Damp and moisture meters

Does a tide mark on a wall prove the damp-proof course has failed?

No. A tide mark is a stain line. It can come from moisture rising from the ground, from salts left after an old leak, or from water that ran down and stopped. A home survey can record the line and a meter reading. It does not, by itself, prove the damp-proof course has failed. The Property Care Association treats damp as a diagnosis of the building, not a mark on the plaster.

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?

Rising damp, penetrating damp and condensation can all leave a mark a buyer would call a tide. The useful questions are where the mark sits, whether the outside ground or the render bridges the course, and whether the plaster has been replaced. Government guidance on damp and mould is about finding the cause. It does not say a horizontal stain is a failed course, and it does not set a price cut.

Ask the surveyor what else they saw: salts, soft skirtings, a high path, a leak above. If the only evidence is the line, do not buy a chemical injection from that line. There is no RICS tariff for a new damp-proof course. A specialist who opens the wall is a different instruction from the home survey.

What should you ask your surveyor?

Where does the mark stop, and what is above it?
Did you see bridging outside, or only the stain inside?
Are you diagnosing a failed damp-proof course, or recording a pattern?

What are the typical next steps?

Do not instruct a damp-proofing firm from the photograph alone.
Compare the mark with the outside ground level before you negotiate.
If you need the wall opened, agree that with the seller. The survey did not do 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

Can hygroscopic salts leave a tide after the wall has dried?

Yes. Salts can keep a meter high and a plaster mark visible. That is why a mark is not a live leak.

Is a tide mark at picture-rail height rising damp?

Rising damp is looked for near the base. A line high up needs another explanation. Do not rename it.