The short answer
Gutters overflow for four reasons: something is blocking the run, the run no longer slopes towards the downpipe, the gutter is too small for the roof draining into it, or a valley is sending a whole slope of water to one spot. Leaves, moss and slate grit cause most blockages. A hosepipe test on a dry day shows which of the four you have.
Water spilling over the lip of a gutter is a message from the roof. The cause is usually easy to find, and it decides between a clean and a refit.
A blockage is the first thing to rule out
Gutters carry rainwater from the edge of the roof to the downpipe, the vertical pipe that takes it down to the drain. Anything that narrows that route makes the water back up and spill over the lip. Leaves, moss washed off the roof, grit from ageing slates and a stray football from next door all do it.
Why is the spill in one place only?
Look at where the water leaves. If it drops over the edge in one place, the blockage is usually close by, often at the outlet where the gutter meets the downpipe. A clump of wet leaves sitting over that outlet acts like a bath plug. A gutter that spills along its whole length points to a different cause, covered below.
Moss deserves a mention. A roof that sheds moss sends it straight into the gutter below, where it packs down and holds water like a sponge. Seeds then sprout in the silt, and the gutter grows its own little garden. Clearing it is a ground-level or ladder job, and our answer on how often gutters should be cleaned covers the timing, especially under trees.
A gutter that holds water has lost its fall
A gutter is never laid flat. It is set to drop gently towards the outlet so the flow keeps going. Brett Martin, a gutter manufacturer, advises 1mm of drop for every 600mm of run in its installation guidance. That is barely visible to the eye, which is why a small sag matters so much.
What makes a gutter sag?
Brackets are the usual culprit. They are the clips that carry the run along the fascia, the timber board your gutters are screwed to. If a bracket has worked loose, rusted or lost its screw, or if the fascia behind it has gone soft with rot, that stretch of gutter drops and the water pools instead of draining. A heavy winter load of wet leaves can pull a bracket down as well.
You can spot a pooling gutter once the rain stops. A dry spell leaves a tide mark or a puddle of silt at the low point, and green algae show where water has sat. Our answer on gutter fall explains how a run is set out, and why re-hanging it sometimes makes more sense than replacing it.
When the gutter is too small for the roof
Every gutter can only carry so much water. A small half-round profile fitted under a long, wide slope may cope in light rain and fail in a downpour. Extensions, new dormers and joined-up roof areas can end up draining into a gutter that was sized for something smaller. There may be too few outlets as well, with one downpipe asked to serve a long run.
Why does water jump the gutter at a valley?
A valley forms where two slopes meet at a dip in the roofline, and rain from both sides funnels down it together. At the bottom, that water leaves the roof in one concentrated stream. The stream can shoot straight across a shallow gutter and over the far side, even when the gutter is clean and correctly sloped. A deeper gutter, a splash guard (a small upstand fixed at the valley mouth) or an extra outlet nearby is the usual answer.
Only a look at the roof settles it. The survey takes in the roof area feeding the gutter, the outlets and the downpipes, and the findings come back to you as pictures of your own gutters.
Checks from the ground, and jobs for a roofer
From the ground, wait for steady rain and watch from a safe spot. Note where the water escapes: at an outlet, at a joint, at a corner or under a valley. Take a phone photo of the wall below as well, because a stained or green patch shows where a spill has been going on for some time, and the pattern of staining often points to the exact outlet or joint at fault.
Can I fix an overflowing gutter myself?
Clearing leaves from a low gutter with a stable ladder and a trowel is within reach of a careful homeowner. Anything at first-floor height, beside a conservatory roof or over a glazed porch is better left to a roofer, because coming off a ladder hurts far more than a wet wall does. A blocked downpipe has its own routine, set out in our answer on how to unblock a downpipe.
Should the cause prove to be fall, size or a failing fascia, that is work for our gutters, soffits and fascias service. Book the survey, and the run is checked end to end, photographed and priced in writing before anything is ordered.
More on overflowing gutters
Why do my gutters only overflow in heavy rain?
A gutter that copes with drizzle but spills in a downpour is usually too small, short of outlets, or partly blocked so its real capacity is lower than it looks. A valley above the spill point is another cause. Heavy rain exposes the weakness, so that is the right time to watch where the water leaves.
Will overflowing gutters damage my house?
They can. Water running down the wall soaks the brickwork, stains it green, and can reach the fascia and soffit, the underside panel of the eaves, where timber rots. In winter, overflow can freeze on the wall face. A spill that goes on for months deserves a proper look rather than another tidy-up.
The full picture: Gutters, soffits and fascias in Taunton ยท More short answers