What causes cracks in walls?
Most cracks in walls are cosmetic: the result of normal building movement, seasonal temperature changes, or plaster shrinkage. Some are not. Knowing which is which determines whether you redecorate or call an engineer.
The main causes of cracking
Thermal movement
Walls expand and contract with temperature changes. In the UK climate, this can create horizontal or diagonal cracks, particularly at junctions between materials. These are usually cosmetic and seasonal.
Plaster and render shrinkage
New plaster and render lose moisture as they cure, causing fine hairline cracks. This is normal in newly plastered walls and properties that have recently been replastered. Not a structural concern.
Clay soil shrinkage (London)
Most of London sits on London Clay. Clay shrinks in dry conditions and swells in wet ones, causing seasonal foundation movement. Long, dry summers, increasingly common in London, can cause significant clay shrinkage, leading to diagonal cracks at window and door corners.
Foundation settlement
All buildings settle into the ground after construction. Minor initial settlement is normal and typically produces fine cracks in the first years after building. Progressive or differential settlement, where different parts of the building settle at different rates, is more concerning.
Tree root activity
Tree roots draw moisture from clay soil, causing it to dry out and shrink beneath foundations. This is particularly significant for London properties near large trees (oak, poplar, willow) within 15 metres. The pattern of cracking can be distinctive: diagonal cracks on the side of the building closest to the tree.
Leaking drains
Leaking underground drains wash away soil beneath foundations, causing foundation movement and cracking. CCTV drain surveys are often recommended when subsidence is suspected.
Structural failure
Removal of load-bearing walls without proper structural support, overloaded structures, and deteriorating structural elements can cause cracking due to genuine structural failure. This category requires urgent professional investigation.
Neighbouring works
Excavation, demolition, or new basement construction by neighbours can cause ground movement and cracking. You may have a legal claim against the neighbour or their contractor.
Crack width guide
Crack width is one indicator of severity, but pattern, location, and rate of change matter just as much. Use this as a guide, not a definitive assessment.
| Type | Width | Severity | Action |
|---|---|---|---|
| Hairline cracks | < 0.1mm | Cosmetic | Monitor. No structural concern. Redecorate when convenient. |
| Fine cracks | 0.1–1mm | Low | Monitor over one complete seasonal cycle (12 months). Fill and redecorate if stable. |
| Moderate cracks | 1–5mm | Medium | Get a professional assessment. May be cosmetic or may indicate foundation movement. |
| Wide cracks | 5–15mm | High | Commission a structural assessment urgently. Do not leave without investigation. |
| Very wide cracks | > 15mm | Severe | Seek immediate professional advice. May indicate danger. |
Warning signs that warrant investigation
Regardless of crack width, the following patterns suggest the crack should be professionally assessed:
- Diagonal cracks running from the corners of windows or doors
- Stepped cracks following the mortar joints in brickwork
- Cracks that are wider at one end than the other
- Cracks that have appeared suddenly or changed rapidly
- Cracks accompanied by sticking doors or windows
- Cracks on the external face of a wall
- Any crack that has appeared after nearby construction work
- A crack that has opened more than 1mm over a few weeks
When to call a structural engineer
If you have moderate or wide cracks, rapidly developing cracks, or cracks displaying any of the warning patterns above, commission a structural reportand flag “cracks or movement” at checkout. The engineer will tell you definitively whether the cracks are cosmetic or structural, identify the cause, and specify what (if anything) needs to be done.
If you’re not sure whether your cracks warrant a full report, start with a site visit consultation (£390). An engineer will assess them in person and tell you honestly what they see.