How to Fix Cracks in Walls in South Africa (and When to Worry)

Most cracks in South African homes are cosmetic: plaster shrinking as it dried, or a house settling the way every house does. A few are the first visible sign of a wall or foundation that's moving. The crack itself usually tells you which kind you have, through its width, its direction and whether it's still growing. So the most useful thing you can do before reaching for filler is read it properly. Filling a structural crack hides it for a season. Ignoring a cosmetic one just leaves an eyesore.

What Causes Cracks in Walls

Five causes account for almost every crack you'll see. Shrinkage happens when plaster dries too fast, from sun, wind or a mix with too much water, and leaves fine random cracks, often within the first year. Settlement is the ground compacting under a new house or extension, which shows up as cracks around door and window corners. Thermal movement is walls expanding in summer heat and contracting at night. Long, unbroken walls with no movement joints crack at their weakest point. Moisture from a leaking pipe, a blocked gutter or rising damp softens plaster and swells or shrinks the material behind it. Foundation or structural movement is the serious one: part of the house moving relative to the rest. That's usually caused by soil, drainage or an overloaded wall.

Types of Cracks in Walls and What Each One Tells You

Horizontal vs Vertical Cracks

Direction matters more than length. A vertical crack is usually the wall shrinking or settling along its height, and an even-width vertical crack that isn't growing is rarely structural. A horizontal crack means something is pushing on the wall or pulling it apart along its length. That could be soil behind a retaining wall, a roof spreading at the wall plate, or a steel lintel rusting and lifting the brick course above it. So a long vertical hairline crack in a passage is usually less worrying than a short horizontal or stair-step crack in an outside wall.

Are Cracks in Walls Serious? When to Worry

Cracks are normal. Almost every house has some, and most never get worse. A crack needs a professional rather than a tube of filler when any of these are true:

Cracks in ceilings follow the same logic. A straight crack along a board joint in a ceiling, or a hairline gap where the cornice meets the wall, usually comes from roof timbers moving with the seasons and is cosmetic. A ceiling that's sagging, a crack that runs down from the ceiling into the wall below, or cracks that appear at the same time as a wall crack are worth having checked. They can mean the roof is pushing on the walls or the walls are moving under it.

None of these automatically means the house is failing. But all of them mean the wall is moving, and you need to know why before anything gets filled. On a new home, raise it with the builder straight away, in writing. NHBRC-enrolled homes are covered for minor defects reported within three months of moving in, and for major structural defects reported within five years. A small plaster crack only falls under the five-year cover if a structural problem is causing it.

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Settlement Cracks in New Houses and Extensions

A new house, or a new extension, settles as the ground under its foundations compacts. Fine cracks around doors, windows and ceiling corners in the first year or two are expected. Let them finish moving before you fill and repaint. Fill too early and you'll be doing it twice. The crack worth watching is the one at the junction where an extension meets the original house. The two sit on different foundations and settle at different rates, so that joint will move. A well-built extension allows for it with a movement joint. One that doesn't will keep cracking along that line, and filler won't stop it.

Which Filler to Use for Cracks in Plaster Walls

The best filler is the one matched to the crack, not one particular brand:

How to Fix Hairline and Small Plaster Cracks

Small, stable cracks are a reasonable DIY job, as long as you don't skip the step everyone skips. Rake the crack open slightly with a crack chaser or the corner of a scraper. Filler pushed into a hairline crack has nothing to grip and falls out with the next movement. Brush out the dust, dampen the crack lightly or prime it, then press in a fine interior crack filler. Leave it slightly proud of the surface, sand it flush once it's dry, and prime before painting. Paint the whole wall, not just the patch. A touched-up square always shows.

How to Repair Deep or Large Cracks in Plaster Walls

Cracks wider than a few millimetres need a different material and a bit more preparation. Cut the crack back to sound plaster and undercut the edges slightly, so the repair is wider at the back than at the face and can't pop out. Then fill it in layers with a cement-based repair mortar, not a decorative filler. Where a crack keeps reopening in the same spot, bridge it with fibreglass mesh or crack-bridging tape set into a skim coat. That spreads the movement over a wider area instead of concentrating it on one line. If the plaster sounds hollow when you tap it, or comes away in pieces, it has lost its bond with the wall. Filling won't hold. That section has to be cut out and replastered, which is properly a job for a plastering contractor in Cape Town. Patching a hollow area only hides the failure for a while.

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How to Fix Large Cracks in Brick Walls

When the crack is in the brickwork itself, not just the plaster on top, filler is the wrong tool entirely. A stable stair-step crack in a garden or boundary wall is usually repaired by raking out the cracked mortar joints and repointing them. Cracked bricks are cut out and replaced. Where a wall has split right through, the standard fix is crack stitching: stainless-steel helical bars grouted into slots cut across the crack, so the wall acts as one piece again. Long boundary walls often crack because they were built without movement joints. When that's the cause, the repair should add a joint rather than just rebuild the same weakness. If the wall is leaning or bulging, it's past repointing, and rebuilding that section is usually the honest answer. That's bricklaying work, not a cosmetic job.

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How to Fix Structural Cracks in Walls

A structural crack is a symptom, so the fix starts with the cause. That usually means a structural engineer's assessment of the foundation, the soil and the drainage around the house. Typical causes include a leaking stormwater pipe washing out soil beside a footing, a tree root system drying out clay, or a lintel that has rusted and lifted the brickwork above it. Only once the movement has been stopped, and confirmed as stopped, does it make sense to stitch the brickwork, replaster and paint. Doing it in the other order is how people pay for the same crack twice.

Cracks in the Walls of an Old House

Older Cape Town houses, especially ones built before cement became standard, were often laid and plastered with lime rather than cement. Lime is softer and lets moisture escape through the wall. Patching it with hard cement plaster or mortar is a common mistake. The cement is stiffer than the wall around it, so new cracks form along the edges of the repair, and it traps damp behind it, which makes the old plaster crumble faster. Repairs to an old lime-plastered wall should use a lime-based plaster that moves and breathes the same way as the original. In heritage areas, check whether the work needs council or heritage approval before anyone starts.

Why Cracks Keep Coming Back in Cape Town Homes

Unlike most of South Africa, Cape Town gets its rain in winter, and houses on clay soils move with that cycle. Clay swells when it's wet and shrinks as it dries, so walls on clay can show cracks that open in summer and close up again in winter. Fill one of those cracks in February and it often reappears by the next summer, because the house never stopped moving. If your crack behaves like that, the long-term fix is about controlling water around the foundations: gutters, downpipes, paving falls and garden beds against the walls. Filler alone won't do it. In older homes nearer the coast, crumbling plaster with white salt deposits usually points to damp drawing salts through the wall, not to movement. That needs the damp dealt with before any replastering will last.

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Common Crack Repair Mistakes

Are Wall Cracks Covered by Insurance?

Sometimes, and it depends on the cause. Most South African homeowner's policies treat gradual settlement, shrinkage and wear as maintenance and exclude them. Sudden damage, like a burst pipe or a storm, is more likely to be covered, and subsidence or landslip cover is often an optional extra. Before you claim, find out what caused the crack. An engineer's report or a contractor's written assessment is what the insurer will ask for anyway. The wording varies between insurers, so check what your own policy says about subsidence and gradual damage before you assume either way.

Getting a Crack Looked At

Most cracks turn out to be cosmetic. The ones that aren't get more expensive the longer they're filled and painted over. SmartCurve has been repairing plaster and brickwork across Cape Town since 2017, on more than 100 completed projects. During a free site visit we'll tell you whether your crack is a filler job, a plaster or brickwork repair, or something that needs an engineer first, before any quote is written. When it's our work, you get a fixed price with no hidden costs. Book your free site visit or call us on +27 62 907 0802.