Why gutters overflow, and what it does to a house
A gutter has one job: to catch the rain off the roof and take it to a drain. When it overflows, the water goes down the wall instead, and that is the whole problem.
The four causes
Blockage
Leaves, moss, silt, a tennis ball, a bird’s nest. Usually at the outlet, because everything in the gutter ends up there. The gutter upstream fills and spills at its lowest point — which may be a long way from the blockage.
Lost fall
Guttering should drop gently towards the outlet. When brackets sag or the fascia moves, the gutter develops low spots that hold water, grow moss and overflow in heavy rain long before the outlet is reached.
Too much water
A long run with one small outlet, or a roof that was extended without adding a downpipe, cannot carry a downpour. The gutter is clear and still overflows. The fix is another outlet, not another clean.
A mossy roof
The roof sheds moss into the gutter every time it rains, so a gutter cleared in October is full again by February. The gutter is not the problem; the roof above it is.
What an overflowing gutter does
Brick, stone and render are made to shed rain, not to be hosed for an hour every time it pours. Water running down the face of a wall soaks in, the mortar softens, frost spalls the brick face, and a green or black streak of algae marks the route. At the bottom, a permanently wet patch of ground beside the house is how water gets under a damp-proof course or into a cellar.
Inside, the same stretch of wall is cold and damp. Black mould on an inside wall, peeling paper and a musty smell in one room often trace back to a gutter outside it.
The wall pays.
Water that goes over a gutter edge runs down the brickwork, soaks the wall at the base and splashes the foundations. Damp inside a house very often starts here.
More guides
How often should gutters be cleaned?
Twice a year for most houses. Which houses need more, which can go longer, and how to tell.
02Moss on the roof: why it matters more than it looks
What moss does to tiles and gutters, why some roofs grow it and others do not, and how it should come off.
03Signs of a roof leak, inside and out
What to look for on the ceiling, in the loft and from the garden, and why the leak is rarely where the stain is.
04After a storm: what to check from the ground
A ten-minute walk round the house that finds most storm damage before the next rain does.
05What a roof and gutter inspection covers
The six things that get looked at, and what each one tells you.