Cracks in render: which ones matter and which ones do not
2 August 2026 · 4 min read
Hairline crazing, map cracking, diagonal cracks at window corners and stepped cracks in brickwork all mean different things. How to read them before you paint.
Almost every rendered house in Perth has cracks. Most of them are cosmetic and can simply be filled and painted. A few are telling you something structural, and painting over those wastes your money and hides the evidence.
Here is how to tell them apart from the ground.
Hairline crazing
Very fine, shallow, random cracks across a whole panel — like the surface of an old ceramic glaze. Usually less than 0.3mm wide.
**What it means:** shrinkage in the render as it cured. Extremely common, entirely cosmetic.
**What to do:** a flexible high-build coating bridges it. This is a paint problem, not a building problem.
Map cracking
Similar to crazing but larger — cracks that form rough polygons across the surface, typically 0.5 to 1mm.
**What it means:** usually a render mix that was too rich, or that dried too fast in the sun. Still generally cosmetic, but wider cracks let water in.
**What to do:** rake out, fill, and use a flexible membrane coating rather than a standard acrylic.
Straight vertical cracks in a consistent position
A crack running straight down, in the same place on multiple storeys or at regular intervals.
**What it means:** often a control joint doing exactly its job, or the edge of a sheet substrate. Sometimes it is where two different substrates meet.
**What to do:** if it is a control joint, it must not be filled rigid — it needs to stay flexible. Filling a control joint with rigid filler guarantees it cracks again, usually within a season.
Diagonal cracks from window and door corners
A crack running at roughly 45 degrees away from the corner of an opening.
**What it means:** stress concentration at the opening. This one is worth attention. It may be normal settlement, or it may be movement in the structure.
Measure it before you decide
Mark each end of the crack with a pencil and write the date. Photograph it. Check again in three months. A crack that has not moved is historical settlement and can be filled. A crack that has grown is active movement, and it needs a structural opinion before anybody paints.
Stepped cracks following the mortar
Cracks that zig-zag along the mortar joints in brickwork, often near a corner or below a window.
**What it means:** differential foundation movement. On Perth's reactive clay soils this can follow drought and heavy rain cycles, or a tree taking water from under a footing.
**What to do:** this is a structural engineer's question, not a painter's. We can make it disappear for a year. We would rather you spent the money finding out why.
Horizontal cracks
A long horizontal crack, particularly one with any displacement between the two sides.
**What it means:** potentially serious. Can indicate a failed wall tie, lintel movement, or lateral pressure.
**What to do:** stop. Get it looked at properly.
A quick reference
| Crack type | Typical width | Usually cosmetic? | Action |
|---|---|---|---|
| Hairline crazing | under 0.3mm | Yes | Flexible coating |
| Map cracking | 0.5-1mm | Yes | Rake, fill, membrane coat |
| Vertical at joint | varies | Yes | Keep the joint flexible |
| Diagonal from opening | 1-3mm | Sometimes | Monitor 3 months, then decide |
| Stepped in mortar | any | No | Structural opinion first |
| Horizontal with offset | any | No | Structural opinion first |
Why filling badly makes it worse
The most common mistake we see is a rigid cement-based filler pushed into a crack that is still moving. The filler is harder than the render around it. The next movement cycle cracks the render *beside* the repair, and now you have two cracks and a visible ridge.
Moving cracks need a flexible sealant and a flexible coating over the top. Static cracks can be filled rigid and will stay filled.
What we do
We rake out every crack wider than a hairline rather than painting over it — filler needs a key, and a smear across the surface will telegraph through within months. We tell you which cracks we think are cosmetic and which ones we think want another opinion. We will not quote a job that depends on hiding a structural crack.
And one thing that is not a crack
Dark staining that runs downward from a crack, or a white crystalline deposit around it, is efflorescence — salts carried out of the masonry by water moving through it. That is a moisture path, and it tells you the crack is letting water in. Sealing the surface without addressing the water traps it inside, where it will push the coating off later.