Reading a crack
How to tell if a wall crack is structural or just cosmetic.
Not every crack means something is wrong with the building. Most don't. A few signs separate the ones you can fill from your side and the ones that need an engineer to look at first.
Reviewed 7 September 2026 by PremiumWalls operations.
At a glance
How do I know if a crack in my wall is serious?
Look at where it is, which way it runs, and whether it has changed. A fine, hairline crack in a flat, web-like pattern is usually shrinkage in the plaster and can be filled. A crack wider than a couple of millimetres, running diagonally from the corner of a door or window, stepping through brickwork, or one that keeps reopening after filling, needs a structural opinion before anything is painted over it.
Structural or Cosmetic? Reading a Wall Crack
The shrinkage crack
Fine, hairline, spread in a web-like pattern across a plastered surface, usually within the first year or two after plastering. This is a normal part of plaster curing, not a structural sign. It gets filled, levelled and primed like any other surface preparation.
The corner crack
A diagonal crack running from the corner of a door or window frame, roughly at 45 degrees. Common where there is minor movement around an opening. Often cosmetic, but worth watching if it is visibly widening rather than staying the same.
The stepped crack
Follows the mortar joints in a diagonal, stair-step pattern through brick or block work rather than running straight through the plaster. This is usually a sign of differential settlement in the structure, and it is the pattern that most often needs a structural opinion before any cosmetic repair.
Cracks that keep coming back
If a crack has been filled before and has reappeared in the same place, the movement causing it has not stopped. Filling it again without dealing with the cause just repeats the cycle, sometimes for years.
Width, and whether it is still moving
A crack wide enough to catch a fingernail, or one that has visibly widened over a few months, is a different category from a fine surface crack. A simple way to check: mark both ends with a pencil line and the date, and look again in a few weeks. If the crack extends past the marks, it is still active.
When to call a structural engineer instead of a painter
Visible movement in a load-bearing wall, cracks appearing on both faces of the same wall in the same place, cracking around a beam or column, or any crack that genuinely worries you about the building's safety, all belong with a structural engineer first. A painting or repair contractor should tell you this rather than fill over it.
Structural or Cosmetic? Reading a Wall Crack — your questions.
Can I just fill a crack myself?
For small, stable hairline cracks, yes — though a properly opened, filled and reinforced repair holds up better than skimming filler over a flat wall. For anything wider, or a crack that has been growing, get it looked at before filling it.
Does a crack always mean something is wrong with the building?
No. Most cracks are cosmetic — ordinary plaster shrinkage or minor settling that has already happened and stopped. It is the exceptions above, particularly stepped cracks through masonry and cracks that keep reopening, that need attention.
Will painting over a crack make it worse?
Painting itself will not cause damage, but a flexible filler applied without opening the crack out properly tends to fail at the same spot, which then looks like the crack has come back when really the repair just did not hold.
How do I know if a crack is still getting worse?
Mark both ends with a pencil line and the date, or place a strip of tape across it. If it extends past the marks or the tape splits within a few weeks, it is still moving and worth a proper look before it is filled.
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