It's a specific shape that's easy to recognise once you know what to look for: a crack starting at the top corner of a door or window frame and running diagonally, or in a stepped pattern through the mortar joints, up towards the ceiling or the storey above. It's rarely alone — check other openings on the same wall and there's often a fainter version starting to show.
This is one fault where the symptom itself is a strong clue about severity. Cracks at openings concentrate stress from whatever is happening structurally above or around them, so treat any crack in this exact pattern as potentially structural until it's been properly assessed.
Why cracks form specifically at openings
An opening removes material from a wall that would otherwise be carrying load evenly, and the lintel above it has to carry that load across the gap instead. Any weakness in the lintel, or in what's supporting it, concentrates stress at the corners of the opening — which is exactly where these cracks appear.
The likely causes, roughly in order of likelihood
- The lintel has deflected under load, or in some older properties there's no proper lintel at all — historic openings were sometimes formed with minimal support
- The opening was formed at some point without adequate support installed at the time
- Foundation movement beneath the wall, causing the whole wall to distort and concentrate cracking at its weakest points
- Thermal or seasonal movement in a long wall with no movement joint, which shows up at openings because they're the weak points in an otherwise continuous run of masonry
How to tell these apart before anyone touches it
Look at whether the crack is stepped, following the mortar joints in a zig-zag, or runs straight through the bricks themselves. A stepped crack through mortar is often less severe than one running straight through brick or block, which suggests more significant force behind the movement.
Mark the crack with a pencil line and a date, and check it again in a few weeks — a crack that's static is a different situation to one that's visibly widening. Ask whether anything was done to the opening previously, such as a door being enlarged or a window replaced without proper support, since a missing or undersized lintel from past work is one of the more common findings.
When you need a structural engineer, not a decorator
Any crack at an opening that is widening, more than a few millimetres, or appearing at multiple openings on the same wall should be assessed by a structural engineer before any repair is priced. Filling and redecorating over a crack whose cause hasn't been established risks masking movement that's still ongoing, and it can return worse, or the underlying issue can get more expensive to fix while it's hidden.
Common questions
Should I be worried immediately if I see this kind of crack?+
Treat it as worth investigating rather than immediately alarming — many cracks at openings turn out to be a lintel or historic construction issue rather than ongoing foundation movement. But it shouldn't be ignored or simply painted over.
How do I know if the crack is getting worse?+
Mark both ends of the crack with a pencil and note the date, then check the width and length again after a few weeks or months. Genuine widening, rather than the crack simply being more visible in different light, is the sign to act on.
Can weather cause this kind of crack on its own?+
Thermal movement can contribute to cracking at openings in long, unrestrained walls, but it's usually a secondary factor rather than the sole cause. It's still worth ruling out lintel and foundation issues before assuming it's purely seasonal.
Get a structural assessment of the crack
We look at the crack pattern on site and bring in a structural engineer where the movement needs proper assessment before any repair is quoted.