The short answer
Gutters overflow because water cannot leave as fast as it arrives. The usual causes are debris blocking the channel or outlet, a run that has sagged and holds water, a gutter too small for the roof area, a blocked downpipe, or a slipped section. Check in heavy rain where the water spills: it points to the cause.
Read the overflow first
Where water leaves the gutter is the best clue. Stand outside in heavy rain, or run a hose along the roof edge, and watch.
If water pours over the front lip at one spot, that point is probably a low spot in the run, or sits just upstream of a blockage. If it spills along the whole length, the channel is either full of debris or too small for the flow. If water jets out at a joint or corner, the gutter is backing up behind a blockage or a bend, and the joint is the weakest point. If water drips from the downpipe shoe at a trickle while the gutter brims, the downpipe is blocked.
Each pattern has a different fix, which is why guessing and sealing joints rarely helps. Take a photograph of where the water spills, and note whether it happens in every shower or only in a downpour, because those two descriptions lead to different causes. Overflow is a symptom, and the aim is to find which of the causes below is behind it.
Blockages: the commonest cause
Leaves, moss, silt, grit shed by tiles and the occasional ball or nest make a layer that narrows the channel. Because a gutter is shallow, a modest layer is enough to reduce its capacity a great deal.
The outlet is the critical point. Debris is carried along the run and packs against the hole where the water drops into the downpipe. Even a clear channel overflows if that hole is plugged. Test it by pouring a jug of water into the gutter near the outlet: it should drain at once, with no pooling around the hole. Slow drainage there with a clear channel points at the pipe below. Downpipes block too, from the top where leaves enter and at the bottom where silt settles at the shoe or drain.
Cleaning, and then testing with a hose, tells you quickly whether debris was the cause. Our answer on how often gutters should be cleaned covers the routine, and what a visit costs is set out in our gutter cleaning cost answer. If it overflows again after a proper clear, the cause is one of the next three.
Falls, brackets and sagging
A gutter is laid with a gentle slope to the outlet. One manufacturer of small gutters recommends about 1:350 where a fall is used. Over time, brackets loosen, fascias soften and the run sags in the middle, and the low spot holds a pool of water that spills long before the rest of the gutter fills.
You can see a sag by looking along the run from one end, or by seeing a permanent line of stain or algae on the wall below. Standing water after rain has stopped is a giveaway, and so is a gutter whose far end is higher than its middle.
The fix is to refix the brackets and correct the fall, and often to repair the fascia. If the board is soft, new brackets will not hold, as explained in our page on soffit and fascia materials. A run that cannot be levelled may need replacing, which is covered by our answer on whether to repair or replace gutters.
Undersized gutters and heavy rain
An undersized gutter for the slope draining into it will spill over in heavy rain, however clean and level it is. This happens when a roof has been enlarged, an extension has been added whose water drains onto an old run, or a valley or a large slope channels water to a single outlet.
The signs are overflow only in the heaviest downpours, at the same point each time, with a gutter that is spotless and well fixed. A larger profile, an extra downpipe or a better-sited outlet can fix it. Each of those is a matter of calculation for the roof area, so it is a survey job rather than a guess.
Valleys, the internal angles formed where two slopes join, concentrate flow. A gutter beneath the end of a valley takes a big share of the roof's water, and often needs a deeper channel or a different outlet position.
Other causes and what to do
Some causes are less obvious. A downpipe that has been crushed or has a loose joint can slow the flow. Ice in winter blocks outlets, and a gutter that overflows only on cold mornings is likely to be partly frozen. An outlet that has been boxed in by paint or by a badly fitted guard cuts the flow. Roof changes matter as well: a new roof covering that sheds water faster or a new extension that adds a slope can overload a gutter that coped for years.
Overflow also does damage. Water that runs behind the gutter soaks the fascia and soffit, and water down a wall stains it and can soak the stone or brick. We describe the knock-on damage in our guide to blocked gutter damage, and the usual first step is the same: find the cause, clear what can be cleared, and test.
We survey gutters free of charge, with photographs of the problem, and quote a fixed written figure for any repair or new section. Sometimes the cause is just a clear, and we tell you that clearly. See our gutters, soffits and fascias service for the full list of work.
