Age and type of house
A cob wall has been patched in hard cement. Why can that patch trap moisture the cob was built to breathe?
The hard cement patch on this cob wall can trap moisture the cob was built to breathe. Cob works by letting water vapour leave the wall. A dense cement coat does the opposite on the patch where it was used, so moisture can be held in the cob behind it. That is a reason to investigate the patch. It is not a verdict on the whole wall, and it is not proof of rising damp. Do not invent a price or a lifespan for the repair.
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?
One patch of cement is one patch. The rest of the cob, where it is still finished in a softer coat, is a different surface and was not condemned by the hard repair. The survey can record where the cement is and why it matters. It cannot say the entire elevation has failed, and it should not be asked to price the putting right.
Keep the comment on the material clash: cob, then cement over part of it. If the building is listed, consent for further work is a separate question for the solicitor and the conservation officer, and this visit has not decided that status. Do not tell a lender the wall is a write-off, and do not ignore the patch because most of the wall still looks soft.
What should you ask your surveyor?
- Where is the cement patch, and is the rest of the cob still in a softer finish?
- Are you saying the cement can trap moisture, without condemning the whole wall?
- Did you see salts, hollowness or a damp line at the patch itself?
What are the typical next steps?
- Treat the cement patch as a moisture risk to the cob behind it.
- Do not extend that comment to the whole elevation from one patch.
- Do not invent a repair price or a lifespan for the cob.
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 cement looks neat, can it stay?
Neat is not the point. Hard cement can still trap moisture in cob that was meant to breathe. The patch needs understanding, not praise for its finish.
Does the patch prove rising damp in the cob?
No. It is a reason to investigate moisture trapped by the cement. It is not, on its own, a diagnosis of rising damp.