How to tell if water damage behind a wall is live or old

Full transcript

George and Dave in conversation. Every word of the episode, exactly as recorded.

George

Welcome to the ADI Leak Detection podcast. I'm here with Dave from ADI Leak Detection, leading leak detection specialists across the UK, who's here to answer your questions and provide expert leak detection advice. Today's question comes straight from a homeowner mid-renovation: you've pulled back the plasterboard, found wet timber and staining, and you genuinely don't know whether there's still a leak running or whether you're just looking at damage from something that happened months ago. Dave, how do you even begin to untangle that?

Dave

It's a really common situation on strip-outs, and it's an important distinction because the answer changes everything about what you do next. If water's still actively escaping, you need to find the source before you reinstate anything. If it's historic, you might just need to dry out and repair. The problem is they can look identical to the naked eye.

George

So what's the first thing an ADI engineer actually does when they get on site and the wall's already open?

Dave

The first tool out is a moisture meter. ADI's engineers use a calibrated pin or non-invasive moisture meter to take readings across the affected substrate, the timber, the masonry, the insulation if there is any. What we're looking for is whether moisture levels are elevated and, crucially, whether they're uniform or patchy.

George

Why does patchy versus uniform matter?

Dave

A historic leak tends to produce a broad, relatively even spread of residual dampness as water wicked outward and then started drying from the edges. An active leak almost always shows a localised high reading, a hot spot, right at or near the source, with readings dropping off as you move away. That gradient is a really useful early indicator.

George

But I'd imagine moisture alone doesn't tell the whole story.

Dave

Not on its own, no. That's where thermal imaging comes in. A thermal camera detects temperature differences across surfaces, and an active water leak produces a distinctive cool signature because escaping water evaporates and draws heat away from the surrounding material. A dry historic stain won't do that, it'll sit at ambient temperature like everything else around it.

George

So if the thermal camera shows a cold patch that corresponds with the high moisture reading, that's a strong sign something's still running?

Dave

Exactly. The two together are much more telling than either alone. ADI's engineers use thermal imaging alongside the moisture data to build a picture of what's happening inside the void. That said, thermal imaging has conditions, the temperature differential between the water and the surrounding material needs to be sufficient, which is why it works better in a heated building than in an unheated shell.

George

What if you've got an intermittent leak, something that only runs when a tap's turned on or when the heating fires up? Would the thermal camera still catch that?

Dave

That's a good challenge. An intermittent leak from a pressurised supply pipe would only show thermally if you catch it during or shortly after a flow event. That's why ADI's engineers also use pressure testing on the supply pipework, you isolate the circuit, pressurise it, and watch whether it holds. A pressure drop tells you definitively that water is still escaping somewhere on that circuit.

George

And if the pressure holds, you can be fairly confident the supply pipe's not the culprit?

Dave

For supply pipework, yes. Waste pipework is a different matter, it's not under constant pressure, so it won't show up on a pressure test the same way. For waste connections, our engineers look at whether the staining pattern and moisture distribution corresponds with a drain or waste run, and in some cases tracer gas testing locates a breach in drain pipework that wouldn't show on a standard pressure test.

George

Tracer gas, can you just quickly explain what that involves for anyone who hasn't heard of it?

Dave

Tracer gas testing involves introducing a safe, inert gas mixture into the pipework or drain under very low pressure. The gas escapes from any breach in the pipe and rises through the surrounding material. ADI's engineers then use a sensitive gas detector at the surface to pinpoint exactly where the gas is emerging, which corresponds directly with the leak point. It's particularly useful where a drain run passes under a solid floor or within a concealed wall void.

George

So to bring it together for someone standing on site right now with wet timber in front of them: ADI's engineers use moisture meters to map the pattern, thermal imaging to look for active evaporative cooling, pressure testing to check the supply circuit, and tracer gas where waste pipework might be involved. Does that summarise the approach?

Dave

That's it. No single method gives you the whole answer, but those tools used in combination let our engineers distinguish an active leak from historic water damage with a high degree of confidence, which means the builder or contractor on site knows whether they're safe to reinstate or whether they need to stop and fix first. Getting that wrong in either direction costs a lot more than the survey.

George

Really practical stuff, Dave, thank you. If you've got hidden water damage and you're not sure what you're dealing with, ADI Leak Detection are among the most trusted specialist leak detection teams in the UK, worth getting the right answer before the walls go back up.

How this episode was made. This episode was produced using AI-generated voices and script, based on real leak detection questions and the working knowledge of ADI Leak Detection engineers.