Why is my gutter overflowing? The six usual causes, and how to tell which

The six usual causes at a glance

1. Leaves, moss and silt in the gutter

This is the cause most of the time. It isn't only leaves. Moss washed off the roof, seeds, tile grit and dust settle in the bottom of the gutter as a sludge, and once that sludge is deep enough, plants take root in it. Cadw's advice is to clear gutters of "plants, leaves and debris, particularly during autumn and winter".

Gutters under oak, beech or lime fill fastest, and they often fill again within weeks if they're cleared before the trees have finished dropping. Our tree-by-tree guide covers the timing.

2. A blocked outlet or downpipe

Water can only leave a gutter through its outlet. If the outlet or the pipe below it is blocked, even a spotless gutter fills up and spills over, usually close to the outlet. That's why the Department for Communities in Northern Ireland, in its technical note on roof drainage, says: "Always check that the water is running freely in gutters and pipes before looking for any other cause."

A good test in rain: look at the bottom of the downpipe. If the gutter is overflowing but only a trickle is coming out below, the blockage is in the pipe.

3. A sagging gutter, or the wrong slope

A gutter needs a slight, even fall towards its outlet. The Northern Ireland note recommends a fall of at least 1 in 360, which is roughly 3mm for every metre. If a bracket has snapped or worked loose, a length droops, water sits in the dip, and in a downpour it spills from the lowest point.

Cadw's advice is to "check that gutters are not sagging and are correctly aligned to discharge water into the downpipe without ponding". It notes this is a common problem with uPVC, which it says "tends to be the shortest lived of all gutter materials". A sagging run is a repair, not a clean, though it often needs clearing as well.

4. A leaking joint

Lengths of plastic gutter are joined with union brackets that hold a rubber seal. A guttering supplier's guide notes that "seals can dry out, shrink or become dislodged", and water then escapes at the joint. It shows as a steady drip from one spot on the underside, or a stain on the wall below. Resealing or replacing the joint usually fixes it.

5. A gutter set the wrong way, or the wind

The Northern Ireland note lists "incorrect setting of the gutter with the inner edge being lower than the outer". When that happens, water runs over the back of the gutter, behind it and down the wall. That's harder to spot than an overflow at the front, and it's how many damp patches start.

The same note makes a point most people never think of: "a gutter falling against the wind can prevent the movement of water causing it to spill over", even when nothing is blocked. If a gutter only overflows in strong wind and rain, that may be why.

6. A gutter too small for the roof

Cadw says that sometimes "the system may be too small to cope with the volume of water being discharged at times of heavy rainfall". The fix isn't always a bigger gutter. Cadw suggests that "the installation of an additional downpipe and the adjustment of the gutter alignment may be a suitable alternative".

Watch for a downpipe from an upper roof emptying onto a lower one. The Northern Ireland note warns that discharge from one roof onto another can saturate walls, because the water arrives at the lower gutter in one concentrated stream.

How to work out which it is

• Wait for proper rain. Cadw's advice is to "grab an umbrella and stand in the rain", because that's the best time to check gutters and drains.
• Note exactly where the water comes over: along a stretch, at the outlet, at a dip, at a joint, or behind the gutter.
• Look at the bottom of the downpipe. Plenty of water means the pipe is clear.
• Look at the wall below for stains, green streaks or moss, which show it's been getting wet for a while.
• Take a photo or a short video from the ground. It's usually all a roofer needs to tell you what's wrong.

Why it's worth sorting before winter

Cadw warns that overflowing gutters "can lead to damp walls and decay", and that if a leak carries on, "water damage can appear on the inner face of the wall". A blocked gutter is a small job. A wall that has been soaked every time it rained for a whole winter is not.

How we deal with an overflowing gutter

We clear gutters by hand from ladders and check that every downpipe runs freely, so the overflow is gone, not just the leaves. If the real cause is a sagging length, a leaking joint or a gutter set the wrong way, we'll show you on the photos and quote for that repair separately. You'll get before and after photos of every run either way.

Written by Hawkes Roofing, roofers based in St Albans who clear gutters by hand across Hertfordshire, Buckinghamshire and north-west London. Last reviewed 26 October 2026.