Clog in the trough
The obvious one. Debris packs at the low end near the downspout and water backs up behind it. You'll see overflow concentrated in one area rather than along the whole run.
Cleaning fixes it. If it's back within a couple of months, that's a guard conversation.
Blockage at the downspout elbow
Extremely common and easy to miss. The trough looks clean from a ladder, but the elbow at the top of the downspout is packed with a wad of shingle grit and leaf sludge.
Test it by running a hose into the drop. If water backs up in the trough instead of running out the bottom, that's your answer.
Not enough capacity
A 5-inch gutter on a steep roof with one downspout for 45 feet of run will overflow no matter how clean it is. The water arrives faster than the system can move it out.
The fix is usually another downspout, sometimes 6-inch gutter with 3x4 drops. We measure the roof plane before recommending either.
Bad pitch
Water standing in the trough after the rain stops means the fall is wrong. Standing water reduces the working capacity of the gutter before the storm even starts.
Rehanging to a proper quarter-inch-per-ten-feet fall is the correction.
The one that isn't a gutter problem
Water sheeting off the roof edge and clearing the gutter entirely on a very steep pitch. Nothing is wrong with the gutter — the water never got into it.
A gutter apron or drip edge extension, or a wider trough, is what solves that.
Questions people ask
Why do my gutters overflow when they're clean?
Almost always capacity or pitch. Either the roof plane feeds more water than the system can move, or standing water is eating the trough's usable volume.
Can a clogged downspout cause overflow?
Yes, and it's the most commonly missed cause. A clean trough over a packed elbow still overflows.
Will bigger gutters stop the overflow?
Sometimes. Adding a downspout is often the cheaper fix and works as well or better.
Is overflow actually damaging anything?
Yes. Water going over the back lip soaks the fascia, and water dumping at the foundation is how basements get wet.
Free estimate
We measure, we look at the fascia, and you get a number in writing.
Talk to a real St. Louis gutter company
Third generation, family-run since 1947. Somebody picks up, day or night.