Age and type of house
A hard cement plinth has been cast against the base of a cob wall. What moisture path does that hard plinth set up for the cob above?
The hard cement plinth sets up a path that holds moisture against the cob and can drive it into the wall above. Cob needs to dry at its base. Cement cast against that base is dense, so water cannot leave the foot of the wall as it would through a more open finish. The survey has recorded the hard plinth against the cob. It has not proved how wet the cob already is, and it has not specified what should replace the plinth.
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?
Cob is an earth wall. It is harmed by staying wet, and a cement plinth can trap splash and ground damp against it instead of letting the base dry. If the cob suffers, it is often just above the cement, where the earth softens. The survey saw the hard material against the soft wall. It did not cut the plinth away, and a meter reading on the day would not design the repair. The path is that dense junction.
Ask someone used to cob whether the plinth should come off and what breathable base should replace it. Tell the solicitor the note is cement against cob, not a proved failure of the whole wall. Do not let a general builder cap the junction with more cement. The home survey has not specified a mix, and it has not said the cob above is already lost. The moisture path is the finding.
What should you ask your surveyor?
- Does the cement plinth sit hard against the base of the cob?
- Is the moisture path into the cob above that dense plinth?
- Has any replacement detail been left unspecified?
What are the typical next steps?
- Treat the cement plinth as a moisture trap at the base of the cob.
- Ask a cob specialist what should replace the hard plinth.
- Do not allow more cement to be built over the same junction.
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 cob above the plinth feels firm today, has the path gone?
No. Firm cob on one day does not remove a cement plinth cast against the base. The path is the hard material holding moisture where the wall should dry. The survey has not proved the cob is already soft, and it has not proved it will stay firm. The plinth is still there.
Will a chemical damp course in the cob cure the plinth?
The survey has not specified a chemical treatment, and cob is not a masonry wall that is injected in the ordinary way. The note is the hard plinth against the earth base. Adding chemicals does not answer that junction. Ask someone who repairs cob what the base should be, rather than reading this report as a prescription.