Cracks in render: which ones matter and which ones do not

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.