Damp and moisture meters
Black spot mould is only behind a wardrobe on an outside wall. Why is that often a cold-surface and air problem before it is rising damp?
The black spot mould behind the wardrobe is first a cold-surface and still-air problem, not a diagnosis of rising damp. The wardrobe keeps air off a cold outside wall, so moisture from the room can settle there. That pattern is on the surface. It is not a tide mark and it is not salts, and it does not show a failed damp-proof course. A moisture meter is only an aid and does not name where the water came from.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
Outside walls run colder than partitions, and furniture makes the hidden patch colder still. Black spot mould in that shadow fits condensation on a cold face with too little air. Rising damp, if it is really suspected, is a different pattern at the base of the wall. One cupboard does not establish it, and the survey should not borrow the mould as proof.
Start with ventilation and the cold face before anyone talks of injecting the wall. The survey can record where the mould was. It cannot run the house through a winter. If the base of the wall is a separate worry, keep it separate. Salts or a bridged course would be their own reasons to investigate, not this mould rewritten.
What should you ask your surveyor?
- Was the mould only behind the wardrobe, or was there a low tide mark as well?
- Is this an outside wall, and was the rest of that wall free of the same mould?
- Are you describing a cold surface, rather than a failed damp-proof course?
What are the typical next steps?
- Treat the mould as a cold-surface and air problem until a different pattern is shown.
- Do not commission an injected damp-proof course from this patch alone.
- Note how the room is heated and ventilated before you blame the base of the wall.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If the mould wipes off, has the damp-proof course failed?
No. Surface mould behind a wardrobe fits room air on a cold wall. Wiping it away does not test the damp-proof course.
Can a meter on that plaster prove rising damp?
No. A moisture meter is only an aid. A high reading on a cold, mouldy surface does not name rising damp as the cause.