The short answer
A gutter that overflows over the front in heavy rain is usually blocked, too small for the roof it serves, or laid with the wrong slope so water pools instead of reaching the outlet. Blockages are the most common and the least costly to fix. Dripping at the joints is a different fault, and is covered separately.
Blockage, size or slope: the three usual causes
Watch the gutter during a downpour if you safely can, from the ground or from a window. If water spills at one point and the rest of the run is quiet, you probably have a blockage near that spot or at the outlet. If the whole length spills together, the channel is likely undersized, or the water cannot leave fast enough. If one end holds a deep pool while the other end stays dry, the slope is wrong.
Photograph what you see, even on a phone from a window. A short clip of the spill point tells a roofer more than a description does, and it helps you check later whether the repair worked.
This page is about overflow over the front lip. Water that drips from joints or runs down the wall behind the gutter has a different cause, and our guide to leaking gutters covers it.
Water pouring over the front lip lands on the wall, the ground and the foundations of the house. It can stain brickwork, push damp into the wall below, and soak the ground beside the footings. Where it runs back behind the gutter instead, it soaks the fascia, the soffit and the rafter ends.
Timber at the eaves is the first casualty, and rot there is slow and unseen. That is why an overflow is worth fixing even when nothing seems wrong indoors. Paint peeling on the fascia, green streaks on the soffit and a damp patch high on a bedroom wall are all signs the water has been going the wrong way for some time.
Blockages in the gutter and downpipe
Leaves, moss washed from the roof, seeds, bird nesting and the odd lost ball all end up in the channel. Silt builds up at the outlet, where the water slows, and narrows the opening. A downpipe that is partly blocked behaves in the same way, with the gutter filling because the water cannot leave.
Clearing the gutter and flushing the downpipe with a hose settles most cases. If the gutter clears and the water still backs up, the blockage is lower down, in the pipe or the drain it feeds, which our page on a blocked downpipe looks at.
Regular clearing, not scaffold work, is the sensible answer here. Aim for a clean after the autumn leaf fall and another in spring, with an extra look if a storm has brought branches down.
Checks you can make from the ground
A good deal can be learned without a ladder. Walk round after rain and look for dark streaks on the wall under the gutter, which show where water has been going over. Check that each downpipe is carrying water at its base, and that the gully or drain at the bottom is clear. Listen for gurgling, which hints at a restricted pipe.
From an upstairs window you can often see leaves piled at an outlet or a long green line of moss along the channel. Binoculars help. Note where the spill happens, because a spot that overflows every time points to a nearby cause, say a clogged outlet or a low joint.
Never lean a ladder on the gutter itself, and leave work on a high elevation to someone with access equipment.
A channel that is too small for the roof
Gutters are sized to the area of roof draining into them. A modest half round channel can cope with a small lean-to and fail under a big pitched slope. The problem also appears when an extension or dormer sends extra water onto an existing run, or when a downpipe was left out and one outlet now serves two roofs.
The remedies are to fit a deeper profile, add an outlet and downpipe so the run is shorter, or both. Adding an outlet often costs less than replacing the whole run. A jointless aluminium system, covered in our page on seamless gutters, removes joints but does not change the arithmetic of roof area against channel size.
A sudden storm can overwhelm even a correct system, and a brief spill in the worst downpour is not always a fault. A gutter that spills in ordinary rain is.
Slope, brackets and alignment
Every run is laid with a slight slope to its outlet, so the water travels instead of loitering. Plastic systems are generally set at roughly 1:600 to 1:350. A flatter line leaves water lying in the channel, where it gathers silt and spills in a hard shower. A steeper one can lift the front lip above the back at the far end. Brackets that have slipped, a fascia that has dropped, or a run that sags between fixings all disturb the line.
Look at the board behind. If the fascia is rotten, the gutter will not hold its line however many brackets you add, which is a job for the roofline rather than the gutter, as described under fascia and soffit costs. If your home dates from before 2000, take care before disturbing flat soffit boards, and read our note on asbestos soffits.
From survey to repair
On a survey visit we go over the run, the outlets, the downpipes, the slope and the fascia behind, and photograph whatever is wrong. You then receive a written quote. Where a clean and a bracket or two will stop the spill, that is all we propose, since the lightest repair that actually cures it is the right one.
A written 10-year workmanship guarantee covers what we do, and we take our waste with us. To arrange a visit, phone 01793 626096, you can also WhatsApp us a photograph or fill in the enquiry form. More on our gutters, soffits and fascias service.