How to Read a Crack in a Wall: What the Shape Actually Tells You

The direction a crack runs tells you more than its length does. A crack that runs straight, along a ceiling line or down a plasterboard joint, is almost always shrinkage. A crack that steps diagonally through the mortar joints is the one worth having looked at, because that pattern comes from one part of the building moving relative to another.

Here is how to tell the common patterns apart, what the width actually means, how to work out whether it is still moving, and the point at which it is worth calling someone.

What does a straight crack mean?

Usually shrinkage or thermal movement, and usually nothing to act on.

New plaster dries and shrinks. Ceilings and walls are different materials that expand and contract at different rates, and the heating goes on and off all winter. That shows up as a fine, even line along the junction between ceiling and wall, down the joints between plasterboards, or running away from a door frame.

The giveaway is that the width stays the same along its length and does not change from one year to the next. Fill it, redecorate, forget about it.

What does a stepped or staircase crack mean?

It means something underneath has moved, and the wall has split along the weakest path available to it.

Mortar is weaker than brick, so a crack caused by movement rarely runs through the bricks themselves. It zig-zags around them, following the bed joints and the perpends. That staircase pattern is the classic signature of differential movement: one part of the building has dropped, lifted or rotated relative to another.

It does not automatically mean subsidence. It means something moved. Working out what moved, and whether it has stopped, is the actual job.

What does a diagonal crack from a window or door corner mean?

Openings are the weak points in any wall, and a diagonal crack running away from a corner is the wall telling you it is distorting.

A wall with a hole in it has to carry its load around that hole, so stress concentrates at the corners of the opening. When the wall is pushed, pulled or dropped at one end, the crack starts at a corner and runs away on the diagonal.

Which corner it starts from, and whether it widens as it rises or as it falls, is what tells an engineer which part has moved and in which direction. It is also why filling a crack before anyone has seen it is unhelpful.

What about a horizontal crack?

A long horizontal crack following a single mortar course is a different animal, and it deserves prompter attention than the others.

In a cavity wall it can point to wall tie failure, where the metal ties holding the two leaves together corrode and expand, lifting the outer leaf course by course. It tends to show at regular heights up the wall rather than as a one-off.

Horizontal cracking is less common than the other patterns and harder to explain away. It is worth looking at rather than waiting out.

How wide is too wide?

Most UK engineers work to BRE Digest 251, which sorts damage into six categories based on how easily it can be repaired rather than on width alone.

• Category 0, negligible – hairline, under 0.1mm
• Category 1, very slight – up to about 1mm
• Category 2, slight – up to about 5mm. Easy to fill, may be visible outside, doors and windows may stick slightly
• Category 3, moderate – 5 to 15mm, or several cracks over 3mm. Begins to carry structural significance
• Category 4, severe – 15 to 25mm
• Category 5, very severe – usually over 25mm

Two things matter as much as the measurement. How many cracks there are, and where they sit. Cracking over an opening counts for more than the same width in the middle of a blank panel, and a wall with several 3mm cracks can land in a higher category than a wall with one 5mm crack.

Is the crack still moving?

This is the question that decides what happens next, and you can start answering it yourself before anyone visits.

Mark each end with a pencil line and write the date next to it. Photograph it with a ruler or a coin in the frame. Then look again once a season has turned, because clay shrinks through a dry summer and swells again over winter, so movement often shows up as a yearly cycle rather than steady widening.

A crack that opens again after being filled is live. A crack that has sat exactly where it was since you first noticed it is history, and history is a great deal cheaper.

What usually causes it?

On the clay soils common across the East of England, the usual suspects are below ground rather than in the brickwork.

• Trees drawing moisture out of clay near a foundation, especially after a long dry summer
• A leaking drain washing fine material out from under the footing
• Different foundation depths where an extension meets the original house
• Shrinkage and thermal movement in newer plaster and render, which is not structural at all

When should you call a structural engineer?

Any one of these on its own is worth keeping an eye on. Two or three together is worth a phone call.

• The crack steps diagonally through the mortar joints
• It is noticeably wider at one end than the other
• It is wider than about 5mm, or there are several above 3mm
• It shows on the outside of the building as well as the inside
• Doors or windows have started sticking, or a floor has gone out of level
• It reopens every time it is filled

It is also worth it if you are buying, selling or remortgaging. A written structural report is what a lender, an insurer or a buyer solicitor will want to see, and where remedial work has already been done they may ask for a Certificate of Structural Adequacy as well.

What we do about it

Ora Labora Structural Engineers carry out structural surveys and subsidence investigations across Peterborough, London and the East of England.

We look at the crack pattern inside and out, whether it tapers, whether doors and floors have moved with it, and what the drainage and the trees are doing. Where it is not clear what is underneath, a trial pit alongside the foundation shows the depth and condition of the footing, usually in an afternoon.

What comes back is a written structural report saying what caused it, whether it is still moving, and what needs doing, in language you can hand straight to a lender, an insurer or a solicitor.

Got a crack you are unsure about? Send us a photograph with something in it for scale and tell us how long it has been there.