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A hidden leak rarely announces itself. It leaves clues instead — and one honest test settles most of the guesswork.
Short answer first: a damp patch that won't dry, a musty smell, or boiler pressure that keeps sliding down between top-ups are the usual signs. The stopcock test — close it and watch what stops — tells you whether it's your own pipework. Ring 020 4577 2888 any hour if the signs add up.
Damp that won't dry, a musty smell, dropping boiler pressure, a warm patch on the floor, or the sound of running water with every tap off. Any one is worth checking; two together is worth a call.
Some signs are visual: a discoloured patch on a ceiling or the top of a wall that comes back after redecorating, a skirting board that's warped or a laminate floor that's lifted at the edges for no obvious reason. Some are less obvious — a musty smell that lingers in one room regardless of ventilation, or a patch of carpet that always feels slightly cool and damp underfoot even in summer.
Others show up on the boiler rather than the wall: pressure that needs topping up more often than it used to, even though nothing visible has changed. And in houses with underfloor heating or pipework run under a solid floor, a warm patch appearing on the floor itself, distinct from the rest of the room, can be a leaking pipe warming the slab from below. None of these prove a leak on their own, but together they build a fairly convincing case.
Examiner's note: one sign is a maybe. Two or more signs in the same part of the house is a working diagnosis.
Close the stopcock, make sure nothing is using water, then watch the boiler pressure and listen. If the leak was from your own supply, the clues should stop.
With every tap, appliance and the boiler's own use of water accounted for, close the main stopcock fully and wait. If the boiler pressure had been dropping day by day, note the reading now and check it again in a few hours with the stopcock still closed — if it holds steady, the leak sits somewhere on the mains-fed side of the system rather than sealed inside the heating loop, and closing the stopcock has genuinely stopped feeding it.
Listen too. A faint trickling or hissing sound somewhere in the house with everything definitely switched off and the stopcock closed points toward a leak that's still active from stored water — a header tank in the loft, or the heating system itself, which the stopcock doesn't touch. This single test doesn't locate the leak precisely, but it reliably narrows down which side of the system it's on, which is most of the work before a plumber even arrives.
Examiner's note: the stopcock test doesn't find the leak. It tells you which half of the house to start looking in — that's still real progress.
The water is going somewhere you can't see — that's what makes it hidden. Stop topping up repeatedly and get it looked at instead.
A sealed heating system loses water only if it's escaping somewhere, and a slow weep at a joint under a floor, behind a wall, or inside the boiler casing itself can run for weeks without a single visible drop appearing anywhere obvious. The pressure gauge is often the only witness, quietly telling the same story every few days: topped up, dropped again, topped up again.
Topping up repeatedly without investigating doesn't just delay finding the leak — it actively adds fresh, oxygen-rich water to the system each time, which can accelerate corrosion at exactly the joints and radiators that are already weakest. If you've topped up more than once or twice in a short period, that pattern on its own is worth reporting, even with nothing visibly wet anywhere in the house.
Examiner's note: "topped up again" written in a diary three times in a month is itself the evidence. You don't need to see the water to prove the leak.
Yes — a failed gutter, cracked render, or a blocked cavity can all mimic a plumbing leak on an external wall. The stopcock test helps rule your own pipework in or out.
Damp on an external wall, particularly on the side of the house that takes the worst of the weather, is genuinely ambiguous without more information. Rainwater finding its way in through a damaged gutter, a crack in the render, or a blocked cavity tray behaves very similarly to a leaking supply pipe from the outside — a persistent damp patch that seems to track with wet weather rather than water usage is a hint toward the building fabric rather than the plumbing.
This is exactly where the stopcock test earns its keep: if closing the stopcock and monitoring for a day makes no difference to the damp patch at all, plumbing becomes a less likely cause, and it's worth having the outside of the wall checked — guttering, render, flashing — rather than continuing to search indoors for a pipe that was never the problem.
Examiner's note: ruling something out is still an answer. "Not plumbing" saves you paying a plumber to confirm what a roofer could have told you for free.
A damp patch or discolouration on a wall or ceiling that doesn't dry out, a musty smell in one particular area, boiler pressure that keeps sliding down between top-ups, a warm patch on a floor with underfloor heating, or the faint sound of running water when every tap in the house is off. Any one of these on its own is worth a look; two or more together is worth a call.
Persistent damp from an ongoing leak is exactly the condition mould needs, so yes — it's worth taking seriously beyond just the plumbing fault. Fixing the leak is the first step; drying the area properly afterwards and treating any mould that's already appeared is the second, and in bad cases that second step is worth its own separate attention.
Yes — a damp patch on an external wall can be a plumbing leak, but it can equally be rainwater getting in through a failed gutter, a cracked render, or a blocked cavity, none of which a plumber's stopcock test will catch. If the stopcock test rules out your own pipework, it's worth having the outside of the wall checked too before assuming the leak has somehow stopped.
It's going somewhere you can't see, which is precisely what makes it a hidden leak rather than an obvious one — commonly a weeping joint under a floor, behind a wall, or inside the boiler itself. Repeatedly topping up the pressure without investigating just delays finding it, and introduces fresh oxygenated water that can accelerate corrosion elsewhere in the system.
The main page — how the line works and the areas it covers.
Go to home →The first five minutes, in the right order.
Open the guide →Pressure, error codes, oil systems — and the gas rule to know cold.
Open the guide →What to try yourself, and when it's NI Water's sewer instead.
Open the guide →How charging works and the questions to ask before work starts.
Open the guide →Pressure, controls, immersion, oil tank — the checklist in order.
Open the guide →Gentle thawing done properly, and the lagging that prevents it all.
Open the guide →Ring any hour and describe what you've noticed — a local plumber can take it from a hunch to a diagnosis.
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