Damp and moisture meters
Why might a damp patch appear only after the heating has been off for about a week?
Because a week without heat lets a wall go cold, or lets a slow weep show that normally evaporates. The delay is a clue, not a damp category. It is not proof of rising damp. One viewing, heated or cold, cannot settle which mechanism you have.
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?
Heating masks things. Condensation on a marginal wall may only bloom once the heating has been off and the surface has gone cold. A very small plumbing weep may dry between firings and only soak the plaster after a still, cold week. Rising damp does not wait for the boiler to be switched off. If the occupier says the patch arrives only after holidays with the heating off, write that history down and still do not promote it into a failed damp-proof course. Ask where the patch sits: mid-wall, at a pipe, or at the skirting.
The survey is one day. It does not turn the heating off for a week and come back, and it does not guarantee concealed pipes. If the house was heated for the visit, a patch the seller described may be absent, and the report should not claim to have seen it. If the house was cold and the patch was there, the report should not claim it is groundwater. Use the position and any pipe, vent or cold external face. Those details decide whether the next person is a plumber or someone thinking about ventilation.
What should you ask your surveyor?
- Where does the patch sit when it appears: by a pipe, on a cold wall, or at the skirting?
- Was the heating on or off at your visit?
- Are you reporting the occupier's timing, or a cause you could see?
What are the typical next steps?
- Do not call a patch that follows a cold week rising damp.
- Write down where it sits. Position matters more than the story.
- If it was not present on survey day, do not invent a category for it.
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
Does a patch that fades when the heating is on prove condensation?
It suggests surface moisture that heat clears. A slow leak can fade the same way. The position tells you more than the fading.
Should you ask the seller to leave the heating off before a second look?
Only if a second look is agreed. The original survey does not include it.