AAC blocks are stable, accurately made units. When an AAC wall cracks, the cause is almost always one of a handful of construction details — which is good news, because each can be prevented.
The Six Common Causes
| Cause | Where the crack appears | Prevention |
|---|---|---|
| Thick cement mortar joints | Along bed and vertical joints | Thin-bed adhesive; lean mix and thin joints if mortar is used |
| Blocks laid wet | Random cracks as wall dries | Lay dry or lightly dampened; never soaked |
| No gap at beam or slab soffit | Horizontal crack at top of wall | Leave 10–20 mm, fill with flexible filler |
| No ties or mesh at junctions | Vertical crack at column; horizontal at beam | Ties every few courses; mesh before plaster |
| Plastering too early | Map cracks in plaster | Wait at least a week; cure plaster |
| Chasing for conduits and pipes | Along the chase | Cut with a chaser, not a chisel; fill and mesh |
Junctions: Where Most Cracks Start
An AAC wall meets the concrete frame at columns and beams. Concrete and AAC expand, shrink and deflect differently, so the joint between them moves. Two details handle it:
- During blockwork: fix galvanised ties or mesh strips into the column and lay them into the bed joints every few courses, tying the wall to the frame.
- Before plastering: fix fibre or galvanised mesh across the junction, overlapping both materials by at least 150 mm, so the plaster bridges the joint instead of cracking along it.
The Top Course Under the Beam
Beams and slabs deflect slightly under load and as concrete creeps over months. If the top course of blocks is jammed hard against the beam, that deflection crushes or cracks it. Leave a 10–20 mm gap at the top and fill it after the frame has taken load, with a flexible filler or a sloped top course.
Cracks You Should Not Ignore
Infill cracks are usually cosmetic. Call a structural engineer if you see:
- Diagonal cracks running from the corners of doors or windows, especially if widening.
- Cracks wider than a few millimetres, or growing.
- Cracks in columns, beams or slabs.
- Doors and windows that start to stick.
These can point to foundation settlement or overload rather than a laying problem.
Repairing Existing Cracks
For hairline cracks in plaster on AAC: rake the crack open into a V, clean it, fill with a flexible crack filler or polymer-modified mortar, apply mesh tape across it, and re-finish. For cracks at junctions that keep reopening, the joint is moving; a flexible sealant and mesh is better than rigid filler.
The Short Version
Build the wall right and AAC rarely cracks: adhesive joints, dry blocks, a soft joint under the beam, ties and mesh at every junction, and patience before plaster. When cracks are wide, diagonal or growing, look at the structure, not the blocks.
