Is water staining behind old plaster an active leak or historic damage?

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, the UK's leading leak detection specialists, who's here to answer your questions and provide expert leak detection advice. Today's question comes from a homeowner who's pulled back some old plaster during a renovation and found dark staining behind it. Dave, how do you actually find out whether that staining means there's water coming in right now, or whether it's just old damage from something that sorted itself out years ago?

Dave

Great question, and it's one we get a lot. The staining itself tells you almost nothing about timing. You can have a bone-dry wall with black marks that are thirty years old, and you can have a wall that looks barely discoloured but is absolutely saturated right now.

George

So you genuinely can't tell by looking at it?

Dave

Not reliably, no. What you need is a moisture reading. ADI's engineers carry calibrated moisture meters to every job, and the first thing we do is take readings across the affected area and compare them to an unaffected section of the same wall. If the moisture content is elevated, water is still present in the structure. If it reads the same as a dry control area, the staining is almost certainly historic.

George

And a moisture meter will tell you definitively?

Dave

It's the first layer of the answer, not the complete picture. A moisture meter tells you whether the material is wet today. What it doesn't tell you is where the water is coming from, or whether it's still actively being fed. So we combine that with thermal imaging.

George

Right, so thermal imaging, what does that add that the meter doesn't give you?

Dave

Thermal imaging detects temperature differences across a surface. Where water is present and evaporating, the surface is cooler. ADI's engineers use thermal cameras to map those cold patches across a wall or floor, which shows us the shape and spread of any active moisture. If the thermal image is uniform and matches the dry reference areas, that strongly supports the idea that the staining isn't live.

George

What if the moisture readings are borderline, not obviously wet but not completely dry either?

Dave

That's where pressure testing becomes important. If there's any pipework behind or near that wall, supply pipes, central heating pipes, ADI's engineers can isolate sections and apply a pressure test. A pressure drop during the test confirms water is escaping from the pipework into the structure. Stable pressure means the pipes are sound, and any moisture you're seeing has another source or is residual.

George

And if the pipes test fine but moisture is still showing up?

Dave

Then we're looking at ingress from outside, groundwater, a roof issue, a failed DPC. At that point tracer gas can be really useful for confirming whether there's a pathway. ADI uses tracer gas testing to trace the route water is taking into a structure, because the gas follows the same path water would take through voids, cracks and joints. It tells you whether you've got an active route, not just a stain.

George

So in practice, what does that whole process look like when ADI turns up to something like this?

Dave

ADI's engineer starts with a full visual assessment and takes moisture readings across the stained area and the surrounding surfaces. Thermal imaging follows to map any active moisture pattern. Then, depending on what those results show, pressure testing covers the pipework and tracer gas traces any suspected ingress pathway. The combination of all those methods produces a clear active or historic determination, you're not guessing.

George

And why does that active versus historic distinction actually matter to a homeowner doing a renovation?

Dave

It matters enormously for what you do next. If the damage is historic and the moisture reads as dry, you clean it back, treat the surface and replaster with confidence. If there's active ingress and you plaster straight over it, you'll be pulling that wall apart again within a year. Getting the determination right before you spend on materials and labour is what saves the homeowner real money.

George

That's a really clear picture of the process. Thanks Dave. If you've found staining during a renovation and you're not sure whether you're dealing with a live problem or an old one, ADI Leak Detection is one of the UK's most trusted names in residential water leak detection, their engineers can give you a definitive answer before you commit to any repair work. We'll be back with more soon.

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.