Foundation

Downspouts and Your Basement: The Cheapest Fix in St. Louis

By St. Louis Gutter Service · September 11, 2026 · 5 min read

If your basement gets damp after a big rain, go outside during the next one and watch where the downspouts end. In most St. Louis homes, at least one of them dumps two feet from the wall, and that's the leak. Waterproofing contractors here charge five figures for interior drain tile. A downspout extension is forty dollars.

What's under a St. Louis house

The metro sits on clay — a thick layer of it over limestone, with a cap of wind-blown loess on the hills. Clay drains slowly and holds water. Pour a few hundred gallons off a roof next to a basement wall and the soil there stays saturated for days, pushing against the wall and finding every crack in the block or the cold joint at the footing. That's hydrostatic pressure, and it's why the wet spot always shows up at the base of the wall on the side with the bad downspout.

A ten-by-ten patch of roof sheds about 60 gallons in an inch of rain. A two-story home in Webster Groves with 1,200 square feet of roof on the back sends 750 gallons through one downspout in a one-inch storm. If that downspout ends at the foundation, all of it soaks in within three feet of the wall.

How far is far enough

Six feet is the number we use in St. Louis, and it's more than the four feet often quoted for sandier ground. Clay doesn't let water spread sideways fast, so a short extension just moves the puddle. Rules that hold here:

• Six feet minimum from the wall on flat ground, ten on a lot that slopes toward the house.
• Downhill, always. On a lot that slopes toward the house, an extension isn't enough; you need a buried line to daylight or a pop-up emitter at the low side.
• Off the driveway. A downspout onto a driveway is a sheet of ice in January and an algae strip in July.
• Not into the window well. Obvious, and we see it every month.

The options, cheapest first

Dry wells are the one to be careful with. In heavy clay a dry well fills and stays full, and you've built a cistern next to the foundation. On the loess ridges in University City and Clayton they can work; in the bottoms they usually don't.

The sewer question

This is specific to St. Louis. In the city, many older homes have downspouts that go into a cast-iron pipe at grade and drain to the combined sewer, which carries both stormwater and sewage. That's how the system was built and in combined-sewer areas it's generally how it stays, though the Metropolitan St. Louis Sewer District has spent years reducing what goes into those lines during storms.

In St. Louis County, most of the sewers are separate, and MSD does not allow downspouts, sump pumps or footing drains to be tied into the sanitary sewer. It causes backups into basements down the line during heavy rain. If your county home has a downspout disappearing into a pipe at the base and you don't know where it goes, find out before you rely on it; a broken or illegal connection can be both the wet basement and a notice from MSD.

When the downspout isn't the whole story

Fix the downspouts first, then wait for two good rains. If the basement still gets wet, look at grading — soil that has settled back toward the house over thirty years, mulch beds built up above the sill — and at window wells without covers. Only after those do interior drain tile and sump pumps make sense, and by then you'll know exactly which wall is the problem.

Gutters that make it worse

An overflowing gutter is a downspout problem you can't extend. Water going over the edge at a corner lands at the wall no matter what's at the bottom of the downspout. If you see overflow, that has to be sorted at the gutter — why gutters overflow in St. Louis rain explains what to look for — and every gutter repair or installation we do ends with a look at where the water lands, because a new gutter feeding the basement isn't an improvement.