Wall ties and cavity insulation
Nothing on the visit sampled a coal-bunker wall, opened horizontal crack lines or tested ties the bunker wall does not prove have failed in the house. What has to stay undecided?
Whether wall ties have failed in the house stays undecided, and so does the bunker wall itself. Nothing on the visit sampled a coal-bunker wall, opened horizontal crack lines or tested ties. A bunker is a small masonry box, often damp and stained, and cracks there do not prove the house walls have the same tie failure. The survey recorded marks on the bunker. It has not investigated ties in the dwelling and it has not sampled concrete or metal.
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?
Horizontal cracks can be a classic hint of corroding cavity ties in a house wall, because expanding metal lifts the bed joints. The same pattern on a coal bunker can instead be frost, salt in the masonry, a weak mortar or simple lack of restraint in a thin wall that was never built like the house. The visit did not open those lines and did not see a tie. Transferring a bunker crack into a house-tie verdict is the step the report has not taken.
If the house elevations themselves show tie symptoms, that is a separate note and still not a test unless a specialist looks in the cavity. Ask the solicitor not to file the bunker cracks as a completed tie investigation. A wall-tie specialist can inspect the house walls if the instruction is made. Tell a lender the bunker was not sampled and the house ties were not tested. Keep the outbuilding off the dwelling's tie conclusion.
What should you ask your surveyor?
- Were the horizontal lines confined to the coal-bunker wall on this visit?
- Were house wall ties left untested?
- Did the survey avoid treating the bunker as proof of tie failure indoors?
What are the typical next steps?
- Keep bunker cracks separate from the house ties.
- Do not tell a lender the dwelling ties failed because of the bunker.
- Instruct a tie specialist for the house walls if that investigation is needed.
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
Is a coal bunker a cavity wall like the house?
Often it is not. Many bunkers are thin solid walls or poorly tied leaves, built as a store, not as the main cavity construction. This visit did not open the bunker and did not test ties. Even if the bunker were cavity work, its cracks would still not prove the house. The survey left that transfer undone.
Can rust stains on the bunker be read as tie corrosion indoors?
No. A stain on a bunker wall is a stain on the bunker. It was not sampled, and it does not identify metal in the house cavity. Tie corrosion in the dwelling would need the house walls investigated. The survey did not do that. Tell the solicitor the stain stays with the outbuilding note.