If you are asking why is my wall cracking, the useful answer is not “houses move”. It is which kind of movement you are looking at, because that decides whether you fill it or phone an engineer.
Most cracks in South African homes come from one of seven causes. Each leaves a recognisable pattern, and you can narrow it down yourself before anyone quotes you.
Why is my wall cracking: the 7 usual causes
1. Plaster shrinkage and crazing
New plaster loses moisture as it cures. Fine, random, map-like cracking across the surface is shrinkage, not structure.
It is common on recent work, on plaster applied too thickly, or where it dried too fast in wind and sun. Cape Town summers do exactly that.
2. Thermal movement
Materials expand in heat and contract at night. Long straight cracks that appear on sun-facing walls, get slightly wider in summer and close again in winter, are thermal.
They cluster near openings and at changes of material, because those are the weak lines.
3. Settlement
All buildings settle a little in their first years. Normal settlement produces fine, stable cracks that stop growing.
The concerning version is differential settlement, where one part of the house drops more than another. That produces stepped diagonal cracks through the brickwork, usually wider at the top or bottom.
4. Clay soils that swell and shrink
This is the big South African one. Clay-rich soils expand when wet and shrink when they dry, lifting and dropping foundations with the seasons.
Parts of Cape Town and the Northern Suburbs sit on reactive soils. The signature is cracking that gets noticeably worse at the end of a long dry summer and eases after winter rain, year after year.
5. Water, damp and leaks
Leaking gutters, poor drainage, a failed downpipe or rising damp all weaken plaster and brickwork. Cracking with staining, bubbling paint or a tide line at the bottom of the wall is a moisture problem first.
Water also changes soil moisture right against the foundation, which is how a blocked gutter turns into a movement crack.
6. Roof and structural movement
Roof timbers move with moisture and load. Cracks along the top of walls, at the wall-to-ceiling joint, or radiating from where a beam bears, often come from above rather than below.
7. Poor original construction
Missing or badly placed movement joints, a lintel that is too short, inadequate foundations for the soil, or mortar that was too strong for the brick. These cracks appear early, in the same place, and come back after every repair. Mix ratios and workmanship standards are published by the NHBRC.
Telling them apart by shape
- Fine random map pattern: plaster shrinkage – cosmetic
- Straight, seasonal, sun-facing wall: thermal – usually cosmetic
- Vertical, fine, stable: minor settlement – usually cosmetic
- Stepped or diagonal through brickwork: differential movement – get it assessed
- Horizontal in a masonry wall: lateral pressure – assess urgently
- Wider at one end: rotation – assess
- Crack with damp staining: moisture – fix the water first
Width is the quickest test
- Under 2mm: usually cosmetic
- 2mm to 5mm: monitor and note any growth
- Over 5mm: have it investigated
Width alone is not the whole story. A 3mm stepped crack through brickwork that reaches both sides of the wall matters more than a 4mm shrinkage crack in fresh plaster.
When to worry, honestly
Some signs should not be filled and forgotten. Get a professional opinion if you see any of these.
- The crack is wider than 5mm
- It is stepped or diagonal and runs through the bricks, not just the plaster
- It is visible on both sides of the same wall
- Doors or windows have started to stick
- Floors slope or skirtings have opened up
- It has grown measurably in the last few months
- It is horizontal in a load-bearing or retaining wall
A structural engineer’s assessment is commonly quoted at about R5,000 to R15,000 in South Africa. It is a small figure against a foundation repair that can run past R100,000, and it turns guesswork into a decision.
How to monitor a crack properly
- Photograph it with a ruler alongside for scale
- Pencil a line at each end and write the date on the wall
- Record the widest point in millimetres
- Re-check monthly for three months, ideally across a season change
Three months of records tells a professional far more than a single photograph. Movement that stops is very different from movement that continues.
What to do about each cause
- Shrinkage and thermal: fill with a flexible product and repaint
- Moisture: fix the leak or drainage first, dry the wall, then repair
- Minor settlement: monitor, then repair once it is stable
- Clay soil movement: manage water around the house, and get it assessed if it is worsening
- Structural: engineer first, builder second – never the other way round
Managing water is the cheapest useful action for most homeowners. Keep gutters clear, direct downpipes away from the foundation, and avoid large thirsty trees close to the house on clay soils.
Once the cause is settled, the repair method matters. Our guide to wall crack repair covers the V-groove, mesh and re-plastering process and what it costs, and the skimming plaster article explains getting the finished wall smooth again.
Getting a straight answer
Any competent builder should be willing to tell you when a crack is beyond their scope. We would rather send you to an engineer than plaster over a problem that comes back.
We assess and repair cracked walls across Cape Town and the Northern Suburbs as part of our brickwork and plastering work, and we also cover the wider region as building contractors in Durbanville.
Assessments are free and there is no call-out fee. Send photographs with a ruler in the shot and we will tell you what we think you are dealing with.
Frequently asked questions
Why is my wall cracking?
Most commonly plaster shrinkage, thermal movement, settlement, reactive clay soils, moisture from leaks or damp, roof movement, or poor original construction. The shape tells you which. Fine random crazing is shrinkage, straight seasonal cracks on sun-facing walls are thermal, and stepped diagonal cracks through brickwork point to ground movement.
When should you worry about cracks in walls?
Worry when a crack is wider than about 5mm, stepped or diagonal through the brickwork, visible on both sides of the same wall, or still growing. Sticking doors and windows, sloping floors and skirtings pulling away are supporting signs. Any horizontal crack in a load-bearing or retaining wall should be assessed promptly.
Are cracks in walls normal in a new house?
Fine cracks are common in the first few years as plaster dries and the building settles. Shrinkage crazing and thin vertical cracks that stop growing are usually cosmetic. What is not normal is stepped diagonal cracking through brickwork, or cracks that keep widening, which suggest the foundations are moving unevenly.
Do clay soils cause wall cracks in South Africa?
Yes. Clay-rich soils swell when wet and shrink when dry, which lifts and drops foundations seasonally. The signature is cracking that worsens at the end of a long dry summer and eases after winter rain, repeating year after year. Managing drainage and keeping large trees away from the house both reduce the effect.
How do I know if a crack is getting worse?
Photograph it with a ruler for scale, pencil a line at each end with the date, and record the widest point in millimetres. Re-check monthly for three months so the record spans a season change. Growth over that period is the clearest evidence of active movement and is exactly what an engineer will ask for.