Found with equipment rather than by opening anything, which matters most in a wall that belongs to two houses and that neither of them should be cutting into on a hunch.
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In a terrace or a semi the wall between you and next door is not two walls touching. It is one wall, and both houses are built into it. Pipework can run within it from either side, which is ordinary and has been fine for a hundred years.
It stops being uncomplicated the moment damp appears. Water shows on the face of the wall that gives way first, which is a fact about plaster and paint rather than about plumbing. Damp on your side tells you where the water came through. It does not tell you which side it came from, and people reliably read it as though it does.
That matters because the usual next move is the wrong one. Opening a party wall to have a look is an intervention in a structure two houses depend on, it is disruptive and expensive, and if the water turns out to be coming from the other side you have opened your wall for somebody else's fault.
So it gets tested instead. Each property's supply can be isolated independently and the meters watched, which frequently answers it inside twenty minutes with nothing cut into. Acoustic listening and thermal imaging narrow the rest. Only then does anybody open anything, and by then everyone knows why.
Shut the supply off before anything else, then ring. We will stay on the line while you find it.
A party wall is a single structure both properties are built into. There is no gap and no boundary inside it, whatever the title plan shows on paper.
So a leak inside that wall is physically inside two houses at once, and it will do damage to both regardless of which side the fitting was on.
That is the fact everything else on this page follows from.
Which is a fact about plaster rather than about where the water started. Water inside a wall runs down and comes out at the weakest point.
The weakest point is frequently a different room, a different level, or a different house from the failure. Old plaster and render fail unevenly.
So the side that sees the damp is not evidence of the side that has the fault, and treating it as evidence is how these conversations go wrong.
Each property's supply can be shut off separately and the meters watched. That is a two-minute test and it tells you which house is losing water.
Close everything in one property, watch its meter, then do the same next door. A moving dial with everything off means water is leaving on that property's side.
It costs nothing, needs no equipment, and settles the question that would otherwise be settled by opening a wall.
Once you know which property, close the valve on the inlet to its hot water unit and watch again. Still moving means the cold side. Stopped means the unit or the hot line past it.
Acoustic listening and thermal imaging take it from there, both without opening anything, and both work through a party wall.
The point of the sequence is to arrive at a specific place rather than a suspicion, before anybody starts cutting.
It is disruptive, it is structural, and it may be for somebody else's fault. A party wall is not a plasterboard partition and it does not go back easily.
Doing it on the strength of where the damp appeared is the most common expensive mistake on this page.
Locating properly costs a fraction of it and is right more often.
Especially where the conversation is going to be with a neighbour. A written account of what was tested, what was found and where, is a fact both parties can look at.
A verbal opinion relayed second hand across a fence is not, and it is how reasonable people end up disagreeing about something that was actually measured.
We will write one. What we will not do is say who should pay for what, because that is not a plumber's call.
Before opening anything and before anybody has spent money. A leak in a shared wall is a shared problem whether or not either party wants it to be.
Access to test both sides is the practical reason. The meter test above needs both properties, and so does thermal imaging on a shared structure.
It is a much easier conversation at the point of finding out than after one side has committed to a repair.
Rising damp, a failed damp course, a blocked subfloor vent or a downpipe discharging against the wall all produce damp in a party wall and none of them is a plumbing failure.
The meter test separates them cleanly. If neither property is losing water, the damp has another cause and a leak detection visit will find nothing.
That is worth establishing first, because it is the cheapest test and it is frequently the answer.
Both of them, on an attached property, because at some point you may need the neighbour's as well as your own. In this housing the meters are frequently side by side at the front.
Check yours actually turns on a dry afternoon. On properties of this age they seize.
Learn the isolating valves indoors too, because closing one of those handles a specific failure without taking water off the house.
The braided hoses under vanities and behind cisterns last around five years and they do not fail gently. They go all at once, at full mains pressure, frequently while nobody is home.
In an attached building the consequence reaches further than the property it happened in, which makes the argument for replacing them stronger here.
They are cheap and quick to change, and they are the most common single cause of a genuinely ruinous indoor flood.
Acoustic listening picks up the noise a pressurised leak makes through masonry, which a party wall carries well. Thermal imaging finds the heat signature of a hot water leak behind a surface.
Pressure testing isolates sections until the one losing water gives itself away.
Between the three, most leaks in a shared wall get located to a small area before anything is cut, which is the whole objective.
Whatever else is true about a shared wall, the damage is set by how long water runs before somebody closes it off.
So the make-safe comes first, then the photographs, then the diagnosis, then the conversation. In that order.
Nothing on this page is a reason to leave water running while people work out whose it is.
Fixed price agreed before anything starts. These are indicative 2026 ranges.
Water stopped and the situation stabilised, at any hour.
Both supplies isolated and tested, with the findings written up.
Acoustic, pressure and thermal, with nothing opened to find it.
Depending on the pipe, the material and how it has to be reached.
Which side, where, and what we did. Not an extra.
And specifically whether it is on a wall you share. That changes the method before anybody has driven anywhere.
Each supply shut off independently, meters observed. It is the cheapest test available and it frequently ends the question.
Acoustic listening and thermal imaging until it is a point rather than an area, and still nothing has been opened.
What failed, which side it was on, and what it took, at the figure agreed before we started.
Shut it off at the meter and ring. A licensed plumber can be heading toward you while you are still on the call.
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Possibly both, and that is the honest answer rather than a dodge. A party wall in a terrace or a semi is one wall serving two houses. Pipework can run in it from either side, so damp appearing on your face of it does not establish which side the water is coming from. It establishes which side it came through.
By testing rather than by opening it. Each property's supply can be isolated separately and the meters watched, which frequently settles it in twenty minutes without anybody cutting into anything. Thermal imaging and acoustic listening do the rest.
Then we work with what we can access, which is usually enough to say whether the water is leaving your system. If your side isolates clean and the damp continues, that is a useful and defensible finding, and it is the sort of thing worth having in writing.
We would rather you did not, and not only because it makes a mess. Opening a party wall to look is an intervention in a structure two houses rely on, and if the water turns out to be coming from the other side you have opened your wall for somebody else's fault.
Turn the water off at the meter before anything else, then ring. How much a burst costs is almost entirely decided by how long the supply stayed on.
Common. Close every tap and appliance and watch the smallest dial on the meter. If it creeps, water is genuinely leaving on your side, and it can do that underground for months without surfacing anywhere.
Locating a concealed leak is $250 to $600. A party wall investigation with both sides isolated is $350 to $700. A burst repair is $300 to $900. Emergency isolation and make-safe is $200 to $450.
Tell us where it is showing and whether that wall is shared with next door. Those two answers decide the method and, frequently, whose problem it turns out to be.
Isolated, located, and opened once, in the right place, on the right side.
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