When Are Cracks Structural?
Most cracks in a home are harmless, but some signal real movement. Here is how structural engineers tell the difference and when you should ask for advice.
A crack is likely to be structural when it is wider than about 5mm, runs diagonally through brickwork as well as plaster, is wider at one end, appears near doors, windows or extension joints, or keeps growing over time. Fine, stable cracks under about 1mm in plaster alone are usually cosmetic. The most important clue is movement: a crack that continues to widen needs a structural engineer, while one that has stayed the same for years rarely does.
Why buildings crack at all
Every building moves a little. Temperature and humidity make materials expand and contract, plaster shrinks as it dries, and timber and plasterboard react to the seasons. These movements produce the fine cracks most homes show, particularly above doorways, along ceiling joints and where different materials meet. They are a normal part of a building breathing, not a sign of failure.
Structural cracks are different. They are caused by the building's load paths or foundations moving in a way the structure was not designed for, for example foundation settlement, subsidence, overloading, bulging walls or failed lintels. Telling the two apart is the whole point of a crack inspection.
How engineers read a crack
We do not judge a crack on width alone. We look at four things together: width, direction, location and whether it is still moving.
Width
The Building Research Establishment classifies cracking on a scale from negligible to severe. As a practical guide for homeowners: under 1mm is usually cosmetic, 1 to 5mm deserves a closer look depending on where it is, 5 to 15mm often needs investigation, and anything wider points to significant movement that should be assessed without delay.
Direction
Vertical cracks following mortar joints are often shrinkage or thermal movement and are frequently benign. Horizontal cracks in a wall can be more serious, sometimes indicating wall tie failure or lateral pressure. Diagonal cracks, especially staircase cracks stepping through brickwork from the corners of windows and doors, are the classic signature of foundation movement.
Location
Cracks at the junction between an old house and a newer extension, around bay windows, near chimney breasts, or at the corners of openings tend to matter more because these are the structurally sensitive points. Cracks isolated in a single plaster panel matter less.
Movement over time
This is the single most useful test. A crack that has not changed in years is almost always stable and cosmetic. A crack that is visibly widening, or that reopens after being filled, is active and needs an engineer. This is why we always advise photographing and monitoring a crack before filling it.
Warning signs that a crack is structural
- Width greater than about 5mm
- Diagonal or staircase pattern through brick
- Wider at one end than the other
- Visible on both the inside and outside of a wall
- Located near openings or extension joints
- Accompanied by sticking doors or sloping floors
- Still growing, or reopening after repair
Several of these signs overlap with subsidence. If you also notice doors jamming or floors sloping, read our guide on the signs of subsidence in London homes alongside this one.
What to do next
If a crack ticks one or more of the warning boxes, the right step is diagnosis, not decoration. A structural inspection establishes the cause, and only once the cause is understood and movement has stopped do we recommend a repair. Genuine structural cracks are repaired using methods such as crack stitching with helical bars, repointing and rebuilding, all set out in an engineering-led repair specification. For cosmetic cracks, the honest answer is often that no structural work is needed at all, which is worth knowing before you spend money.
Cracks: common questions
What size crack is a structural concern?
As a rough guide, cracks under about 1mm are usually cosmetic, cracks of 1 to 5mm may need investigation depending on pattern and location, and cracks wider than about 5mm, especially diagonal cracks through brickwork, warrant a structural engineer's inspection. Width is only one factor; direction, location and whether the crack is still moving matter just as much.
Are cracks in a new-build or new extension normal?
Fine cracks often appear in the first year or two as new materials dry out and a new extension settles against the existing house. These are usually cosmetic and stabilise once movement finishes. Cracks that keep widening, run diagonally through brickwork, or open at the junction between old and new structure should be checked by an engineer.
Should I fill a crack before getting it checked?
No. Filling a crack hides whether it is still moving, which is the single most useful piece of diagnostic information. Photograph the crack with something for scale, monitor it over a few weeks, and have it inspected before any filling or redecoration so the repair addresses the cause rather than just the appearance.
Not sure if a crack is serious?
Send us photos with something for scale and we will tell you whether it needs an engineer.