Why Your Gutters Overflow in St. Louis Rain (Even When Clean)
The gutters were cleaned in June. Then a line of storms comes across from St. Charles County on a Thursday evening, and there's a curtain of water coming off the back of the house onto the patio like the gutter isn't there. The call we get the next morning always starts the same way: "they can't be clogged, we just had them done." Usually they aren't. They're overwhelmed.
Forty-one inches, delivered badly
St. Louis averages about 41 inches of rain a year, which is ordinary. The problem is that a big share of it arrives in spring and summer thunderstorms that drop an inch or two in an hour. The National Weather Service office in Weldon Spring logs several of those across the metro every May and June. A gutter sized by the old rule of thumb — 5-inch K-style, one 2x3 downspout every 30 or 40 feet — handles a steady half-inch-an-hour rain fine and falls behind at two.
The arithmetic: a 2x3 downspout drains roughly 600 square feet of roof in moderate rain, closer to 400 in a downpour. A two-story colonial in Ballwin or Chesterfield routinely puts 1,000 square feet or more of roof onto the back gutter with one downspout at the end. The water arrives faster than the outlet can take it, the run fills, and it goes over the front lip at the far corner.
The three causes and how to tell them apart from the ground
1. Downspout capacity. The overflow is worst at the end of the run farthest from the downspout, or the whole run sheets over evenly while the downspout at the bottom is gushing. Fix: upsize to 3x4 downspouts, which roughly doubles the outlet, and add a second downspout on any run longer than 35 feet. That's $150–$300 per downspout, not a new gutter.
2. Lost pitch. Gutters need to fall about a quarter inch every ten feet toward the outlet. Spike hangers back out of the fascia a little more each winter as the wood swells and shrinks, the run drops in the middle, and water sits there instead of moving. Look down the run from one end on a dry day, or look for a tide line of dirt and standing water a day after rain. Mosquitoes in July are the other tell — they need a week of standing water, and a properly pitched gutter is dry in a day. Fix: re-hang on hidden screw hangers and reset the pitch, $150–$300 per run.
3. Undersized gutter. Steep roof, big roof area, or a valley funneling two roof planes into one spot. The overflow is right under the valley or where two roof sections meet, and it happens in the first minutes of a storm. Fix: a 6-inch gutter on that run, about 40% more capacity, $12–$16 a foot on a partial replacement.
Sorting it from the driveway
| What you see | Likely cause | Typical fix |
|---|---|---|
| Overflow at the far end from the downspout | Downspout capacity | Add a downspout or go to 3x4 |
| Overflow mid-run, water standing on dry days | Pitch | Re-hang and reset the fall |
| Overflow right below a roof valley | Gutter too small for the surge | 6-inch on that run |
| Water down the fascia, gutter half empty | No drip edge or short shingles | Roof flashing, not a gutter fix |
| Downspout barely trickling in a downpour | It's a clog after all | Flush it |
The one that isn't the gutter's fault
A roof with no drip edge, or with shingles cut short of the gutter, lets water run behind the gutter and down the fascia. From the yard it looks like overflow. Up close the fascia is streaked and the gutter is dry. We see this on 1960s ranches in Florissant and Affton that have been re-roofed twice after hail and never had the flashing added either time. It's a roofing detail, and it's covered in gutter problems that start at the roof.
Why this matters more than a wet patio
Almost every house in St. Louis has a basement, and it sits in clay that holds water. A gutter that overflows at the back corner drops a sheet of water along the foundation wall, and two days later there's a damp line on the basement floor. Basement waterproofing companies here charge $5,000–$15,000 for interior drain tile; fixing the gutter capacity and getting the downspout six feet from the wall is a few hundred dollars. We cover the soil side in downspouts and your St. Louis foundation.
What to do
If you're seeing overflow, ask for a gutter repair inspection rather than another cleaning. It takes ten minutes from a ladder to tell which of the three it is, and the fix for the first two is a morning's work. If the whole system is 1980s sectional aluminum and the pitch has gone on every run, that's a different conversation — repair vs. replacement has the rule we use.
Need gutter work in St. Louis?
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