Gutters are sized for the roof area draining into them and the rainfall intensity where you work. Placing the outlets well matters as much as the gutter size: a big gutter with one outlet at the far end can overflow where a smaller gutter with two well-placed outlets copes easily.
Common sizes
Residential gutters are usually 5" K-style, with 6" K-style for large or steep roofs and heavy-rain areas. Half-round gutters suit older houses. Downspouts are typically 2×3" on 5" gutters and 3×4" on 6" gutters. Match the downpipe size to the gutter outlet; an undersized pipe throttles the whole system.
Roof area and rainfall
The effective roof area draining to a gutter includes an allowance for pitch, because wind-driven rain hits a steep roof's face. Sizing guides from gutter makers and SMACNA tables use the drained area and your local rainfall intensity, in inches per hour. For ordinary houses, the standard sizes above work when outlets are well placed. Large or steep roofs, and valleys that dump water onto one spot, are where a bigger profile or extra outlets pay off.
Outlets
Put an outlet roughly every 30–40 ft of gutter, and close to valleys, which deliver a lot of water to one spot. A central outlet with the gutter falling both ways doubles the capacity of a run, since each half only carries half the roof. Corners near outlets reduce flow, so keep outlets a little away from angles where you can.
Fall
About ¼" per 10 ft is typical: enough to drain, without making the gutter look crooked against the fascia. Set a string line at the right fall before attaching brackets. On long runs, falling both ways to two outlets keeps the gutter from dropping too far below the roof edge.
The full fitting list
- Gutter lengths and connectors or seamless runs
- Hangers about every 24", closer in snow areas
- End caps, internal and external corners, and outlets
- Downspouts, elbows and straps
- Splash blocks or extensions at the bottom
- Leaf guards or outlet cages, if wanted
- Sealant for joints on systems that need it
Measuring
Measure along every eave that needs a gutter, corner to corner. Count separate runs (each with two ends), corners, and the height from gutter to ground at each downpipe. Note where water can discharge: into a storm drain, a dry well, a rain barrel or onto a splash block. That decides the bottom fittings.
A worked example
A house with 120 ft of eaves in two 60 ft runs, 10 ft to the ground: each run needs two outlets at 35 ft maximum, so four downspouts in total. That's 13 sections of 10 ft gutter (with 5% extra), 62 hangers at 24", four end caps, four outlets, four 10 ft downspouts and 12 elbows.
Replacing old gutters
Check the fascia before you quote. Rotten fascia boards have to be replaced before new brackets can go on, so price an allowance or exclude it in writing. Look at the drains too. New gutters won't fix water problems caused by blocked or broken underground drains.
The gutter calculator counts every section, hanger, outlet and fitting for your runs.
Common questions
Can I add a downpipe to an existing gutter?
Yes. Cut in a new outlet where the gutter overflows, and fit a new downspout with somewhere for the water to go.
Why does my gutter overflow at the corner?
Usually a valley is dumping water there, the outlet is too far away, or the gutter is blocked. An extra outlet near the corner, or a deeper profile, normally fixes it.
Materials compared
- Aluminum: light, rust-free, and available seamless, formed on site to any length. Dents more easily.
- PVC / uPVC: cheap, easy to fit, and the most common choice on houses. It can become brittle with age and expands in heat, so leave expansion gaps at joints.
- Steel: strong and long-lasting when coated. Heavier.
- Copper and cast iron: premium options for period and heritage properties, with very long lives and prices to match.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.