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.
Reading the crack: what caused it
Plaster does not crack at random. The pattern tells you what moved, and that decides whether sealing it will hold.
| Pattern | Cause | Does sealing hold? |
|---|---|---|
| Fine random map cracking over a whole panel | Plaster shrinkage, usually within the first two years | Yes — a breathable coating is normally enough |
| Straight horizontal line at a floor or lintel level | Differential movement between materials at a junction | Yes, if a flexible sealant is used |
| Diagonal from the corner of a window or door | Foundation or lintel movement | No — get it assessed before any cosmetic work |
| Vertical, stepping through the mortar joints | Structural: settlement or heave | No |
| Long vertical crack in a boundary or retaining wall | Thermal movement over an unjointed run | Yes, but a movement joint is the real fix |
| Crack following a chase or a pipe run | Backfilled chase shrinking away from the surrounding plaster | Yes |
Why this matters more in Cape Town than most places
Two loads act on a Cape Town elevation that do not act on an inland one, and between them they open cracks that a Highveld wall would never develop.
- Horizontal winter rain. A north-westerly front drives rain sideways for hours at a time. A vertical crack that would shed water on a still day becomes a direct inlet, and the elevation facing the front takes essentially all of it.
- Summer thermal cycling. Months of dry south-easter and strong sun expand and contract long plastered runs daily. Unjointed boundary and parapet walls are the worst affected, which is why they crack in the same places on every street.
- Coastal salt. On the Atlantic Seaboard, Blaauwberg and the Helderberg, salt in the air crystallises in the pores of plaster and paint and forces the surface apart from the inside. Salt-contaminated plaster has to come off and be replaced with a salt-retardant mix, which is part of damp proofing in Cape Town.
The practical consequence is that a crack repair on the weather elevation is worth doing properly and a crack repair on the sheltered elevation often is not urgent at all. We will tell you which is which rather than quoting the whole building.
Expansion joints — the crack that is supposed to be there
Not every straight vertical line in a wall is a defect. Buildings over about six metres long are designed with expansion joints — deliberate gaps that let the structure grow and shrink with temperature without tearing itself. A joint doing its job is a straight, full-height gap of consistent width, usually filled with a flexible sealant over a backing rod.
The failure is almost never the joint. It is the sealant in it, which has a service life of roughly ten to fifteen years in Cape sun before it goes hard, shrinks and lets go at one edge. Once that happens the joint is an open channel straight through the wall, and on an exposed elevation it will put water inside faster than any crack.
- How to tell it apart from a crack: an expansion joint is straight, runs the full height, and has parallel edges and a consistent width. A structural crack wanders, tapers, and usually steps through the mortar courses.
- How it is repaired: rake out the perished sealant completely, clean both faces, fit a closed-cell backing rod to the correct depth, and re-gun with a polyurethane or MS-polymer sealant. The backing rod is not optional — it sets the depth-to-width ratio that lets the sealant stretch instead of tearing.
- What not to do: fill it with mortar, crack filler or any rigid product. That converts a working joint into a fixed one, and the wall then cracks somewhere it was not designed to.
The same principle applies to the smaller movement joints in tiled balconies, screeds and around the perimeter of a slab. If a crack keeps reopening in the same place after being filled, it is almost certainly a movement joint that nobody recognised as one.
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 reason we are called back to a wall someone else has already sealed. The treatment is covered under damp proofing in Cape Town.
Related services
- waterproofing systems
- roof leak repair in Cape Town
- damp proofing in Cape Town
- waterproofing prices
- choosing a waterproofing contractor
- a free inspection
- contact us
Common questions
Which wall should I do first if I cannot do them all?
The elevation that faces north-west, because that is the one the winter fronts drive rain against. On most Cape Town houses one elevation takes the great majority of the water load and the others are comparatively sheltered. Doing the weather elevation properly beats doing all four cheaply.
When is a crack a structural engineer's problem, not mine?
When it runs diagonally from the corner of a window or door, when it steps through the mortar joints rather than sitting in the plaster, when it is over about 3mm and tapers, or when doors and windows near it have started sticking. Any of those and we will not seal it — sealing hides the evidence while the movement continues.
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.