Subsidence, settlement and heave
What does it mean if a crack is wider indoors than outdoors, or the other way round?
It means the two faces have not broken in the same way, so it does not prove which way the ground moved and it does not date the crack. Plaster, render and brick open differently, and a home survey only records the widths it could see. It does not test the soil, and one visit does not monitor the crack.
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?
Plaster is a thin coat and often splits more than the masonry behind it. Render can bridge a joint or hide it until the coat itself fails. An outside crack can look narrow because someone pointed over it, while the inside crack was left. The reverse happens when a fresh skim covers an older split in the brick. None of those patterns is a measurement of subsidence. The Association of British Insurers describes subsidence as downward movement of the ground. A difference between indoor and outdoor width is not that test.
Ask the surveyor to say what they actually compared: bare brick, painted render, or plaster. A photograph of one face does not stand in for the other. If both faces were seen, the report can still only describe that day. It cannot tell you the crack is growing, and it cannot tell you it has stopped. Do not write to the seller that the wider side has diagnosed the cause. The cause is still untested.
What should you ask your surveyor?
- Did you measure both faces, or only the one in the photograph?
- Was either face recently plastered, rendered or pointed?
- Are you describing the crack, or diagnosing movement of the ground?
What are the typical next steps?
- Keep the indoor and outdoor descriptions separate when you speak to a builder.
- Do not treat the wider side as proof of subsidence or of safety.
- If the purchase turns on whether the crack is live, that needs monitoring agreed in writing, not another look at the same photograph.
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
Does a hairline outside and a wide crack inside mean the wall is hollow?
No. It may only mean the plaster has parted. Hollow plaster is a different check. Ask what was tapped, and do not invent a cavity.
Should you fill the wider side before anyone else looks?
No. Filling destroys the comparison. Leave both faces until you have decided whether they need to be watched.