Wall ties and cavity insulation
What is a tell-tale on a crack, and why does it not answer a wall-tie question?
A tell-tale is a gauge fixed across a crack to see if the crack moves. It measures movement at that point over time. It does not look into the cavity, identify a tie, or say why the crack opened. Leaving one in place for a week before exchange rarely proves anything.
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?
Surveyors and engineers use tell-tales, glass slips or metal discs when they need a record of change. Wall-tie cracking is diagnosed by the pattern in the courses and, properly, by seeing the ties. A gauge on one crack cannot see a rusty tie two metres away. If the seller already fitted gauges, ask for the readings and the dates. A gauge with no record is decoration.
Crack monitoring that matters usually runs for months, which may not fit a purchase. The Association of British Insurers’ subsidence material is about ground movement after you are the owner and insured. Before exchange, decide whether you need an engineer or a tie inspection now, not whether a plastic slide looks reassuring. The survey does not read a tell-tale as a pass.
What should you ask your surveyor?
- Is there a dated record for the gauge, or only the gauge?
- Does the crack pattern look like bed-joint tie cracks or like movement over an opening?
- What investigation would actually tell ties from ground movement here?
What are the typical next steps?
- Ask the seller for any readings. Do not restart the clock casually and call it monitoring.
- Do not treat a single gauge as a substitute for seeing the wall.
- If you need an answer before exchange, instruct the inspection that matches the pattern.
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 you fit tell-tales yourself before you buy?
You would need the seller’s permission, and a short record will not satisfy a lender or an insurer. Agree anything like that in writing.
Does a stable gauge mean the ties are fine?
It means that crack did not move while it was measured. Ties can be rusty in a wall that is not moving this month.