-inch gutters already overflow in heavy rain. Downspouts matter as much as the gutter: a 3 by 4 inch downspout has twice the opening area of a 2 by 3 inch one.
How sizing works
- Find the area draining to each run. Measure the roof plane or planes that shed water into that gutter, using their footprint dimensions.
- Adjust for pitch. Steeper roofs catch more wind-driven rain and deliver water faster, so sizing methods add a factor for pitch.
- Use local rainfall intensity. Gutters are sized for short, intense bursts, not daily averages. North Georgia thunderstorms can be intense, which favors larger gutters on big roof planes.
- Compare to capacity. Manufacturers and trade guides publish how much roof area each gutter and downspout size can serve at a given rainfall intensity.
Practical rules that work
- Typical one and two story homes with moderate pitch: 5-inch gutters, ideally with 3 by 4 inch downspouts.
- Steep roofs, big unbroken planes, or long runs: 6-inch gutters.
- Valleys dumping into a run: a downspout near the valley and a splash guard at the corner.
- Runs longer than about 30 to 40 feet: a downspout at each end or an extra one in the middle.
Signs your current size is wrong
Overflow in heavy rain with clean gutters and clear downspouts, water sheeting over inside corners, and erosion under one stretch every storm. Often the cheapest fix is bigger or additional downspouts before replacing the gutters. See gutter sizing, seamless gutters and guards and how roofers measure a roof.