Mould on a wall is usually treated before it is understood. It gets wiped off, the wall is repainted, and weeks later the same dark patch is back. The mould is not the problem. It is the sign that a surface has stayed damp long enough for spores to settle and grow, and until the moisture feeding it is found, nothing painted over the top will hold.
Two causes account for most mould on internal walls in London homes: condensation, and water escaping from a pipe or getting in through the building fabric. On the surface they look alike. Where the patch sits, when it appears, how warm the wall is and what a moisture meter reads all differ. This guide goes through those differences and what a survey measures.
Why does mould grow on a wall in the first place?
Mould needs three things: spores, which are in the air of every building; something to feed on, such as emulsion, wallpaper paste, plaster or dust; and moisture. The first two are always there, so moisture is the only one you can change.
Government guidance for rented housing providers groups damp into four kinds: condensation damp, where moisture generated inside the home cools and condenses onto colder parts of the building; penetrating damp, where water enters from outside through defects in walls, roofs, windows or floors; rising damp, where ground moisture rises up through the building; and traumatic damp, meaning water from leaking waste and heating pipes, overflowing baths or sinks, and burst pipes.
That last group is the one people overlook. A weeping joint on a buried heating pipe, a failed shower tray seal on the other side of a partition, or a waste pipe dripping inside a boxed duct will keep plaster wet indefinitely, and mould follows the wet plaster.
What does the pattern of the mould tell you?
Shape and edges are the first clue, and they are free to read. Condensation forms wherever a surface is cold enough for moisture in the air to settle on it, so it follows the cold parts of the room. Water from a leak comes out of one point, so it has a centre.
| What you see | Points to condensation | Points to a leak |
|---|---|---|
| Shape of the patch | Diffuse, speckled, often several patches in one room | One patch with a denser middle, fading outwards |
| Where it sits | Corners, behind furniture, window reveals, ceiling edges, external walls | Along a pipe run, below a bathroom, beside a soil stack, at a radiator tail |
| Surface condition | Paint intact, mould sitting on top of it | Blown plaster, bubbling paint, a tide line, salts on the surface |
| Timing | Worse in winter, better in summer, worse after cooking or showering | Present all year, or tracks the heating season on a heating pipe |
| Other rooms | Similar patches in other cold rooms, and on windows | One room only, often one wall only |
None of these is decisive alone. A leak on a cold external wall produces condensation as well, so a room can show both patterns at once. They are the reason to take readings, not a substitute for them.
Where on the wall is it, and why does that matter?
Position is the strongest clue in a normal flat or house, because the two causes are drawn to different places.
- Room corners where two external walls meet. The coldest part of the wall surface, and the classic home of condensation mould.
- Behind a wardrobe or sofa against an external wall. Air cannot circulate, the surface runs colder, and mould grows on the wall and on the furniture.
- A vertical band running floor to ceiling. That is the shape of a pipe chase or a soil stack, not the shape of a cold surface.
- A patch on an internal partition, away from any outside face. Internal walls are rarely cold enough for condensation, so a wet one usually means water is getting into it.
- A patch that matches a bathroom, kitchen or shower on the other side. Worth checking before anything else.
Our guide to the signs of a leak inside a wall covers what else shows up when a pipe is the cause.
What do surface temperature and humidity readings show?
This is where guessing stops. Condensation is a temperature problem before it is a water problem. When air touches a surface below its dew point, moisture comes out of the air and onto that surface. So the survey asks one question: is this wall cold enough, given the air in this room, for condensation to form on it? Three measurements answer it.
- Air temperature and relative humidity in the room, which give the dew point temperature for that air.
- Surface temperature of the wall at the mouldy patch, read with a thermal camera or a contact probe.
- Moisture content of the material, taken with a non-invasive meter and, where appropriate, a pin meter at depth.
If the wall surface sits at or below the dew point of the room air, condensation is expected there and no leak is needed to explain it. If the surface is warmer than the dew point and the material is still wet, the water is coming from somewhere else. A wall that reads wet several centimetres into the plaster, rather than only at the face, also points away from condensation, because condensation wets the surface first. Readings are taken across a grid, so the result is a map: a leak has a wettest point, while a cold wall usually just has a cold half.
What is a cold bridge, and how is it different from a traced pipe run?
A cold bridge, or thermal bridge, is a part of the structure that conducts heat out faster than the material around it: concrete lintels, steel beams, balcony slabs passing through the wall, and floor to wall junctions. On a thermal image it shows as a cool line or block that follows the building, and it stays put whether the heating is on or off.
A leaking pipe looks different. A heating pipe leaking hot water shows as a warm line with a cooler damp halo around it, and it changes when the heating is switched on and off. A cold water or waste leak shows as a cool area that matches no structural feature, and it will usually still read wet hours after the room has warmed up, because evaporation from a wet wall keeps it cool.
That is why thermal imaging is used alongside moisture readings rather than on its own. The camera shows temperature, not water, so it needs the meter readings next to it before anyone can say what a cool patch means.
What does a damp and mould survey actually measure?
A survey on a mouldy wall works from the room outwards.
- The environment. Temperature and relative humidity in the affected room and in a control room, plus the calculated dew point.
- The surface. Thermal images of the wall and surrounding structure, to separate structural features from anomalies.
- The material. Non-invasive and pin moisture readings across a grid, at more than one depth where the construction allows.
- The building. External inspection for cracked render, failed pointing, blocked gutters, damaged flashings and defective seals.
- The services. Where readings point at plumbing, a check of the pipework in that area, which can include isolating and pressure testing the section, or tracer gas if the pipe is buried.
- The ventilation and heating. Whether extract fans work, whether trickle vents are open, and whether the room is heated evenly.
Doing all of that in one visit matters because the answer is often mixed. A cold external wall in a poorly ventilated bedroom can be pure condensation. The same wall with a leaking pipe in it is both, and treating only the ventilation leaves a wall that never dries.
What should you do next?
- Photograph the patch with something for scale and note the date. Repeat every couple of weeks so you can see whether it is spreading.
- Write down when it is worst. After showers, in cold weather, when the heating runs, after heavy rain. Each points somewhere different.
- Move furniture off the wall so you can see the full extent and let air reach it.
- Check the other side of the wall, the room above and the room below, including any boxed pipework.
- Check the outside face for cracked render, an overflowing gutter, a leaking downpipe or missing pointing at the same height.
- Do not paint over it or apply an anti-mould coating before the cause is known. It hides the evidence a survey needs and the patch returns anyway.
Mould is not something to live with while you work through this. Government guidance is clear that damp and mould mainly affect the airways and lungs and can cause or worsen conditions such as asthma, and that people with existing respiratory problems, weakened immune systems, young children and older adults are more at risk. The same guidance says it is unreasonable to put damp and mould down to lifestyle, and points instead to building deficiencies, poor ventilation, inadequate heating and poor energy efficiency.
When should you call a specialist?
Call when the mould keeps returning after cleaning, when it sits on an internal wall, when the plaster is blown or the paint is bubbling, when the wall is wet in summer as well as winter, or when it lines up with a bathroom or a pipe run. Also call when you need the cause written down for a landlord, an agent, a freeholder or the flat above.
What a leak detection survey does is find and document the water. We take the readings, trace the source, and set out what is making the wall wet. We do not take laboratory samples or air samples, we do not carry out mould removal, and we do not issue any form of mould clearance certificate. Where the cause turns out to be a building defect or ventilation rather than a leak, the report says so and a builder, roofer or ventilation installer takes it from there.
Our damp and mould survey is £250 to £450, and a thermal imaging survey on its own is £150 to £250. Our guide to telling damp from a leak covers the wider picture, including rising and penetrating damp. Call 020 3912 0200 with your photographs and notes to hand.
Frequently asked questions
Is mould always a sign of a leak?
No. Condensation on cold surfaces is the most common cause of mould in homes. But a leak produces mould too, and the two are told apart by where the patch sits, how wet the material is below the surface, and whether the wall is cold enough for condensation to form on it.
How can I tell condensation mould from a leak myself?
Look at pattern and position. Speckled mould in corners, behind furniture and around windows, worse in winter, fits condensation. A single patch on an internal wall, next to a pipe run or below a bathroom, with blown plaster, fits a leak.
Why does mould keep coming back after I clean the wall?
Because cleaning removes the growth but not the moisture. Whether that moisture is condensation on a cold surface or water from a pipe, it is still there after the wall is wiped, so the patch returns in the same place.
Can mould grow from a leak that has already been repaired?
Yes. Plaster and masonry hold water long after the source is fixed, and a wall can stay damp enough to support mould for weeks while it dries. Readings taken over time show whether it is drying out.
Does black mould mean something different from other mould?
Colour is not a reliable guide to the cause, and identifying a species needs laboratory work we do not carry out. What matters is the same either way: find the moisture, stop it, then deal with the growth.
