The short answer
Gutters overflow because water arrives faster than they can carry it away. The usual causes are a blockage of leaves, silt or moss in the channel or at the outlet, a gutter that sags or lacks fall so water stands, a downpipe that is blocked or too small, and a run that is too small for the roof area above it. Look at where the water spills: over the front, at a joint or at one corner shows which cause applies.
Read the pattern of the overflow
Watch the gutters in heavy rain, from the ground or a window, and note where water leaves them. A single spill point, usually near an outlet or a corner, suggests a local blockage or a low spot. Water pouring over the whole front lip of a run means the channel is full along its length, which points to a blocked outlet or downpipe, or to an undersized run. Water dripping from joints is a leaking seal and not an overflow. Streaks down the wall under one spot show where it has been happening for a while.
Timing helps as well. An overflow only in the heaviest rain suggests capacity is short. An overflow in light rain means something is blocking the flow. A problem that starts after autumn points to leaves. A problem that started after building work suggests that a new roof area is draining into an old run.
Blockages and how they form
Leaves, twigs, moss from the roof and grit from ageing concrete tiles wash into the gutter and settle at low points and around outlets. Wet debris turns to a thick mat that catches more. Plants and seedlings take root in it. A bird's nest, a ball or a dead leaf plug at the outlet can stop the flow altogether. If the downpipe is blocked, the whole run fills and spills.
The fix is to clear the channel, flush it with water and check the outlet and pipe, as described in how to unblock a downpipe. Do it from a stable platform, not by leaning off a ladder. A routine prevents a repeat, and how often gutters should be cleaned suggests one.
Trees deserve a special mention. Overhanging branches drop leaves, seeds, sap and twigs directly into the channel, and a gutter beneath a mature tree can fill within weeks of leaf fall. Cutting back branches that touch the roof helps, as does a guard on the worst runs, though neither removes the need to look. Moss is a second source: a mossy roof sheds clumps into the gutter every time it rains, and it is worth treating the roof as well as clearing the channel.
Fall, sag and size
A gutter needs a gentle fall towards the outlet, so water runs. If brackets have loosened or the fascia behind has softened, the channel dips and holds a pond. Water that stands overflows at the lowest point first and freezes in winter. Sagging is explained in why gutters sag, and the repair is to reset or add brackets, or to fit new ones into sound timber.
If the channel is clear and level and still overflows in storms, the run may be too small. A bigger roof, a long run with a single outlet, an extension that sends extra water onto an old gutter and a flat roof draining into the eaves are all causes. Larger gutters, an extra downpipe or an outlet in the middle of the run can solve it; what size guttering you need explains the logic. Overshoot can also be caused by the gutter sitting at the wrong height relative to the tile edge, in which case see water running behind a gutter.
Check the downpipe end too. A pipe that discharges into a blocked gully, a full water butt or a clogged drain backs up the whole system, and the first sign appears at the gutter. Pour a bucket of water into the top of the pipe and watch the bottom. If it comes out slowly, or not at all, the blockage is below and the gutter is only showing the symptom.
What overflow does, and how we help
Overflowing water soaks the wall, stains the render, and can reach the damp-proof course and the foundations. At the top, it wets the fascia and soffit and rots them, as described in rotten fascia boards. A full gutter is heavy and pulls on its brackets, which makes the next sag worse. In frost, standing water freezes and splits joints.
The repair depends on the cause. A blocked run is cleared, a sagging one reset, an undersized one replaced with a larger profile, and a leaking joint resealed, as in how to fix a leaking gutter joint. If the gutters are old and failing in several ways, replacement is more sensible than patching, and signs your gutters need replacing lists the signs.
Our free survey photographs the overflow points, the outlets, the brackets and the fascia behind, and tells you plainly which of these applies. If a clear-out is enough, we say so. The guttering, soffits and fascias page describes the service, with fixed written figures after the visit.
Every roof is different. A free survey with photos settles it for yours.
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