Not every crack in a brick wall is structural. Some are cosmetic shrinkage that closes back up with the weather. Others are early warnings of foundation movement, corroded steelwork, or failed wall ties, and waiting on those is expensive. The trick is knowing which crack you’re looking at.
Direction, location, and pattern do most of the diagnostic work. Here are the five most common crack types we see on Sydney brickwork, what causes each, and which ones need urgent attention.
1. Stair-step cracks
Diagonal cracks that step down through the mortar joints, following the brick coursing. The crack often starts wide at the top or bottom of an elevation and tapers along its length.
Cause: foundation settlement, lintel failure below, or differential movement between two parts of the building. The taper points toward the moving section.
How serious: stair-step cracks are almost always structural. If the crack is more than 2–3mm wide or growing, get a diagnosis quickly. Helibar stitching combined with the underlying fix (lintel, drainage, or engineering review) is the usual repair.
2. Horizontal cracks at regular vertical intervals
A series of horizontal cracks running through mortar joints, evenly spaced 450–900mm apart, often the full length of a wall.
Cause: failed cavity wall ties, almost without exception. The original galvanised iron ties have rusted, expanded, and forced the joints apart. The pattern matches the original tie spacing.
How serious: structural. The outer leaf has lost its connection to the inner leaf and is at risk of bowing or, in extreme cases, separation. Remedial wall tie installation through the existing wall is the standard fix, no demolition required.
3. Vertical cracks at corners or above openings
Single vertical cracks running through joints (and sometimes through bricks), often appearing at building corners, at the junction between two construction periods, or directly above windows and doors.
Cause: thermal movement combined with missing or inadequate expansion joints. Long brickwork walls expand and contract with the seasons; if there’s nowhere for that movement to go, something has to give.
How serious: cosmetic to moderate. They rarely indicate structural failure but they let water in, so they should be repaired. Diamond saw-cut control joints can be retrofitted in long unbroken runs to prevent recurrence.
4. Cracks that follow the brick face (not the joints)
Cracks that pass through the bricks themselves rather than the mortar around them. Often accompanied by flaking, spalling, or face erosion.
Cause: the brick is failing, salt attack, frost damage on softer bricks, or hard cement mortar trapping moisture. The wall is not moving; the brick fabric is breaking down.
How serious: moderate. The wall isn’t in danger, but the affected bricks need cutting out and replacing before they spread. Combine the cut-out repair with the underlying cause, usually rising damp, hard mortar, or coastal exposure.
5. Cracks above windows and doors with rust staining
Stair-step cracking centred on the corners of an opening, often with brown weep marks running down from a horizontal line above the window.
Cause: a corroded steel lintel. As the lintel rusts, it expands to up to seven times its original volume. The expansion lifts the brickwork above and cracks the bricks below. Rust staining on the underside of the lintel confirms it.
How serious: structural and progressive. The lintel will continue to expand until it fails completely. Extraction of the corroded lintel and installation of a new galvanised or stainless replacement is the only durable fix.
When to act immediately
Most cracks are watchable, you can sketch them, mark the ends with a pencil and a date, and see if they grow over a few weeks. Some shouldn’t be watched:
- Cracks wider than 3mm
- Cracks that grew noticeably after a storm or ground works
- Bowing or bulging brickwork (sight along the wall from a corner)
- Cracks combined with sticking doors and windows
- Cracks on chimneys, parapets, or any free-standing wall
If any of those describe what you’re seeing, get someone on site. We offer a free site assessment across Sydney Metro and can usually diagnose the cause within 30 minutes on site, or remotely from clear photos through our Project Brief form.