Age and type of house
A cob wall has been coated in hard cement that is map-cracked. Why is that coat a moisture risk to the cob, and what mix of the wall was not sampled?
The hard cement is a moisture risk to the cob because it holds water against a wall that needs to dry, and the mix of that wall was not sampled. The coat is map-cracked. Cob is an earth wall. A dense cement skin is less able to let moisture leave than the cob behind it, and cracks in the skin are routes for rain to get in and then stay. The survey can see the map cracking and name the mismatch. It did not cut a sample, did not analyse the earth mix, and did not measure how wet the cob is behind the coat.
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?
Map cracking is a pattern of fine cracks in a coat that is too rigid for the softer wall under it. On cob, that pattern is a warning about moisture as well as about the finish. Water that passes a crack meets earth and straw, then finds the cement slow to let it out. The wall can soften while the coat still looks largely in place. The inspection did not open the coat and did not send the cob to be described.
Ask a specialist in earth walls what should happen to the cement coat, and do not scrape or break it off on the strength of this report. The solicitor can ask whether any previous cob repair is documented. A moisture meter, if one is later used, is still not a sample of the mix. The survey recorded a cement coat on cob. It did not test what the wall is made of.
What should you ask your surveyor?
- Where is the cement coat map-cracked, and what cob was actually visible?
- Did you sample the wall mix, or only describe the coat?
- What moisture signs, if any, did you record without opening the cob?
What are the typical next steps?
- Treat the cement coat as a moisture risk to the cob, not as a sample.
- Ask an earth-wall specialist before anyone alters the coat.
- Ask the solicitor whether any earlier cob repair is on paper.
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 is painted, does the paint seal the risk?
Paint on the cement does not remove the mismatch, and it was not a sample of the cob. The survey saw a hard coat that is map-cracked. Paint may slow rain on the face. It does not let the earth wall dry, and it does not describe the mix behind the cracks.
Can the map cracks be filled and the cob left unexamined?
Filling is a decoration of the cement, not an examination of the cob, and the survey has not specified it. The mix of the wall was not sampled. A filled crack in a hard coat can leave the same moisture trap in place. The report identified the risk. It did not design a filler or test the earth.