Nothing worries a homeowner or buyer quite like a crack. Across a decade of surveying I have lost count of the phone calls that start with “there’s a crack in the wall” — and the number of times the honest answer, once I have actually looked at it, is that it is 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. Long walls facing the sun are the worst offenders. Thermal cracking is typically fine, follows a predictable pattern, and does not get progressively worse year on year.
Shrinkage. New plaster, render, concrete, timber and mortar all shrink as they cure and dry. 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 has just had an extension built.
Differential movement. Where two materials meet — masonry against timber, an extension against the original house — they move at different rates. Hairline cracking at that junction is close to inevitable and means very little.
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 is the one people mean when they say subsidence, and it is genuinely rare relative to the other four.
What subsidence actually is
Subsidence is downward movement of the ground supporting the foundations. When part of a building loses support and part does not, the structure distorts, and that distortion shows as characteristic cracking.
The usual causes in this part of the country:
- Clay shrinkage. Shrinkable clay soils lose volume as they dry out. This is the classic cause, strongly associated with trees and large shrubs drawing moisture from near the foundations, and it peaks in dry summers.
- Leaking drains. Escaping water softens or washes away fine material beneath a foundation, particularly in granular soils. It is one of the most common causes and one of the most fixable.
- Trees. Both directly, through moisture extraction from clay, and indirectly, through root damage to drains.
- Mining legacy and made ground. A genuine consideration across much of Yorkshire and the North West, where historic workings and filled ground can settle long after the house was built.
Two things often confused with it: heave is the ground swelling upwards, frequently after a large tree is removed from clay soil, and settlement is the building compressing the ground under its own weight — usually early in its life, and usually long finished.
Reading a crack
When I inspect a cracked wall I am reading several things together, never any one of them alone.
Less concerning:
- hairline width, under about 1 mm
- confined to plaster or surface finishes
- uniform width along its length
- old, previously decorated over, and not reopening
More concerning:
- wider than about 3 mm, or visibly widening over time
- diagonal, stepping through the mortar joints rather than straight through the brick — the classic pattern near openings
- tapered: distinctly wider at one end than the other, which indicates rotation
- visible in the same place on both the inside and the outside of the wall
- accompanied by sticking doors and windows, sloping floors, or gaps opening at skirtings
- concentrated near a large tree, or following the line of a drain
Then there is context, which changes everything. A mature, thirsty tree species on shrinkable clay puts the same crack in a completely different light from one 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.
And above all, progression. A crack that has been the same width for five years is a very different proposition from one that has visibly widened in the last few months. A dramatic-looking but stable historic crack is usually a lower concern than a modest crack that is demonstrably growing.
None of these on its own is proof. Diagonal stepped cracking through mortar can equally 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.
What a real investigation involves
If the pattern, the context and the progression genuinely add up to possible ground movement, the honest next step is investigation, not a guess dressed up as a verdict. That proceeds in stages:
- Visual assessment and mapping — recording the damage, its distribution, and its relationship to trees, drains, slopes and previous alterations.
- Drainage survey — a CCTV survey of nearby drains, since defective drainage is both a common cause and a comparatively cheap fix.
- Site investigation — boreholes or trial pits to establish soil type, foundation depth and moisture profile, with root analysis where vegetation is suspected.
- Monitoring — precise level and crack-width monitoring over months, often through a full seasonal cycle, to establish whether movement is ongoing, seasonal or historic.
Only with that evidence can anyone honestly answer the questions that matter: is it moving, why, and what response is proportionate. Underpinning — the remedy everyone fears — is needed in only a minority of cases. Removing or managing a tree, or repairing a drain, resolves a great many claims.
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 house over cracking that would have settled on its own.
If you are buying a property with cracking
Do not let a crack panic you out of a good house, and do not let an agent’s “it’s just settlement” talk you past a real problem. Get the cracking assessed independently before exchange.
Where there is a subsidence history, ask for the paperwork: investigation reports, monitoring data, completion certificates for any repairs, and the Certificate of Structural Adequacy if the work went through an insurance claim. A properly investigated and repaired property is a very different proposition from an unexplained live crack.
If it is your own home
Photograph the cracks with a ruler or tape in shot for scale, date the photographs, and repeat monthly. That record is worth more than any single opinion, because it answers the progression question that nothing else can.
If a crack is wider than about 3 mm, stepping through brickwork, visible inside and out, or clearly growing, get an independent assessment. If the cause does look like subsidence, contact your buildings insurer — it is a standard insured peril — but going in with independent evidence puts you in a far stronger position to see the claim handled properly.
The reassuring truth
The overwhelming majority of cracks I am called out to see are thermal, shrinkage, or old and stable. Subsidence gets the headlines because it is the expensive, frightening outcome, but it is the exception rather than the rule.
What actually protects you is not assuming the worst or the best. It is getting the pattern read properly by someone who has seen enough of them to know the difference — and who has no remedial work to sell you either way.
Further reading: what subsidence actually is and how to check for it, and whether subsidence can be fixed without underpinning. To see where a particular crack sits on the BRE damage scale, try our cracks in walls checker.