St. Louis homeowner guide

What size gutter does a St. Louis roof need?

By Gutter Installation St. Louis, a referral service. Updated November 3, 2026.

Short answer

Many smaller St. Louis roofs manage with 5-inch K-style gutters. Large or steep roof sections, long runs and valleys that dump into one corner are better served by 6-inch gutters with bigger downspouts. Lambert averages 11 days a year with an inch or more of precipitation, so ask a contractor to size from your roof areas and pitch, not by habit.

Gutter roll-forming machine on a work van in a brick alley with a fresh length of white gutter

Worth remembering

One inch of rain on 1,000 square feet of roof is about 623 gallons.
Roof area and pitch decide size, and valleys concentrate flow.
Downspouts are often the real bottleneck.
April through June are the wettest months.

The rain to plan for

1991-2020 normals, St. Louis Lambert
Measure	Normal
Precipitation	41.70 inches a year
Wettest months	May 4.82, April 4.73, June 4.49 inches
Days with 0.5 inch or more	28.4 a year
Days with 1 inch or more	11.0 a year

Those are daily totals. Gutters overflow during the worst few minutes of a storm, so a contractor or designer should think about short, hard bursts as well. If your project is big enough to need formal calculations, ask a design professional to use current rainfall intensity data. (NOAA normals)

What decides the size

Roof area draining to each run, measured in plan, plus an allowance for pitch.
Pitch. A steep roof sheds water faster and can overshoot a small gutter.
Valleys. Two roof planes meeting can send most of a roof to one corner.
Run length and outlets. A long run with one outlet needs more capacity and has to fall farther.
Upper roofs draining to lower roofs. That water arrives at the lower gutter all at once.
Debris. Wider troughs and larger outlets clog less quickly under heavy trees.

Downspouts

A gutter empties only as fast as its outlet allows. Common sizes are 2x3 and 3x4 inches, with larger or additional outlets available. Adding a downspout can solve an overflow for less than a bigger gutter, if there's somewhere sensible for it to go. On tight city lots, that's the catch.

How the house types differ

Soft-brick townhouse: tall wall, small plan area, sometimes a low-slope rear roof. The looks and fixings may matter more than capacity.
Bungalow or four-family: moderate roof, short eaves, little overhang, so an overflow lands right on the wall.
Postwar ranch: long runs. Check fall and outlet count.
Newer two-story: big planes and valleys. This is where 6-inch often earns its keep.

Telling if yours is too small

Go out in a hard rain with a raincoat. If water overshoots at a valley corner, or overflows along the run while the downspout runs full, you have a capacity problem. If it overflows only near a clog or a sag, you don't. Fix that first.

Questions

Is bigger always better?

Not always. On small roofs, an oversized gutter can look out of place and costs more. Inside a historic district, profile and size can be review questions. Size to the roof.

Can I replace only the corner that overflows?

Sometimes. A valley splash guard, an extra outlet or a larger downspout may solve a corner problem without replacing the whole run. Ask for that option first.

Does a half-round carry as much?

Generally less than a K-style of the same width. If you want half-round on a big roof, ask the contractor about going up in size or adding outlets.

Do I need a calculation for a house?

Not usually a formal one, but a contractor should be able to explain the choice in terms of roof area and pitch. For commercial or large multi-family buildings, ask for it to be calculated.

Sources

NOAA NCEI: 1991-2020 normals, St. Louis Lambert

General information for St. Louis-area homeowners. Not a quote, and not engineering, legal or insurance advice.