Work backwards from the hole in the wall
You are standing outside looking at a pipe you have never noticed before, and it is dripping. The useful question is not whether it is serious. It is which thing inside the house that pipe is attached to, because an overflow pipe is not a general drain — it is a dedicated outlet from one vessel or one valve, and nothing else feeds it.
That makes it traceable. An overflow runs by gravity or by pressure from its source to open air, in as short and as straight a line as the building allowed. So the exit point is evidence. Note four things before you go inside:
- Height. Roughly at eaves, at first-floor ceiling, at first-floor waist height, or low down near the ground.
- Material and bore. White plastic of about 22mm, white plastic of about 32mm, or copper of about 15mm.
- Which room is behind it. Stand inside and work out what is on the other side of that patch of wall.
- Temperature of the water, tested from the side with the back of a finger. Cold is a tank or a cistern. Hot changes everything, and is covered in is a leaking overflow pipe dangerous.
The height map
This is the fastest way to a shortlist. It is not infallible — pipes get rerouted during extensions and loft conversions — but it is right far more often than not.
| Where it exits | Most likely source | How to confirm from inside |
|---|
| At or just below the eaves, white plastic, often turned down and cut at an angle | The cold water storage cistern in the loft, or a smaller feed-and-expansion tank beside it | Go into the loft. Look for a tank with a plastic pipe leaving its side an inch or two below the rim, heading towards that wall |
| First floor, white plastic, directly behind or beside a bathroom or cloakroom | A toilet cistern. Its float valve is not shutting off | Lift the cistern lid. Water sitting at or above the internal overflow slot, or trickling into the pan, confirms it |
| Ground floor, white plastic, behind a downstairs loo or a utility room | A ground-floor cistern, or occasionally a water softener or a header tank in a cupboard | Same check. Also open any airing or utility cupboard on that wall |
| Copper, about 15mm, near or below a boiler, ending turned down at the wall | The boiler pressure relief valve discharge | Watch the boiler pressure gauge over a heating cycle. A climb when hot and a low reading when cold points here |
| Larger bore, water warm or hot, from a property with a sealed cylinder and no loft tank | An unvented cylinder's temperature or pressure relief discharge | Look for the tundish — an open gap in the pipework near the cylinder. Anything running through it is the cylinder |
Two things cause most of the confusion. A boiler condensate pipe is small white plastic, runs to a drain rather than open air, and drips whenever the boiler fires — that is normal, not an overflow. And a loft tank overflow in a converted roof sometimes exits at first-floor level, because the run was taken down a stud wall during the conversion.
Isolate one feed and watch
A shortlist is not proof. Proof is cheap: cut the supply to your best candidate and see whether the drip stops.
- Dry the end of the pipe outside with a cloth, and put a jam jar or a yoghurt pot underneath it. You want a clean baseline, because a pipe that has been running keeps weeping for a while after it stops being fed.
- Close the service valve on that one feed. Toilet cisterns almost always have one: a small in-line valve on the supply just before the cistern, turned with a flat screwdriver. A quarter turn so the slot sits across the pipe is off. Loft tanks are fed from the rising main and may have a gate valve or lever valve on the inlet instead.
- Wait, and look at the pot. Half an hour is usually enough. A dry pot means you have found it. Water still collecting means it is not that feed.
- If it was not that one, reopen the valve and move to the next candidate. Work one at a time — closing three valves at once only tells you that one of three was responsible.
Two shortcuts worth knowing. A few drops of food colouring in the suspect cistern or tank will show up at the pipe end within minutes if that is the source, which is faster than waiting out a slow drip. And an intermittent overflow — dry all morning, running every evening — is almost always a cistern, because it only overflows after someone uses and refills it. A loft tank or a relief valve tends to run steadily.
Flats, and pipes that come out where they should not
In a converted house or a purpose-built block, the pipe leaving the wall outside your window may not be yours at all. Communal cold water tanks in the roof space of a block serve every flat below, and their overflows run down the outside of the building past other people's windows. A drip appearing outside a second-floor flat can be fed by a tank three storeys up.
Before you start lifting your own cistern lid, look up. If the pipe runs vertically past your window and carries on upwards, it is a riser from somewhere above and it belongs to the freeholder or the managing agent, not to you. That is a report-it job, not a fix-it-yourself job, and it is worth reporting in writing so the date is on record.
The same applies the other way round. If the overflow is yours but the water lands on a neighbour's path, wall or extension roof, you want it stopped for reasons beyond the bill: it stains render, soaks into brickwork, and a freezing night turns a dripped-on path into ice.
What it is costing you while you work it out
If the property is on a meter, an overflow is metered water leaving the building and going nowhere. You can measure it rather than guess. Turn off everything that uses water inside, take a reading, leave it an hour without touching a tap, and read again. Movement with everything off means something is running continuously, and an overflow is one of the easier causes to confirm, because you can stand outside and watch it. There is more on that in what a moving meter with everything off means and in the water bills hub.
There is a second reason not to sit on it. Thames Water's leak allowance is discretionary, not automatic: the company says it may cover the cost of the lost water if the leak is fixed within four weeks of you being made aware of it, and the claim has to be made within three months of the repair, backed by a plumber's invoice or a Thames Water job number. Read that clock carefully. It starts when you are made aware — not when you decide to act.
The awkward part: someone else usually notices first
Overflow pipes discharge onto the outside of a building, which is the one part of it you do not look at. A neighbour, a postman, a window cleaner or the person who parks under it will see it before you do, and they often assume you already know.
So if someone has just told you about yours, take the date seriously — it is the date the clock above starts, and on a tenanted property it is also the date you were put on notice as the landlord. Write it down, tell whoever is responsible for the repair in writing, and keep the message. If you are a tenant, report it to the landlord or agent the same way rather than trying to close valves you did not install.
When tracing stops working, and it becomes our job
Most of what is on this page is not a leak detection job and we will say so on the phone. A float valve is a twenty-minute fix and a cheap part. A relief valve or an expansion vessel on a gas boiler belongs to a Gas Safe engineer. An unvented cylinder is G3 work.
Where it turns into something for us is when the tracing does not resolve:
- Every candidate feed has been isolated in turn and the pipe still drips.
- The overflow has been fixed, and the meter still moves with everything off.
- There is damp inside — a stained ceiling, a wet patch at the top of a wall — that the overflow does not account for, because an overflow discharges outside.
- The pipe is dry but the boiler keeps losing pressure, which means the water is going somewhere that is not the discharge.
At that point it is a trace rather than a guess: isolate and pressure-test to confirm a real loss and size it, thermal imaging along the pipe runs, tracer gas on a drained circuit where thermal cannot see. That is £150 per hour, agreed before we attend, no call-out fee, and if we do not find the leak you do not pay for the looking time. Where a floor or ceiling has to come up, our own handymen and builders put it back. The visit comes with a written report prepared to support a trace-and-access claim, though what your policy covers is the insurer's decision. See water leak repair and pricing.
