Why is my gutter overflowing?
Water coming over the edge has six common causes, and most of them can be identified from the ground while it is actually raining.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 3 min
- Safety
- warning
Short answer
Watch it in heavy rain and note where the water comes over. Overflowing at one low point is a blockage or a sag. Overflowing along the whole run is undersized guttering or a downpipe that cannot take the flow. Water over the back edge is a roof-edge problem, not a gutter one.
The instinct is to get a ladder out immediately. The better move is to wait for rain and watch, because the pattern of where the water escapes narrows six possible causes down to one or two, and most of that observation happens with both feet on the ground.
Safety
Step by step
- Watch it in heavy rain and note exactly where it comes over.One localised waterfall, an even curtain along the whole length, or water appearing behind the gutter against the wall. These are three different faults and the rest of the diagnosis follows from which one you see.
- Overflow at a single point usually means a blockage.Leaves, moss washed off the roof, or a nest. Water backs up behind it and spills at the low point. GuideHQ has a guide on clearing a blocked downpipe, which is where the debris usually ends up.
- Overflow at a sag means the fall has gone.A section that has dropped between brackets holds standing water, and the run no longer drains to the outlet. Look along the gutter from a distance — a dip is easier to see from the ground than from a ladder.
- Overflow along the whole run means the system is at capacity.Either the guttering is undersized for the roof area it serves, or one downpipe is doing the work of two, or the outlet is partly restricted. Common on extensions and on roofs that have been re-covered with heavier tiles.
- Check the downpipe is actually flowing.In rain, put a hand near the bottom of the downpipe and listen. A downpipe that is silent while the gutter is full is blocked, and that alone accounts for a large share of overflows.
- Look at where the water leaves the roof.Water shooting over the front lip in heavy rain means it is arriving too fast and too far out. That is a roof-edge geometry problem, and GuideHQ has a separate guide on it.
- Look for water behind the gutter, against the wall.Different fault again, and a more damaging one, because the water is running down the fascia and the wall rather than into the gutter.
- Check the gully at the bottom.A downpipe discharging into a blocked gully backs up and surcharges the whole system. It is the easiest thing to check and it is at ground level.
- Deal with it before it becomes a damp problem.Persistent overflow soaks one part of a wall repeatedly, and that is a common cause of a damp patch inside. GuideHQ has a guide on making that connection.
If it doesn't work
Overflows only in very heavy rain, never in light rain.
Cause: Capacity rather than blockage — the system is adequate until it is not. — Fix: Check for a partial restriction first, then consider whether the roof area needs a second downpipe or larger section.
Overflows in all rain, at one spot.
Cause: Blockage or a sagged section holding water. — Fix: Clear the run and check the fall and bracket spacing.
Water dripping from the joints as well.
Cause: The gutter is running full, so joints that normally stay dry are under water. — Fix: Fix the flow problem first — the joints may stop leaking once the gutter is not brimming.
Questions people ask
Can I just fit a bigger gutter?
Sometimes that is genuinely the answer, particularly where a roof has been extended or re-covered. But confirm it is a capacity problem first, because a bigger gutter does nothing about a blocked downpipe.
Is moss on the roof the cause?
It is a very common contributor. Moss washes into the gutter, breaks down and forms a silt that holds water and blocks outlets, so it turns up as blockages every autumn.