Diagonal cracking (brickwork)

Diagonal cracking is the defect where cracks run at an angle across brickwork courses, typically stepping along the bed joints and perpends and radiating outward from window and door corners. It is the most commonly raised defect at PCI on masonry-veneer houses and almost always traces back to one of three root causes.

The three root causes:

Missing or undersized articulation joints. Brickwork shrinks (clay) or expands (concrete masonry) and wants to move. Without continuous vertical articulation joints at the right spacing (typically 5 to 6 m max in clay, less in concrete masonry), the wall cracks at the weakest line, which is the corner of an opening.
Mortar-bridged “articulation” joints. The detail was provided but mortar was buttered across the joint during the lay, effectively glueing both sides together. The wall behaves as if there were no joint at all.
Footing movement. The footing under part of the wall has settled, heaved, or rotated relative to the rest. Common on Class M or H sites where the slab edge moves seasonally; less common on Class A or S sites.

How to tell the three apart:

Crack opens and closes seasonally (visible after the first wet season then again after first dry season): footing-movement cause. Likely Class M+ site with ground movement.
Crack first appears within 6-12 months of lay at a window corner with no nearby articulation joint: missing-joint cause.
Crack at a notional joint line that visually exists but has hairline mortar across it: bridged-joint cause. Probe with a knife to confirm.

Where diagonal cracks typically appear:

At window corners: bottom corner (sill) most often; top corner (head) next.
At door corners: top corners on internal doors in masonry; both top and bottom on external doorways.
At wall returns: where a long wall meets a perpendicular wall, the corner can step-crack.
At step-overs: where brickwork crosses an intermediate slab beam or step in foundation level.

The HIA / state-published Guides to Standards and Tolerances address acceptable hairline cracking thresholds. A typical residential threshold:

Less than 0.3 mm wide cracks that remain stable: cosmetic; acceptable.
0.3 to 1.0 mm wide with no recurring growth: monitor over 6 months; rectification often required at PCI.
Greater than 1.0 mm or visibly growing: structural concern; engineer’s assessment.

Rectification:

Cosmetic cracks: rake out the mortar to 10-15 mm depth, repoint with colour-matched mortar.
Structural cracks from footing movement: address the soil/footing first; cosmetic repair only after movement stabilises (typically 2 wet-dry cycles).
Missing-joint cracks: cut a vertical chase the full height of the wall, install a proper articulation joint with backer rod and sealant, re-point the brickwork either side.

For builders.

Spec articulation joints at design stage: locations and spacings on the structural drawings, not left to the brickie’s judgement on the day.
Watch the brickie at corners and joint lines: bridged joints are an install-quality issue that’s preventable on the day and expensive to fix later.
Set client expectations: hairline cracks in some locations are within acceptable tolerances. The Guide to Standards and Tolerances is the reference for the conversation.

Also known as: step cracking, stepped cracking, diagonal step crack.

Category: Practical / masonry / defects.

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