A damp patch and no idea where it is coming from? Find it before lifting a single board

Detection is a process of elimination. A property has several separate water systems: the cold mains, the hot water, the central heating and the wastes, plus rainwater from outside. The first job is to work out which one is leaking, by isolating and pressure testing each in turn. Then the leaking system is traced along its route with equipment that sees or hears through the building fabric, until the wet point is pinned down to a small area. Only then is anything opened up, and only there.

A boiler that keeps losing pressure with no visible drip. A damp patch on a wall or ceiling that dries and returns. Skirting or flooring that has lifted or gone soft. A warm area on a floor with no heating pipe you know of. Mould in a corner with no condensation to explain it. A water meter that turns when everything is off. A bill that has jumped. A faint hiss at the stopcock at night. A musty smell in one room. None of these is proof on its own, and all of them are worth a survey before they become a ceiling on the floor.

Thermal imaging reads the surface temperature of floors and walls. A leaking heating pipe shows as a warm plume; a mains leak shows as a cold patch where evaporation is cooling the surface. Acoustic listening uses ground microphones to hear the noise of water escaping under pressure, which is the main tool for mains leaks under solid floors. Tracer gas is used when the others cannot reach: the pipe is drained, filled with a harmless hydrogen-nitrogen mix, and a detector finds where the gas rises through the floor. Moisture mapping with capacitance and pin meters outlines the wet area and its wettest point. Endoscope cameras look inside voids through a small drilled hole. Dye testing confirms wastes, shower trays and bath seals. Several methods are used on each survey and the results cross-checked before anything is opened.

Most home and buildings insurance policies include trace and access cover: the cost of finding a leak and of making good what has to be opened up to reach it. What the insurer needs is a proper report: the symptoms, the methods used, the findings, photographs and thermal images, the location of the leak and the recommended repair. The survey is written up in that form, so a claim is straightforward. The repair of the leaking pipe itself is normally not covered by insurance, and that is quoted to you separately and honestly.

Once the source is known, the repair is usually small: a joint remade, a section of pipe replaced, a valve changed, a shower tray resealed. The access is kept to the minimum, one or two floorboards or tiles rather than a whole floor, and the opening is left ready for making good. Where the leak is on a pipe buried in a concrete screed and the pipe is old, the honest advice is sometimes to abandon that run and re-route it in an accessible place rather than patch a pipe that will fail again a metre further on.

The report from the survey is written for the claim: what was found, where, how, with the evidence. Where the insurer needs a repair quote, one is provided. Where the leak is in another flat or in a communal part of a building, the report says so clearly, because who pays depends on where the pipe is, and a clear finding avoids a long argument between neighbours and managing agents.

Water pouring through a ceiling is an emergency first and a detection job second. The emergency visit isolates the supply, makes the property safe and stops the damage getting worse. The detection survey follows in daylight with the equipment, because tracing a leak at midnight with a torch is how floors get lifted for nothing. If the emergency has already been dealt with by someone else, the survey can be booked directly.

In London housing the usual suspects are: compression joints on heating pipes under floors that have worked loose over decades of expansion; copper heating pipes chased into concrete screed and corroded from the outside; push-fit joints that were never fully inserted; shower trays and bath seals that only leak when someone is standing in them; waste pipes with a joint that has dropped; toilet pan connectors; the mains pipe between the street and the stopcock, often still lead in older properties; and pinholes in the bottom seam of old radiators. Flat roofs, gutters and window seals account for a fair share of "leaks" that are not plumbing at all, and the survey rules those in or out too.