Cracks in your walls: when to worry about subsidence

Few things unsettle a homeowner like a crack appearing in a wall — and few words unsettle them like “subsidence”. The reassuring truth is that most cracking in buildings has nothing to do with foundation movement. The important skill is telling the ordinary from the significant.

Why buildings crack

Buildings move constantly. Materials expand and contract with temperature and moisture; new materials shrink as they dry out; different materials move at different rates where they meet. All of this produces cracking that is cosmetically annoying but structurally meaningless.

Common benign causes include:

• Shrinkage — new plaster, new concrete and new timber all shrink as they dry, producing fine cracks in the first year or two, especially in new builds and fresh renovations.
• Thermal movement — long walls, particularly those facing the sun, expand and contract daily and seasonally.
• Differential movement — where an extension meets the original house, or masonry meets timber, hairline cracking at the junction is close to inevitable.
• Lintel deflection and minor settlement — small, historic and non-progressive movement that has long since stabilised.

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 the distortion shows as characteristic cracking.

The usual causes in the UK:

• Clay shrinkage — shrinkable clay soils lose volume as they dry. This is the classic cause, strongly associated with trees and large shrubs extracting moisture near foundations, and it peaks in dry summers.
• Leaking drains — escaping water can soften or wash away fine material beneath foundations, particularly in granular soils.
• Trees — both directly (moisture extraction from clay) and indirectly (root damage to drains).
• Mining legacy and made ground — a genuine consideration across parts of Yorkshire and the North, where historic workings and filled ground can settle long after construction.

Related but distinct: heave (ground swelling upwards, often after a large tree is removed from clay soil) and settlement (the building compressing the ground under its own weight, usually early in its life and usually finished).

Reading a crack

No single feature is conclusive, but investigators weigh a consistent set of signals:

Less concerning:

• hairline width (under about 1 mm)
• confined to plaster or finishes
• uniform width along the crack
• old, stable, previously decorated over and not reopening

More concerning:

• wider than about 3 mm, or visibly widening over time
• diagonal, stepping through brickwork mortar joints — the classic pattern near openings
• tapered — wider at the top than the bottom (or vice versa), indicating rotation
• visible both inside and outside at the same location
• accompanied by sticking doors and windows, sloping floors, or gaps opening at skirtings
• concentrated near a large tree, or above the line of a drain

Progression matters more than appearance. A dramatic-looking but stable historic crack is usually a lower concern than a modest crack that is demonstrably growing.

What a subsidence investigation involves

If the pattern of damage genuinely suggests foundation movement, investigation proceeds in stages:

1. Visual assessment and mapping — recording the damage, its distribution and its relationship to trees, drains, slopes and previous alterations.
2. Drainage survey — a CCTV survey of nearby drains, since defective drainage is one of the most common and most fixable causes.
3. Site investigation — boreholes or trial pits to establish the soil type, foundation depth and moisture profile; root analysis where vegetation is suspected.
4. Monitoring — precise level monitoring and crack-width monitoring over months, 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 intervention (if any) 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 many claims.

If you’re 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 by someone independent before exchange. Where there is a subsidence history, ask for the documentation: investigation reports, monitoring data, completion certificates for repairs, and any Certificate of Structural Adequacy issued through an insurance claim. A properly investigated and repaired property is a very different proposition from an unexplained live crack.

If it’s your home

Photograph cracks with a ruler or tape for scale, date the photos, and repeat monthly. If cracking is wider than about 3 mm, stepping through brickwork, visible inside and out, or clearly growing, get an independent assessment. If the cause looks like subsidence, contact your buildings insurer — subsidence is a standard insured peril — but going in with independent evidence puts you in a far stronger position to see the claim handled properly.