Cosmetic or structural?
| Sign | Likely cosmetic | Likely structural |
|---|---|---|
| Width | Under 2mm, even | Over 3mm, or tapering |
| Pattern | Random map cracking, shrinkage | Diagonal from a corner of an opening |
| Through the brick | No — plaster only | Yes — stepped through the mortar joints |
| Doors and windows | Operating normally | Sticking or out of square |
| Movement | Stable over months | Visibly widening |
| Action | Rout, seal, re-coat | Structural engineer before any cosmetic repair |
We will not seal over a crack we think is structural. Sealing it hides the evidence and the underlying movement carries on regardless.
How cracks are repaired properly
- Rout the crack out to a consistent V or U profile, at least 6mm wide and 6mm deep — sealant needs a joint with a shape, not a hairline.
- Clean out all dust and loose material.
- Prime the joint faces.
- Fill with a flexible polyurethane or hybrid sealant that can accommodate movement, not a rigid filler that will simply crack again next to the old crack.
- Skim and make good so the repair is invisible.
- Re-coat the elevation, ideally with a breathable water-repellent so the whole wall resists driven rain rather than just the repaired line.
Sealing the whole elevation
Where the plaster is generally porous rather than cracked in one place — common on older and on coastal properties — the answer is a penetrating silane or siloxane cream applied across the elevation. It soaks in and lines the pores so liquid water cannot pass, while still allowing water vapour out.
That last property matters enormously. A non-breathable sealer on a wall that already holds moisture traps it, and the plaster comes off in sheets a season or two later. This is the most common cause of the penetrating damp we are called to fix a second time.
Common questions
How much does wall crack repair cost?
About R180–R450 per linear metre for routing, sealing and making good. A full breathable water-repellent coating over an elevation runs roughly R90–R180 per m² on top of that.
Why do cracks keep coming back in the same place?
Because a rigid filler was used on a joint that moves. Plaster cracks are usually movement joints the building made for itself, and the repair has to flex. A cement-based filler will crack again, generally right alongside the old repair.
Can I just paint over a crack?
Paint bridges a hairline crack for a season at most. Once the crack opens under thermal or wind loading, the paint film splits and water goes straight in. Rout it, seal it, then paint.