The short answer
Gutters overflow for one of four reasons: they are blocked with leaves, moss or silt; they are not laid to the right fall so water sits in the run; they are too small for the roof; or a roof valley or upper roof is discharging a flood into one spot. A clear gutter that still overflows points to fall, size or a valley, and each of those has its own fix.
The four common reasons gutters overflow in heavy rain, how to tell which one you have, and what puts it right.
A blockage is the first thing to rule out
Leaves, moss washed off the tiles, seed pods, and the grit that sheds from concrete tiles all end up in the gutter. In time they build into a soft dam. Water cannot pass the dam, backs up behind it and spills over the front edge, usually at the same spot every time.
The telltale sign is overflow near an outlet or a bend, with clean water elsewhere. A green stain on the brickwork under the gutter marks where the spill lands. Plants sprouting from the gutter are a sure clue too. A downpipe can be the culprit as well: if the gutter looks clear but water pours over the top near an outlet, something may have lodged in the pipe or at the bend below.
Clearing is a simple job by hand, though it should be done from a tower or a ladder fitted with a stand-off, not by leaning off a ladder against the plastic. Our answer on a blocked downpipe goes through the pipe itself. If your house has trees close by, the gutter will refill, so plan on clearing it every autumn.
Gutters laid to the wrong fall
A gutter needs a slight slope towards its outlet, so water runs along it instead of lying in it. Brett Martin's installation guidance gives the figure as 1mm for every 600mm. That is a very gentle fall, which is why a gutter that has dropped by a few millimetres in the wrong place can stand full of water.
A sagging run is the usual cause. Brackets work loose, fascia boards soften, or the gutter has been overfilled by years of debris and bent under the weight. The water then settles in the low spot, spills over the edge and, in winter, can freeze and drag the gutter down further.
You can spot it after rain. Look along the run: if water stands in a stretch of the gutter once the rain has stopped, the fall is wrong there. A level held along the back of the gutter will show it too.
Refitting means taking the run down or lifting it on to new brackets, and checking the fascia beside it. Our answer on fixing a sagging gutter goes into the signs of failing brackets.
Gutters that are too small for the roof
A gutter has a limit to how much water it can carry. If it is too narrow for the area of roof draining into it, or has too few downpipes for the length, a heavy shower will fill it faster than it can empty. The roof does not need to be large, either: a big roof sending its water to a single short run, or an extension added without changing the guttering, will overload a small channel.
The sign is overflow only in very heavy rain, with clean, level gutters. The gutter looks fine in a normal shower and then pours over in a downpour. Another clue is a run that was fitted long ago, before a loft conversion or extension altered where the rain falls.
The fix is to move up a size, add a downpipe, or both. Sizing follows the roof area and the outlets, with BS EN 12056-3 as the reference standard. A new run is a bigger job: in Checkatrade's October 2026 table, 15 to 20 metres with three downpipes typically costs £650 to £1,700, and your own written total follows our survey. See gutter replacement cost for what moves that figure.
Roof valleys and upper roofs that dump water
A valley is the internal angle where two slopes join, and all the rain from both slopes funnels down it in one narrow stream. If a gutter sits directly under that point, it takes the whole load in a single spot and overshoots the far side. The same happens where an upper roof drains onto a lower one and the water then runs off its edge into the gutter below.
The giveaway is a fan of water leaping clear of the gutter in the same place, even when the gutter is clean and level. It may be worse in heavy rain and fine in light showers.
Fixes include fitting a valley or rainwater diverter that spreads the flow, raising or widening the gutter at that point, adding a deeper section, or putting a downpipe closer to where the water lands.
To find out which of the four causes you have, email a short video of the spill in heavy rain, filmed from the ground, with the address. The survey covers the gutter, the fall and the valley, and the fix is priced as a single written total. The wider job sits on our gutters, soffits and fascias page.
More on this in Gutters, Soffits & Fascias in Crawley
Two more overflow questions
Why do my gutters only overflow in heavy rain?
That points to capacity rather than a blockage. A gutter that copes with a shower but spills in a downpour is usually too small for the roof, short of downpipes, or catching a valley flow. Watch where the water leaves the gutter and note whether it is at one spot. Our survey checks the size and outlets against the roof.
Will cleaning the gutters stop them overflowing?
It will if a blockage was the cause, and it is always the right first step. If the gutter overflows again soon afterwards, look at the fall, the size and the roof valleys. Standing water after rain in a clear gutter means the slope is wrong, and cleaning will not change that.