Volodymyr Lukaniuk 
Gutters overflow in heavy rain when water reaches one part of the gutter faster than that part can carry it away. The useful clue is where the water comes over. A spill along the whole front lip, a jet off one inside corner and a sheet running down behind the gutter are three different problems with three different fixes, and the wrong fix costs money and leaves the spill exactly where it was.
Start with one question: does it spill in ordinary rain, or only in a cloudburst? Water coming over in a steady, moderate rain almost always means something is in the way or the gutter is holding water at a low spot. Water that only comes over in the hardest few minutes of a summer storm points at capacity, at a valley dumping into one corner, or at a gutter set too low to catch fast water.
How do you find the cause from the ground?
Watch one storm. It takes ten minutes and tells you more than any guess made on a dry afternoon. You do not need a ladder, and you should not use one in the rain.
- Wait for a real downpour, not a drizzle, and go out in daylight if you can. At dusk, a flashlight aimed along the gutter line shows the falling water clearly.
- Walk the house slowly at a distance where you can see the roof edge, and look at each run of gutter from the end, not from straight below.
- For every spill, note three things: which run it is on, whether it comes over the front or down the back, and whether it is near a corner, a downspout or the middle.
- Check the bottom of each downspout. A strong steady stream means that outlet is working. A weak trickle while the gutter above is full means a blockage somewhere in the pipe or at the outlet hole.
- Take a short phone video of each spot. A gutter contractor can tell a lot from fifteen seconds of footage, and you will not be waiting for the next storm to show them.
What each spill location usually means
Once you know where the water leaves, most cases fall into one of the rows below. The last column is there because it is where homeowners lose the most money.
| Where the water comes over | What it usually means | The fix that works | The fix that does not |
|---|---|---|---|
| Over the front lip along most of a run, only in hard rain | The trough or its downspouts are too small for the roof area above | Another downspout, a larger downspout, or a 6-inch gutter on that run | Another cleaning; guards |
| A jet off one inside corner | A roof valley is sending two slopes of water into one short stretch at speed | A splash guard at the corner, or an outlet at the corner | Sealing the corner miter |
| Down the back of the gutter, against the fascia | The gutter hangs too low under the roof edge, or there is a gap between the drip edge and the trough | Resetting the gutter higher; gutter apron or flashing at the edge | A bigger gutter at the same height |
| One spot in the middle of a run, even in light rain | A sag or reversed slope holding water, or a debris dam | Clear it, then reset the hangers to restore the slope | Adding a downspout at the other end |
| Next to the downspout, while the downspout barely runs | A clog at the outlet, in an elbow, or in a buried drain line | Clear the outlet and pipe; check the buried line | Upsizing the gutter |
Two rows can be true on the same house. A corner under a valley, on a long run that drains to a single downspout at the far end, spills for both reasons at once, and fixing only one of them leaves part of the spill.
Why the corner under a valley spills even on a new gutter
A valley collects water from two roof planes and delivers all of it to a few feet of gutter. On a steep roof that water arrives fast, so it can carry straight over the front edge before it ever settles into the trough. That is why a brand new, perfectly clean gutter can still throw a sheet of water off one inside corner. You can see the valley side of this on our page about open, closed-cut and woven roof valleys.
There are three ways to deal with it, and they fit different situations:
- Splash guard at the corner
- A strip of metal fixed to the front of the gutter where the valley water lands, standing higher than the lip so the water hits it and drops back in. It works when the trough has room for the water and the problem is speed. It does nothing if that stretch is already full.
- An outlet and downspout at the corner
- Takes the valley water out of the gutter right where it arrives. This is the better fix when the corner is full rather than just splashing, and it also takes load off the rest of the run.
- A larger gutter on that run
- Makes sense when the run is undersized along its whole length, not only at the corner. On its own it may still splash at the valley, so it is often paired with a splash guard.
Water down the back of the gutter is a height problem
When water runs behind the gutter instead of over the front, the gutter is usually set too low, or the roof edge does not reach far enough over it. In a light rain the water clings to the shingle edge and drips into the trough. In a downpour some of it follows the underside of the drip edge and runs down the face of the fascia board. More capacity does not change that, because the water never gets into the trough.
The fix is to reset the gutter so its back sits up under the roof edge and, where there is a gap, to add a gutter apron so water has a path into the trough. Our guide on gutter flashing and the gutter apron shows what that piece looks like. Leave it long enough and the back of the fascia rots where you cannot see it; the signs are on our soffit and fascia page.
Clean gutters that still overflow are usually too small or too few
If the gutters are clear, the slope is right and the water comes over the front along a whole run in hard rain, the system does not have the capacity for that roof. Fine Homebuilding notes that residential gutters are often planned to handle "the most intense five-minute burst of rain", which in our area is a summer thunderstorm, not an average rainy day. A gutter sized for less spills in exactly those five minutes.
On the homes we work on, 5-inch K-style gutters with 2x3-inch downspouts handle most roofs. Large or steep roofs, and roofs cut up by valleys and dormers, get 6-inch gutters with 3x4-inch downspouts. Our article on 5-inch vs 6-inch gutters walks through which one a house needs, and gutter pitch and downspout placement covers where the outlets go. The cheapest change is often an extra outlet. The same Fine Homebuilding piece says "going from one downspout to two doubles the watershed that a section of gutter can drain."
When a run needs more than an outlet, we replace it with seamless gutters sized for the roof above it. You can see how we approach that on our seamless gutter installation page.
Do the watching from the ground or a window. A wet ladder against a gutter full of moving water is how people get hurt, and nothing you can see from the top of it in a storm is worth that. The phone video from the lawn is enough for a diagnosis.
If it only spills in light rain, start with what is inside the gutter
Overflow in an ordinary rain is the easy case. Leaves, maple seeds and shingle granules collect at the outlet and in low spots, and the water backs up behind them. Our gutter cleaning service clears the troughs and the downspouts and checks the water runs out the bottom. If one stretch fills up again between cleanings, the slope is usually the reason: a hanger has let go and the gutter now dips. Our page on sagging gutters and failed hangers covers that repair.
Gutter guards help with the debris part and nothing more. They will not fix a gutter that is too small, too low or too flat, so it pays to know which problem you have before buying them. We cover the trade-offs in our honest take on gutter guards.
Questions about gutters that overflow in a downpour
Is it normal for gutters to overflow a little in a cloudburst?
A few seconds of spill at the peak of a very hard storm can happen on a system that is otherwise fine. Water coming over in steady or moderate rain, or the same spot spilling every storm, is not normal and has a cause you can find.
Why do my gutters overflow right after I had them cleaned?
Cleaning fixes debris and nothing else. If the gutter is too small for the roof above it, sits too low under the roof edge, holds water at a sag, or catches a valley at the corner, it will spill on the first heavy rain after the cleaning.
Will gutter guards stop my gutters from overflowing?
Only when debris was the cause. A guard does not add capacity, change the slope or move the gutter up under the roof edge. In a downpour, a guard with leaves matted on top can make water sheet over the front.
Does adding one more downspout really help?
Often it is the cheapest real fix. Fine Homebuilding puts it plainly: going from one downspout to two doubles the watershed a section of gutter can drain. The new outlet goes where the water collects, which is usually the corner under a valley or the far end of a long run.
Can water spilling behind the gutter damage the house?
Yes. That water runs down the back of the fascia board and the wall, where nobody sees it. Paint peeling on the fascia, stains on the soffit and wet spots at the foundation under one stretch of gutter are the usual signs.
Seeing water come over the gutters in the last storm? Book a free inspection or call (866) 992-2982, Monday to Friday, 8:00 AM to 5:00 PM. Send us the phone video and we can usually tell you what we are looking at before we arrive. We work across the north and northwest Chicago suburbs.