Most overflow comes from something in the water’s way: leaves in the trough, a clog in the downspout, or a low spot where the gutter has lost its slope. Less often, the gutter is too small for the roof above it. Where the water spills over tells you which one you have, so watch the next rain before you pay for anything.
Read the overflow first
Where the water spills tells you most of the answer, so look during a steady rain rather than after.
| Where the water spills | Most likely cause | Fix |
|---|---|---|
| Along the whole gutter | Debris, a clogged downspout or a gutter too small for the roof | Clean it, then test the downspouts |
| Right beside a downspout, gutter clean | A clog below the outlet, usually at an elbow | Clear the downspout |
| An inside corner, under two roof slopes | A roof valley | An extra outlet at the corner, or a valley splash guard |
| The far end of a run, away from the outlet | The slope has settled the wrong way | Rehang to fall toward the outlet |
| The middle of a run, water standing after rain | A sag or low spot | Rehang at that point |
| Over the front edge, gutter not full | Overshoot from a steep roof or a gutter hung too low | Reposition the gutter |
| Down the wall behind the gutter | A gap at the roof edge or a gutter pulling away | Drip edge, apron or rehang |
1. Debris in the gutter
Leaves, maple seeds, cottonwood fluff and shingle grit collect at the outlets first. Once water can’t get out, it rises and spills over the front. This is the cheapest fix and the first thing to check. Most houses here need two cleanings a year, and timing them to the trees matters more than the price.
2. A clogged downspout
A gutter can look spotless and still overflow beside the downspout, because the clog sits lower down. Leaves and seeds wash into the outlet and jam at the first elbow, at the bottom bend, or where a buried line takes over.
The test takes two minutes. Run a garden hose into the gutter just upstream of the outlet. If the gutter fills and only a trickle comes out the bottom, the downspout is blocked. Tap along it to find the dead-sounding section, pull the elbow if you can reach it, or flush from the bottom up. A clog at the start of a buried line is a job for someone with a drain snake.
3. A sag or a lost slope
A gutter should fall toward each outlet by at least 1/16 inch per foot, about 5/8 inch over 10 feet. Loose hangers, soft fascia and the weight of snow and ice can pull a section down until it holds water instead of moving it, or leave a whole run level or sloping away from its outlet so the water pools at the far end.
The sign is water still sitting in one stretch a day after the rain stops, and there is a clear line between how much standing water is normal and how much is not. Rehanging and resetting the slope fixes most sags, once you know whether the hanger or the board has failed. If the fascia behind it is rotten, that board has to be replaced first, or new fasteners won’t hold. It is still a repair job, not a replacement.
4. A roof valley pouring into one spot
Where two roof slopes meet, the valley gathers water from both and drops it into a few feet of gutter. The result is a waterfall at an inside corner while the rest of the run drains fine.
The numbers explain it. Gutters here are sized for 7.4 inches of rain an hour, the peak five minutes of a ten-year storm. A valley draining 400 square feet of roof at that rate delivers about 30 gallons a minute to a few feet of gutter. The gutter is not too small for the house. It is too small for that corner.
Watch what the water does, because a valley fails in two ways. If it fills the gutter at the corner and spills over, the problem is volume: add an outlet and downspout right at the corner, or upsize the one already there. If it shoots across the gutter and over the front without filling it, the problem is speed: a valley splash guard, sometimes sold as a gutter valley shield, is a short upright of metal on the outside of the gutter that catches the water and drops it in. Bigger gutters across the whole house cure neither.
5. Water shooting past the gutter
From the ground, overshoot looks like overflow. Check whether the gutter is actually full. If it isn’t and water still lands in the flower bed, the rain is jumping the front lip.
Steep roofs, valleys and gutters hung too far below the roof edge all cause it. Cleaning won’t change anything. The gutter has to be moved to where the water actually comes off the roof.
6. Too few or too small downspouts
One outlet can’t drain a long run under a big roof in a summer storm. Plan on one downspout for every 30 to 40 feet of gutter.
Size matters as much as count. A 2×3 downspout handles about 600 square feet of roof and a 3×4 about 1,200. Adding an outlet costs far less than replacing a gutter. Where that water goes once it reaches the ground matters just as much.
7. Gutters too small for the roof
Rule out the first six causes before you blame size. If the gutter overflows along its whole length only in hard rain, with the outlets clear and the slope sound, it may be undersized.
Sioux Falls storms test gutters. The airport recorded 5.67 inches in 24 hours on August 6 and 7, 2022, the most on record there. A 6-inch gutter drains almost half again as much roof as a 5-inch. It still won’t fix a clog, a sag or a bad slope. Sizing for Sioux Falls rain is simple arithmetic, and the choice between 5-inch and 6-inch usually comes down to one roof plane.
8. Guards that shed the water
Some guards let heavy rain slide across the top and off the front. Hood-style guards are the worst for it in a downpour. Fine mesh can mat over with cottonwood fluff and grit, and the gutter under any guard can still clog.
If the overflow started after guards went on, watch whether the water goes into the gutter or runs over it. Some guard types handle a downpour far better than others.
Water running behind the gutter
This is a separate problem. Water that drips between the gutter and the house usually means the gutter has pulled away, or the roof edge has no drip edge to carry water into the trough. It has its own causes and fixes, and none of them is cleaning. Left alone, it rots the fascia, so have it looked at before winter.
Can overflowing gutters flood a basement?
They can add to it. Water that pours over a gutter lands right against the foundation. Basement water can also come from grading, cracks, window wells or a sump problem, so fixing the gutter may not stop it on its own. Downspouts should put water at least 5 feet from the house, on ground that falls away from it. If the basement is already wet, match the wet corner to the downspout above it.
What to check from the ground
On the next rainy day, look for these without climbing anything:
- Does it spill everywhere or in one spot?
- Is the gutter full, or is water jumping past it?
- Is water coming out of the bottom of each downspout?
- Does it happen in every rain or only in heavy storms?
- Is water still sitting in any section after the rain stops?
- Is there a roof valley above the problem spot?
Questions about overflowing gutters
Why do my gutters overflow when they’re clean?
The clog is probably in the downspout, or the gutter has lost its slope. A roof valley or too few outlets can do it too.
Why do they overflow only in heavy rain?
The system is reaching its limit. A partial clog, too few downspouts or a large roof feeding one run shows up only when the rain is hard.
Will bigger gutters stop the overflow?
Only if the gutter is too small. A larger gutter won’t clear a clogged downspout, fix a sag or catch water shooting past it.
Should water stand in my gutter after it rains?
A little around a seam is normal. A pool that sits in one section for a day points to a sag or a lost slope.
Get a quote
If the overflow keeps coming back and you’re not sure what the gutters need, tell us about it with a photo of the spot and a note on when it happens, and a local gutter contractor will call, text or email to set up a visit. The quote is free. Request a free quote