The count, and the pattern inside it
| Outcome | Investigations |
|---|
| No plumbing leak found | 5 |
| Pipework sound, fault in a boiler component | 1 |
| A leak found | 18 |
| Not established on the visit | 1 |
| Total | 25 |
The interesting part is not the five. It is what the five had in common. Every one of them was reported as damp, mould, a stain or a noise. Not one of them was reported as visible running water, an active drip, or water coming through a ceiling.
That is a small sample and we are not claiming it as a law. But it matches the physics, and it is the most actionable thing in this dataset: the symptom you describe when you book largely determines how likely it is that there is anything for a plumber to fix.
The five
Mould in several rooms of a flat
Mould inside a cylinder cupboard, behind kitchen units, behind a washing machine and behind a dishwasher. Both bathrooms were inspected with a torch and a moisture meter and were dry. Hot and cold were isolated and pressure tested for a minimum of fifteen minutes each, with no drop on either. The waste system was tested by running each appliance and watching the connections. Nothing escaped anywhere. The conclusion was historic moisture rather than an active fault, with a recommendation for controlled drying and remediation.
Damp reported for years
A property where damp had been reported over a long period and previous engineers had attended without identifying a source. The hot circuit was isolated and pressure tested, and held. The cold was isolated and tested, and held. The heating circuit was tested at the occupier's request because they reported occasional pressure reductions, and it held too. Tracer gas was then introduced into all three circuits in turn, and nothing was detected escaping from any of them. The recommendation was a damp assessment and an external fabric survey, because whatever is causing that damp is not the plumbing.
A water company notice with nothing behind it
An extra 168 litres a week, investigated over three visits with visual inspection, pressure testing, thermal imaging, an inspection camera under the bathroom floor and tracer gas on the cold system. Every test negative. The external stopcock turned out to serve two flats, and a continuous waste discharge from the property above accounted for the volume. The investigation did also find serious historic water damage: swollen floor tiles, saturated joists and a section of bathroom floor that had sunk and would give way under load. Real damage, from leaks that had already stopped.
Damp marks with an endoscope behind them
Damp marks investigated with an endoscopic camera through small openings. No sign of an active leak. Recorded as either a past leak already resolved, or ingress through an external wall.
A dripping sound behind a wall
A constant dripping or running-water sound reported as coming from a concealed cistern, with the occupier's reasonable assumption that something inside it was leaking. The cistern was inspected and tested, the toilet operated, and no water was escaping anywhere. The noise was condensation forming inside the concealed cistern and running down its internal face.
The sixth: sound pipework, failed component
One more deserves its own line because it is the classic false positive. A heating system would not hold pressure: every time it was topped up, the gauge fell back within minutes. A full visual inspection of radiators, valves, joints and accessible pipework showed nothing. The system was drained, tracer gas introduced, and the property surveyed. Nothing. The test was repeated a second time to be sure. Still nothing.
The system was then drained again, vented and re-pressurised, and at that point water discharged continuously through the boiler's pressure relief valve. A system that cannot absorb the expansion of its own water will do exactly that, and the usual reason is an expansion vessel that has lost its charge or become waterlogged. The job was referred on to a Gas Safe registered engineer for the vessel and the relief valve, and no pipework repair was needed.
If your pressure drops and nothing anywhere is wet, read the expansion vessel article before you book detection. Two negative tracer gas runs is an expensive way to reach the same conclusion.
Why damp so often is not a leak
A plumbing leak is one of at least five things that produce damp inside a London property, and it is the only one a leak engineer can fix:
- Condensation. Warm moist air meeting a cold surface. It appears on external walls, in corners, behind furniture, in unventilated cupboards and, in one of these jobs, on the inside face of a concealed cistern. It is the single most common cause of mould in a flat.
- Penetrating damp. Water crossing the building fabric from outside, through failed pointing, a cracked render, a blocked gutter or a defective sill. It is usually worse after rain, which is the tell.
- Drainage defects. Waste and soil pipes are not pressurised, so they only leak when something upstream is used. One job in this set traced widespread damp across two floors to a hole in an external soil pipe that only discharged when the bathroom above was used, and another to a blocked gully soaking the masonry.
- A historic leak. Already fixed, or stopped on its own, with the substrate still holding moisture. Two of the five above were recorded this way, and one of them had left a structurally unsafe floor behind.
- An actual leak. Which is why the investigation happens.
A pressure test on each circuit and a tracer gas run is how you take the last of those off the list. Once it is off, the remaining four are a damp surveyor's work, not ours, and saying so is part of the job.
What a no-leak result gets you
People assume a negative is a wasted visit. It is not, for three reasons.
First, it is documented. The report lists each circuit, how long it was held, at what pressure, and what did not happen. That is the evidence a managing agent, a landlord, an insurer or the next trade needs to stop treating the plumbing as the suspect. In the case with years of unexplained damp, previous engineers had attended and no one had written down what they had ruled out, which is why it kept being investigated.
Second, it redirects the money. Drying, remediation, ventilation and fabric repair are the correct spend on a condensation problem, and none of it happens while the owner is still paying people to look for a pipe.
Third, you do not pay for it. Our rate is £150 per hour, agreed before we attend, and under no find, no fee you do not pay for the time spent looking if we do not find the leak. What that does and does not cover is set out in full here, and it is worth reading, because the phrase is used loosely in this trade.
When not to call us
Call a damp surveyor rather than a leak engineer if all of the following are true: the damp is on an external wall or in a corner, it is worse in winter or after rain, no fitting anywhere is dripping, your boiler holds pressure, and your meter does not move with everything turned off. That combination describes condensation or penetrating damp, and no amount of pressure testing will improve it.
Call us if any one of those is not true. A moving meter, a falling boiler gauge, damp that tracks bathroom use, or a warm patch on a floor are all reasons to rule the plumbing out properly rather than assume.
Limits
Five no-leak outcomes out of twenty-five investigations, in one firm's London caseload over nine weeks between 17 July and 18 September 2026, classified by hand from the written reports. Five is a very small number and the proportion would move considerably on another twenty-five jobs. "No leak found" also means no leak found on that day by those tests: in one of these, the occupier reported occasional pressure reductions that could not be reproduced under test conditions, and the report says so rather than pretending otherwise. The full method and exclusions are in the dataset post.
