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 question the next morning is always the same: "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 | Clogged rain gutters or downspout 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. It's common on 1960s ranches in Florissant and Affton that have been re-roofed twice after hail without the flashing being 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.
Watching it during the next storm
The best evidence comes from standing outside, under an umbrella, in the first twenty minutes of a real storm. Most people only see the aftermath. Pick a safe spot away from the eaves, and if there is lightning, watch from a window instead. Then look for four things.
- Where it starts. Water that spills at one point first is telling you where the system is weakest. Note whether that spot is near a valley, at the far end of a run, or in the middle.
- What the downspout is doing. A downspout running full and fast while the gutter overflows means the outlet is too small. A downspout barely dribbling means something inside is blocked.
- How long it lasts. Gutters overflowing in rain only during the heaviest burst usually have a capacity problem. Overflow that keeps going in a gentle rain points to a clog or lost pitch.
- Where it lands. Mark the spot on the ground. If it's within a few feet of the foundation, that's the place the basement is most at risk.
A quick phone video of each side of the house is worth more than any description. It also helps a contractor give you a sensible price before the visit.
Common mistakes that make overflow worse
- Bending the front lip down. Some homeowners push the front of the gutter lower so water spills "away" from the house. It just moves the problem and weakens the hangers.
- Adding guards to fix overflow. A guard keeps leaves out. It doesn't add capacity, and on a run that is already too small it can make water shoot over the edge faster.
- Caulking the corners. If the water is going over the top, sealant at the corners won't touch it.
- Cleaning again and again. If the gutter is clean and still spilling, another cleaning is money spent on the wrong cause.
Questions to ask whoever looks at it
- Which of the three causes do you see, and where?
- How much roof drains to each downspout on this side?
- Would a second downspout or a larger one fix it, or does this run need a bigger gutter?
- Has the pitch dropped anywhere, and why — spikes, rotted fascia or something else?
- Where will the extra water go once it's out of the downspout?
Clear answers to those let you compare two quotes properly. A good answer names a cause and a fix, not just a price.
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 a simple rule for deciding.
Common questions
Is it normal for gutters to overflow in a very heavy storm?
In the worst cloudbursts, a little spill at a valley or a corner can happen even on a well-sized system. Regular overflow in ordinary summer storms is not normal. That usually means a downspout is too small, the pitch has dropped, or the gutter is too small for the roof.
Can I add a downspout to an existing gutter?
Usually yes. A contractor cuts a new outlet into the bottom of the run, fits a downspout and routes it away from the house. It is often one of the cheapest ways to stop overflow on a long run, and it doesn't require replacing the gutter.
Need gutter work in St. Louis?
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