Read the shape first
Almost every ceiling we are called to has already been cleaned two or three times. The useful information is not the mould itself but where it sits and what edge it has, and that is visible before anyone touches anything.
| What you are looking at | Condensation | A leak |
|---|
| Outline | Diffuse speckling that fades out with no clear boundary | One patch with a hard edge, often close to circular or following a pipe run |
| Position | Perimeter, corners, and in parallel stripes where the joists cold-bridge the plaster | Anywhere, frequently mid-ceiling or ringing a downlight |
| Colour underneath | Paint colour, clean, no staining | Brown, yellow or grey tide staining under or around the growth |
| Season | Worst from November to March, recedes in summer | The same all year, or worse when a particular appliance is used |
| Room | Bathrooms, kitchens, cold or unheated bedrooms, north-facing rooms | Any room, most often directly below a bathroom, kitchen or airing cupboard |
| After cleaning | Returns slowly and slightly larger each winter | Returns in the same outline, sometimes within a fortnight |
| Touch | Cool and paper-dry | Cool and slightly soft, and the plaster may bulge or feel spongy |
The two checks worth doing before you call anyone
Press the middle of the patch with a fingertip. Condensation-affected plaster is hard. Plaster that has been wet for a while gives very slightly, and plasterboard that has been wet for months feels like stiff card. If it gives, this is not a ventilation problem.
Look for a tide line. Water spreading through a plasterboard ceiling deposits dissolved salts and dirt at the edge of the wet area as it dries, which leaves a fine brown rim. Condensation leaves nothing behind but the growth. A rim means water arrived from somewhere and then stopped, which also tells you the leak may be intermittent rather than continuous.
One pattern deserves its own warning. Mould in a neat ring around a downlight almost always means water is tracking across the top of the ceiling and escaping through the hole cut for the fitting, because that hole is the lowest point available to it. Do not poke at a wet light fitting. Turn that circuit off at the consumer unit first, and if there is water actually sitting in the fitting, treat it as an emergency rather than a decorating problem.
What is directly above the patch
Another flat
If you are below someone else, the cause is nearly always their bathroom: a shower tray, a waste connection, or the seal where the tray meets the tiling. Tell them in writing and keep a copy, photograph the ceiling with the date visible, and read what to do about a leak from the flat above before the conversation turns into a dispute about who pays.
A bathroom, kitchen or airing cupboard of your own
This is the most common and the most findable. The shortlist is short: the shower waste, the bath waste, the seal around the tray or bath, a basin trap, the washing machine or dishwasher supply, or a heating pipe crossing the void. Each of those can be isolated and tested one at a time, which is how a detection visit works.
A roof or loft void
If the patch sits under a roof, this is probably not a plumbing job. Rain-driven ingress moves with the weather and appears during or shortly after heavy rain from a particular direction. That is a roofer, and we will say so rather than charge you to find out. The exception is a cold-water tank, its overflow, or heating pipework in the loft, all of which behave like a plumbing leak and do not care about the weather.
An unheated void and nothing else
Mould over a cold void with no water anywhere near it is a cold bridge, usually a gap in the insulation or a poorly draught-sealed loft hatch. The fix is insulation and ventilation, not detection.
The condensation cases you can fix yourself
If everything above points at condensation, do not spend money on an engineer. The National Residential Landlords Association separates condensation from rising damp and penetrating damp by the mechanism: condensation forms when, in its words, moist warm air meets a colder surface such as a window or external wall, while penetrating damp comes through the structure from damaged roofs, blocked guttering, cracked brickwork or defective pointing.
- Run the extractor for fifteen minutes after every shower, and check it actually moves air by holding a sheet of paper to it. A fan that cannot hold paper is doing nothing.
- Close the bathroom door while showering rather than opening it to let the steam out into cooler rooms.
- Keep unused rooms above about the same temperature as the rest of the flat. Cold rooms collect the moisture generated everywhere else.
- Dry washing outdoors or in a vented tumble dryer. An indoor airer is one of the largest single moisture sources in a London flat.
- Leave a gap behind wardrobes and beds on external walls so air can move across the plaster.
Clean the growth with a proprietary mould treatment rather than bleach on a porous surface, and repaint only once it has stayed clear through a full winter.
If you rent
The Homes (Fitness for Human Habitation) Act 2018 inserted new sections 9A to 9C into the Landlord and Tenant Act 1985, and the matters considered when deciding whether a dwelling is unfit include freedom from damp along with repair, ventilation, water supply and drainage. The practical effect is that the cause has to be identified rather than the mould simply painted over. The NRLA tells its own members the same thing, and lists plumbing leaks explicitly, describing slow leaks behind walls, under sinks or from poorly sealed bathrooms saturating plaster and timber for months before anyone notices.
Report it in writing, photograph it with something showing the date, and keep the thread. If a landlord or managing agent sends a decorator and the patch comes back in the same outline, that record is what moves the conversation on.
When to stop cleaning and have it looked at
- The patch has a defined edge and a stain rather than a diffuse spread.
- The plaster gives when you press it, or it has bulged.
- It reappears in the same shape within a month of being treated.
- It is directly below a bathroom, kitchen or airing cupboard.
- Mould is ringing a light fitting.
- It is no better in August than it was in February.
What finding it involves
A ceiling is the easiest surface in a property to survey and the most expensive one to open blind. The sequence is moisture mapping across the whole ceiling to find the wettest point rather than the most visible one, thermal imaging to follow pipe runs and to show the temperature difference where water is evaporating, and then isolation testing: each appliance and each circuit is run and stopped in turn while the moisture readings are watched. Where the pipework can be drained, tracer gas finds what nothing else will, because it will come up through a plaster ceiling at the exact point of loss.
The reason that matters is the difference between opening one 300mm square in a ceiling and taking a whole ceiling down. And once it is found, we repair the pipe and then put the ceiling back: reboard, skim, and paint. Most firms stop at the first half of that. Ceiling leak repair covers the whole job.
What it costs
Detection is charged at our hourly rate of £150, agreed before we attend, and if we do not find the leak you do not pay for the time spent looking. Repair and making good are quoted separately once the cause is known. There is more detail on pricing, and if the ceiling is actively dripping rather than stained, start with water coming through a ceiling instead.
