Cracks in house walls: when to worry

Are cracks in house walls normal?

For the most part, yes. Almost every house in England and Wales has some. Plaster shrinks as it dries, and clay soils swell through the winter and shrink through the summer, lifting and dropping foundations by a few millimetres a year. A hairline crack above a door that opens up in August and closes again by December is a house behaving exactly as it should.

What is not normal is a crack that keeps widening, or one that runs through the brickwork rather than through the plaster laid over it.

How wide is too wide?

Surveyors classify cracks using a standard called BRE Digest 251, published by the Building Research Establishment. It sorts damage into six categories, by width and by what putting it right involves.

Categories 0 to 2 are what the standard calls aesthetic. They are a decorating problem. Categories 3 and 4 are serviceability, meaning they affect whether the building keeps the weather out and whether doors and windows still open properly. Category 5 is a stability problem.

The Building Research Establishment is explicit that width alone does not classify anything. Its own guidance says several factors affect the category, including whether the cracks are getting wider over time. For categories 3 and above it also depends on how many cracks there are, not simply how large any one of them is.

Which is why the measurement you take in three months is worth more than the one you take today. A stable 6mm crack is generally less interesting than a 2mm crack that has doubled since the spring.

So before anything else, put a pencil mark at each end of the crack, write the date beside it, and photograph it with a ruler held against it. Do the same again in three months. That record is worth far more to a surveyor than any description you can give over the telephone, and it costs nothing.

A 20mm crack is severe under any classification. A single 3mm crack in a plastered internal wall that has not changed in two years is the pattern that most often turns out to be decorative, though nobody can confirm as much without looking at it.

How to tell if a wall crack is structural

There is no single test, and anybody offering you one over the internet is guessing. Surveyors do look at a consistent set of characteristics, however, and you can look at the same ones yourself.

Direction. Vertical cracks that follow a mortar line are often shrinkage or ordinary movement with temperature. Diagonal cracks are the ones that matter more, particularly those running at around forty five degrees from the corner of a window or door opening and widening as they travel. That is the classic pattern for foundation movement, and it is the one to photograph.

Depth and location. A crack in the plaster is a different thing from one you can trace through the brickwork outside. Go and look at the same spot on the external wall. A crack visible on both faces of the same wall, in the same place, is telling you something that the plaster alone is not.

What else has changed. Doors and windows that have started to stick, floors gone out of level, a step appearing in the damp proof course. These tend to show up alongside genuine movement. Their absence is one indicator fewer, though it does not rule movement out.

What the house is standing on. Shrinkable clay under the foundations, mature trees close by, leaking drains, made ground or a filled watercourse. None of it is visible on the wall, and it is the part almost nobody checks.

None of them is conclusive alone. A diagonal crack over a window in a house built on gravel with nothing growing near it usually has a duller explanation than the same crack in a Victorian terrace on London Clay with a poplar in the back garden.

Why am I finding cracks in my walls this year?

If you are on clay, the weather is the likeliest answer.

Clay shrinks as it dries. When a long dry spell pulls moisture out of the ground beneath a foundation, the ground drops, and the building drops unevenly with it. That is subsidence. In England it is overwhelmingly a summer and autumn problem, and it shows up months after the weather that caused it, which is why cracks noticed in September were often set in motion in June.

2026 has been a heavy year for it. Insurers paid out £72 million on domestic subsidence claims in the second quarter alone, with the average claim reaching a record £20,000. That is more than £2,000 up on the same quarter last year, which the Association of British Insurers attributes to the warmest spring on record in England and Wales. Subsidence payouts across the whole of 2025 came to £307 million, itself a record and 10 per cent up on 2024. As of 10 August, 71.3 per cent of England was in drought, with 45 million people living in a drought area.

Trees sharpen the effect. A mature broadleaf close to a building, such as a poplar, willow, oak or elm, draws a great deal of water out of clay across a dry summer. The effect falls away with distance, which is why so many subsidence conversations turn out to be about a tree rather than about a house.

If you are not on shrinkable clay, the drought explanation is probably not yours. Clay is not the only cause of ground movement, mind: leaking drains, made ground and old fill account for a fair share of it.

Whether your address sits on shrinkable clay is a matter of public record, through British Geological Survey mapping. It is not a matter of public convenience. Reading it properly needs mapping software rather than a postcode search, which is a large part of why so few people ever look.

When should I actually call somebody?

It is worth getting a professional opinion if any of these apply:

• The crack is wider than about 5mm
• It has grown measurably over the last few months, or appeared suddenly
• It runs diagonally, particularly in a stepped pattern that follows the mortar joints
• It runs from the corner of a door or window opening
• It shows on both faces of the same wall, or you can see daylight through it
• Doors or windows have started sticking, or floors have moved

Who to call depends on what you need.

A structural engineer assesses whether the building is moving and why, and produces a report you can act on. Achartered surveyor covers the overall condition of the building, usually as part of a purchase.

If you have buildings insurance and you suspect subsidence, insurers normally instruct their own loss adjuster and engineer. They generally want to inspect before any repair work happens, because work carried out first removes the evidence they would be assessing.

Note what is not on that list: your builder's opinion, and the free assessment offered by a company that sells underpinning.

What we cannot tell you

Whether the crack in your own house is serious. Nobody can settle that from a description or a photograph, and you should be sceptical of anybody who offers to.

What is answerable is the ground question underneath it, and that is where a good deal of the diagnosis sits before anybody has looked at a wall.

We produce data and consultancy reports on the ground and weather conditions around a property in England and Wales. The Ground module covers what a house is standing on, its shrink swell potential, nearby trees, and how the recent drought record reads at that address. It is not a survey. We do not visit, and we do not assess the condition of the building. Where the indicators above are present, a structural engineer is the relevant professional, and nothing in our report changes that.

Contains public sector information licensed under the Open Government Licence v3.0. Contains British Geological Survey and Environment Agency data © Crown copyright and database right.