The short answer
A gutter overflows when the rain it receives outpaces what it can carry off. The commonest cause is a blockage in the gutter or at the outlet. Other causes are a lost or missing fall, an undersized gutter or outlet for the roof area, a sagging run that holds a pool, and moss or grit washing in from the roof. Each has a different cure.
Blockages and debris
The simplest cause is also the commonest. Leaves, twigs, moss, silt, seeds and the odd ball or bird's nest block the gutter or, more often, the outlet where water drops into the downpipe. Water backs up behind the blockage and spills over the lowest edge.
Look at where the water comes over. If it pours over the front near an outlet, the outlet or the top of the downpipe is blocked. If it spills along the length, the gutter is clogged with debris. If it gushes from one joint, there may be a joint failure as well.
The downpipe can be the problem even when the gutter is clear. A blocked pipe backs water up into the gutter. Tip a bucket of water down the outlet and see whether it clears straight away. A slow trickle or a rising level tells you the pipe or drain below is blocked.
Clearing is a simple job when it can be done safely from a stable ladder or tower, working gloved and lifting debris into a bucket. Where the gutter is high, over a porch or on a steep drop, a tower or a roofer is the safe choice.
Our page on how often gutters should be cleaned sets out a routine that keeps blockages from building up.
Fall, sag and undersized systems
A gutter needs a gentle slope to carry water to the outlet. Brett Martin's guide of 1 mm per 600 mm of gutter is hard to see by eye but keeps water moving. If the slope is missing or reversed, water collects in the low spots and overflows there.
Falls are lost when brackets work loose, when the fascia rots and the gutter drops, or when the gutter has sagged under the weight of debris and standing water. A sagging section forms a pond, and a pond in a plastic gutter stresses its joints.
Size is another cause. A gutter and its outlets must suit the area of roof that drains into them. BS EN 12056-3:2000 is the standard behind roof drainage layout, and the system should be sized to suit the roof. A small gutter on a large roof, or too few outlets on a long run, cannot cope in a downpour even when clean.
Changes to the roof can overwhelm an old system. A new extension draining onto an existing run, a re-roof that sheds water faster, or a downpipe that was removed for other work can all push a gutter beyond its capacity.
The signs are overflow in heavy rain only, with the gutter clear and flowing normally in light rain. Water pooling in a visible dip is the clue for sag.
Moss, grit and the roof above
The roof can be the source of what blocks the gutter. Moss growing on tiles and slates loosens and washes down, and clumps accumulate in the gutter and block the outlet. Weathering concrete tiles let go of grit, and clay and slate roofs can shed fragments, all of which settle into a sludge along the gutter bottom.
If your gutter fills with moss and grit soon after cleaning, treat the roof as well as the gutter. Moss is best removed by a roofer using gentle methods; Marley says high-powered jet washing is not recommended for tiles and slates. Our page on roof cleaning and moss removal covers that side.
Overhanging branches add leaves, seeds and twigs. Trimming back branches, where the tree is yours or you have permission, cuts the load. Where a conservation area applies, tell the council before any tree work.
A guard over the gutter can keep large debris out, though it will not stop fine grit and moss. Our page on gutter guards weighs the trade-offs.
When the spill is at the back of the gutter, the water goes behind it into the eaves. That is a more serious problem, because it can soak the fascia and the roof timbers.
Diagnosing and fixing the cause
Match the symptom to the cause. Overflow at an outlet with a full gutter means a blockage. Overflow at a dip along the run means a sag or lost fall. Overflow only in the heaviest rain, with the gutter clean, points to undersizing. Overflow soon after cleaning with debris from the roof points to moss and grit.
A short test helps. In dry weather, feed a hose in at the high end of the gutter and follow the water along. It should flow steadily to the outlet and away down the pipe. Ponding shows a fall problem, and a slow outlet shows a blockage below.
The fixes follow. Clear blockages, refix or add brackets, re-hang the run to the right fall, enlarge or add outlets and downpipes, fit larger gutters where the roof area demands it, and treat the roof where it is the source. If the fascia has rotted, it is repaired first, so the gutter has firm fixings.
We check the whole system on the survey, photograph the problem points and explain the cause in plain words. A single written price is given for the cure, with access and VAT stated.
Left alone, overflow soaks the wall and fascia, as our page on damp walls from a leaking gutter shows, so it is worth tackling early. The gutters, soffits and fascias hub covers every part of the system.
