Cracks in Walls: When to Worry and When to Leave Them Alone
Cracks in walls are one of the most common concerns homeowners bring to a structural engineer. Most cracks in walls in UK homes are cosmetic, but a minority indicate real structural problems. The good news is that most cracks in UK homes are cosmetic to caused by normal thermal movement, settling or shrinkage as a building ages. The minority that genuinely need attention have specific patterns that distinguish them from harmless cosmetic cracking. This guide tells you which is which.
If you are worried about a crack right now, the fastest way to get clarity is a structural engineer inspection report. But read this first so you know what questions to ask.
Types of Cracks in Walls and What They Mean
Hairline cracks (less than 0.1mm wide)
These are almost always cosmetic. They appear on plaster surfaces as the building expands and contracts with seasonal temperature changes, or as the plaster dries out after building work. Hairline cracks in ceilings and walls after a new build or after plastering are normal and expected. Fill with a flexible filler and redecorate. No structural concern.
Fine cracks (0.1mm to 1mm wide)
Still likely to be cosmetic, particularly if they are vertical or horizontal and have been stable for years. Fine cracks in plaster that follow the joints between plasterboard sheets are a common example. Worth monitoring but rarely structural. If a fine crack has appeared suddenly in a property you have owned for a long time, or has appeared after building work next door, mention it to an engineer.
Moderate cracks (1mm to 5mm wide)
These need attention. A crack wider than 1mm in a masonry wall or above a door or window opening should be assessed by a structural engineer, particularly if it is diagonal, stepped through the mortar joints, or accompanied by out-of-plumb walls or sticking doors. These are the cracks that sometimes indicate foundation movement, lintel failure or other structural problems that need early intervention.
Wide or severe cracks (over 5mm wide)
Get a structural engineer involved without delay. Wide cracks, cracks with displacement (where one side of the crack is higher or further forward than the other) and cracks accompanied by visible movement of the wall are signs of potentially serious structural distress. These do not fix themselves.
Crack Patterns and What They Suggest
| Crack Pattern | Likely Cause | Action |
|---|---|---|
| Diagonal crack from corner of door or window | Lintel failure or differential settlement | Structural engineer assessment |
| Stepped crack following mortar joints in brickwork | Foundation settlement or subsidence | Structural engineer assessment |
| Horizontal crack in mid-wall | Lateral pressure, often in retaining or basement walls | Structural engineer assessment urgently |
| Vertical crack at junction of extension and main house | Differential settlement between old and new structure | Monitor and review if widening |
| Fine cracks in plaster following plasterboard joints | Normal thermal movement, cosmetic | Fill and redecorate |
| Cracks in corners of rooms | Normal building movement, cosmetic | Fill and redecorate |
The Crack Patterns That Concern Structural Engineers Most
Diagonal cracks from the corners of windows and doors
A diagonal crack running at roughly 45 degrees from the corner of a window or door opening is a classic sign of lintel failure or foundation settlement. The lintel is the beam above the opening that carries the wall above it. Original Victorian and Edwardian terraces often have timber lintels above window and door openings to these can rot, deflect and fail over time, causing diagonal cracking from the opening corners. This is the crack pattern that looks alarming but is often straightforward to fix: a new steel lintel and some repointing. A structural engineer confirms whether that is all it is, or whether there is deeper foundation movement involved.
Stepped cracks in external brickwork
A crack that steps through the mortar joints diagonally across the face of a brick wall almost always indicates differential settlement to part of the foundation has moved down more than another part. In clay soils this is often caused by tree root activity drawing moisture from the clay, causing it to shrink. In older properties it can be historic settlement that happened decades ago and has long since stabilised. A structural engineer distinguishes between active movement and historic settlement through the crack width, crack displacement and whether the crack is clean or stained with weathering.
Horizontal cracks in walls
These are the cracks that concern engineers most. A horizontal crack in a wall often indicates that lateral pressure is pushing the wall outward to this occurs in retaining walls, basement walls and sometimes in cavity walls where the wall ties have corroded and the two leaves are separating. Horizontal cracks in the middle of a wall should be assessed by a structural engineer without delay.
When Cracks Are NOT a Structural Problem
To give a balanced picture: most cracks you will encounter in a UK home are not structural. Here are the common non-structural causes:
- New build movement: most new properties move as they dry out and settle in the first few years. Fine cracks in plaster and at junctions are normal and expected.
- After building work: new plaster cracks as it dries. Cracks at the junction between new and old plasterwork are very common and cosmetic.
- Seasonal movement: timber floors, door frames and plaster expand and contract with humidity changes through the year. Hairline cracks that open in winter and close in summer are almost always thermal movement.
- Shrinkage in plasterboard: the joints between plasterboard sheets are a common location for fine cracks. These are cosmetic and fixed with jointing tape and filler.
What a Structural Engineer Does When Assessing Cracks
A structural engineer visits the property and looks at the crack pattern systematically. The assessment covers: the width of the crack (measured with a crack gauge), whether there is displacement across the crack, the pattern and direction of cracking, the location relative to openings and foundations, the soil type and drainage around the property, and any nearby trees or recent building work that could have caused movement.
The engineer then produces a written report explaining the cause, whether the movement is active or historic, the severity, and the recommended action. For a structural survey in the context of a property purchase, the report helps buyers understand exactly what they are taking on before exchanging contracts.
See our full guide: structural engineer report and inspection costs.
FAQ: Cracks in Walls
Are cracks in walls dangerous?
Most cracks in UK homes are cosmetic and not dangerous. The cracks that can indicate structural problems are wide (over 1mm), diagonal from openings, stepped through brickwork, horizontal, or accompanied by displacement. If in any doubt, a structural engineer can give you a clear answer from a site visit.
How do I know if a crack is getting worse?
Mark the ends of the crack with a pencil or marker, note the date, and check back in two to four weeks. If the crack has extended beyond your marks it is active. Crack monitors (small plastic tell-tales available from builders' merchants) are a more precise way to track movement over time.
Should I fill a crack before getting it inspected?
If you are planning to call a structural engineer, wait until after the inspection before filling. A fresh fill tells the engineer nothing about the crack history. If the crack has already been filled and has re-opened, that is useful information to it suggests ongoing movement rather than a one-off event.
What does it cost to fix a structural crack?
That depends entirely on the cause. A failed lintel replacement costs £500 to £1,500 including the structural engineer's calculations and builder's labour. Foundation repairs for active subsidence can cost £10,000 to £30,000 or more. This is why getting an accurate diagnosis first is important to many cracks that look alarming turn out to have simple, inexpensive causes.