Where this starts
A wall with a hole in it is weaker at the corners of the hole, in the same way a sheet of paper tears from a notch. So when anything makes a wall move — the building drying out, a lintel deflecting, the ground beneath shifting, thermal expansion along a long elevation — the corner of a window or door is where it shows. The position tells you almost nothing on its own.
WIDTH IS THE FIRST READING and it is the one with published guidance behind it. Hairline cracking a fraction of a millimetre across is in the range that ordinary shrinkage and seasonal movement produce in almost every building. Cracks of a few millimetres are worth monitoring and explaining. Cracks wide enough to put a finger into, or that have moved the wall out of plane, are a different conversation and one for somebody who can look at the foundations.
THE PATH IS THE SECOND READING, and it distinguishes a mortar problem from a structural one. A crack that follows the joints — up one perpend, along a bed, up the next — is taking the path of least resistance through the weakest material, which is what a modest movement does. A crack that runs STRAIGHT THROUGH the bricks themselves had enough force behind it to break the units, and that is a stronger signal.
THE DIRECTION IS THE THIRD. A crack that runs diagonally UP and OUT from the top corner of an opening usually means the wall below is dropping relative to the wall above, or the opening is spreading. A crack under a sill running down and out often means the opposite. A horizontal crack along a bed joint at a consistent height is a different family again and can indicate cavity ties or a restraint issue.
WHAT NOBODY CAN TELL FROM A PHOTOGRAPH is whether it is still moving, and that is the most valuable fact available. A crack that has been the same width for years, with old paint still bridging it, is history. A crack with clean, sharp arrises and fresh debris is current. The cheap way to settle it is to record it and look again in a few months.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Thermal and moisture movement in the wall
Most commonFine cracking that opens in cold weather and closes in warm, often on long elevations or south-facing walls, following mortar joints rather than breaking bricks.
How to check: Whether the width changes with the season. Record it in winter and again in summer before concluding anything.
Shrinkage in a new or recently altered building
Most commonFine cracks appearing in the first year or two after construction or after an extension, often at the junction between new and old work, and stabilising.
How to check: The age of the work. New construction dries and shrinks, and the corner of an opening is where that shows.
A lintel deflecting or an inadequate bearing
CommonCracking concentrated above the opening, sometimes with a slight sag in the line of brickwork over it, and often with the crack starting at the very end of the lintel.
How to check: Sight along the courses above the opening. A dip in the line that follows the lintel is the lintel, not the wall.
Wall tie failure in a cavity wall
CommonHorizontal cracking along bed joints at a regular vertical spacing, sometimes with the outer leaf bulging, usually in walls of a certain age.
How to check: Whether the cracking is HORIZONTAL and regularly spaced. Corroding ties expand and lift the courses they sit in.
Ground movement under part of the building
Less commonA tapering crack, wider at one end, running diagonally and often through bricks rather than joints, frequently with doors or windows becoming hard to operate.
How to check: Whether the crack tapers and whether openings nearby have started sticking. Movement large enough to rack a frame is not cosmetic.
Something loading the wall that did not before
Less commonNew cracking that follows an alteration — a removed wall, a loft conversion, a new opening — rather than following a season.
How to check: Whether anything structural changed before it appeared. A new load path is the most direct explanation available.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Width is the single most informative number and the only one that can be compared later. Without a dated record, a crack cannot be shown to be stable, which is what most of them are.
- 2
Mortar is the weakest path and modest movement takes it. A crack through the units needed more force, and that raises the question rather than settling it.
- 3
A parallel-sided crack is usually movement back and forth; a tapering one suggests rotation, which points at one part of the wall moving relative to another.
- 4
A dip in the courses over a window localises the problem to the lintel, which is a contained repair rather than a question about the building.
- 5
Frames that have started to bind are a second, independent sign of movement, and two agreeing signs are worth far more than one.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a crack is more than a few millimetres wide, tapers noticeably, or runs through the bricks rather than around them.
- Where doors or windows near the crack have started to stick or no longer close, since that means a frame has been pushed out of square.
- Where a crack is widening on a dated record, which is the one observation that turns a monitoring exercise into an investigation.
- Where horizontal cracking appears at a regular vertical spacing, or the outer leaf bulges, both of which point at the ties rather than at the surface.
- Where the cracking followed an alteration — a removed wall, a new opening, a loft conversion — because the load path changed and the wall is reporting it.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.