Overflowing gutters
Why is my gutter overflowing?
The short answer
Most often because something is stopping the water leaving: a blockage in the run, a blocked outlet or downpipe, or a slope that carries water away from the outlet. Brackets that have slipped, joints that have opened and a roof that delivers more water than the gutter can carry can do it too. Where the water spills from tells you the cause, and a short look from the ground usually shows it.
Reading the overflow: what each pattern of spilling water points to, and which fixes are quick and which need a survey.
Start with where the water comes over
The place where water leaves the gutter is the best clue. Watch the run during a downpour and note whether the spill is at one end, along the whole length, over the outlet, at a joint or from the back against the wall. Each pattern points to a different fault, and guessing wastes a ladder trip.
Spilling at the outlet, the hole that feeds the downpipe, usually means debris is plugging it. Spilling along the whole front edge points to a gutter full of silt, or one that has lost its fall. Water dripping from joints is a separate problem of failed seals, covered in our page on leaking gutter joints. Water pouring over the back of the gutter, behind it, suggests the gutter is too low against the roof edge or the roof tiles do not reach into it. Write down what you see and take a photograph; it helps whoever surveys it.
Blockages in the run and at the outlet
This is the most frequent cause, and the quickest to put right. Leaves, moss, grit and nesting material settle at the lowest point of a run, which is normally next to the outlet. Once a layer of silt forms, new debris builds on it, and the effective depth of the channel shrinks until a modest shower spills over. Our guide to how often gutters should be cleaned explains the rhythm that prevents it.
A clean fixes the problem only if the water can then leave. If the gutter is clear and still overflows, look lower. The downpipe, the upright pipe leading to the drain, can block with the same debris that failed to pass the outlet, or with a build-up at a bend. A pipe that gurgles, or a gutter that holds water long after rain has stopped, suggests a blockage inside it. Rodding or flushing from the top usually clears it, and our page on blocked downpipes covers how. Where the pipe feeds a drain that is itself blocked, the gutter will back up however clean it is, and that is a matter for a drainage contractor.
Fall, brackets and a gutter that holds water
A gutter must slope gently towards its outlet. Manufacturer guidance sets this at very small amounts: Brett Martin describes 1mm for every 600mm, and FloPlast gives 1:350 for its MiniFlo system. Brackets are spaced to hold that line, and Brett Martin puts supports no more than 900mm apart. These are manufacturer figures for their own products, not a legal standard, and the right figures for your system are in its instructions.
If brackets have loosened, the run sags between them and water pools instead of draining. Pools freeze in cold weather, add weight and open joints. In time the pooled water finds the lowest point and spills over the front. A run that is level or tilted away from the outlet will never drain well, whatever you clear from it. Our pages on gutter fall and bracket spacing show how it is set out, and the sagging gutter answer explains the usual causes of drooping.
A gutter that is too small for the roof it serves
Sometimes the gutter is clean, level and still overwhelmed. A large roof slope, a roof that was extended or re-pitched, or a flat-roof extension discharging onto a pitched roof above the gutter can all deliver more water than a small half-round run can pass. In very heavy rain, even a sound gutter can overshoot at an inside corner, or at the point where two runs meet and the flow doubles.
Remedies range from a bigger gutter section, an extra outlet to share the load, a deeper profile on the busiest run or a downpipe at a corner where it makes sense. Our page on gutters overflowing in heavy rain goes further into this. We would not recommend replacing a whole roofline for a single inside corner that overshoots. We trace the cause, photograph it and propose the lightest repair that works; the written quote, one total, arrives once we have surveyed.
Why an overflow should not be left alone
Water spilling over a gutter does not harm the gutter much. It harms what is underneath. A wall that is wetted each time it rains can stain, grow algae and in time hold moisture. Timber fascias and soffits that stay wet can rot, and rafter ends behind them can be reached. A path or flowerbed below takes a steady drumming that the ground was never designed for, and a doorway beneath the spill becomes unpleasant to use in every shower.
It is also a sign that the gutter has stopped doing its job, and the same fault may be pulling the brackets loose or loading the fascia. Our guide to damp from a blocked gutter explains what follows when it goes on. If you are unsure, send us photographs and a note of where the spill happens through the enquiry form or WhatsApp. The full list of gutter work is on the guttering, soffits and fascias page.
Related questions
Overflow questions
Why does my gutter only overflow in heavy rain?
It is probably partly blocked or undersized rather than completely stopped. Light rain passes through the narrowed channel, but a downpour delivers more water than it can pass. A clean, then a look at the fall, usually separates a blockage from a capacity problem.
Can I clear an overflow from the ground with a hose?
A hose can test the flow but rarely clears a real blockage, and it throws dirty water on the wall. Safe access to the gutter is the better answer. Never climb on a roof to reach a run, and do not work from a ladder alone.