Nothing worries a homeowner or buyer quite like a crack. In twenty years of surveying, I have lost count of the panicked phone calls that start with “there’s a crack in the wall” — and the number of times the honest answer, once I’ve actually looked at it, is that it’s nothing to lose sleep over. Subsidence is real, and it does happen, but it is genuinely the least common explanation for cracking in a wall. Here is how to tell the difference before you call a structural engineer or, worse, an insurer.
Start with what a crack actually is
A crack is a wall telling you it has moved. The question is never really “has it moved” — almost every building moves a little, all the time. The question is why, how much, and whether it has stopped.
The usual suspects
Thermal movement. Materials expand and contract with temperature. It shows up most at junctions — where an extension meets the original house, around door and window openings, at the corners of bay windows. Thermal cracking is typically fine, follows a predictable pattern, and doesn’t get progressively worse year on year.
Shrinkage. New plaster, render, or masonry mortar shrinks as it cures and dries out. This is completely normal in the first one to three years of new work, and it is one of the most common reasons I get called to a property that’s just had an extension built.
Historic settlement. Many older buildings settled decades or centuries ago and have been stable ever since. The crack is real, but it is a record of something that already happened, not a warning of something still happening.
Ground-related movement — the least common cause. This includes subsidence (the ground beneath the foundation shrinking or moving away), heave (the ground swelling and pushing up), and landslip. This is what people worry about, and it is genuinely rare relative to the other three.
What I’m actually looking for
When I inspect a cracked wall, I’m reading several things together, not any one of them alone:
- Direction and path. Diagonal cracks that step through mortar joints (rather than running straight through the brick or block itself) are more consistent with ground movement than most other causes.
- Width pattern. A crack that’s noticeably wider at the top than the bottom is a more classic subsidence indicator than one of even width.
- Correspondence. Does the crack appear in the same place on both the inside and outside of the wall? Genuine structural movement usually shows on both faces.
- Context. What’s near the wall? A mature, thirsty tree species on shrinkable clay soil changes the picture completely from the same crack on a modern estate with no significant vegetation nearby. So does a leaking drain, a recent excavation next door, or a mining history in the area.
- Is it still moving? A crack that’s been the same width for five years is a very different proposition from one that’s visibly widened in the last few months.
None of these on their own is proof. Diagonal stepped cracking through mortar can also be pure shrinkage in a badly built extension. The skill is in weighing the whole pattern against the building’s age, construction, setting and history — which is exactly why a proper inspection matters more than a photo sent to a forum.
When it genuinely needs more than a look
If the pattern, context and progression add up to a real possibility of ground movement, the honest next step is investigation, not a guess dressed up as a verdict. That typically means monitoring the crack over months — sometimes through a full seasonal cycle — plus, where warranted, drainage checks, root investigation or trial pits to establish soil type and foundation depth.
This is slower than anyone wants, buyer or seller. But committing to expensive remedial work — or walking away from a purchase — on the strength of a single visual impression is how people waste real money in both directions: paying for underpinning that was never needed, or abandoning a purchase over cracking that would have settled down on its own.
The reassuring truth
The overwhelming majority of cracks I’m called out to see are thermal, shrinkage, or old and stable. Subsidence gets the headlines because it’s the expensive, frightening outcome — but it is the exception, not the rule. What actually protects you isn’t assuming the worst or the best; it’s getting the pattern read properly by someone who has seen enough of them to know the difference.
If a crack is worrying you, the right first step is a proper assessment — not months of anxiety, and not a builder’s quote for underpinning before anyone has established it’s actually needed.