Cracks in render: which ones to worry about

This guide helps you sort the cracks you can live with from the ones to act on. For the repair itself, see our render repairs page, and for what it costs, render repair costs in Sydney.

Why does render crack?

Two broad reasons. The render itself shrinks as it cures, especially if it dried too fast in heat or wind, which leaves fine hairline cracks on the surface. Or the wall under it moves, because footings shift with soil moisture, or because different materials meet and move differently. The first is a finish problem. The second is a building problem that shows up in the render.

CSIRO's homeowner's guide to foundation maintenance puts it plainly: most cracking in masonry walls is a cosmetic nuisance only. The point of this guide is to spot the rest.

How wide is too wide?

The CSIRO guide sorts wall damage into five categories, from 0 to 4, using the crack-width limits in the Australian Standard for residential slabs and footings, AS 2870. In short:

• Category 0, negligible: hairline cracks, under about 0.1 mm.
• Category 1, very slight: fine cracks under about 1 mm that don't need repair.
• Category 2, slight: cracks under about 5 mm that are noticeable but easily filled; doors and windows may stick slightly.
• Category 3, moderate: cracks of about 5–15 mm, or several of 3 mm or more together; doors and windows stick and the wall may let water in.
• Category 4, severe: cracks of about 15–25 mm, distorted frames and walls that lean or bulge; sections of wall may need rebuilding.

Read the guide itself for the full descriptions. As a rule of thumb, from category 2 upwards it is worth finding out why the wall is moving before anyone repairs the render.

What do different crack patterns usually mean?

• A web of fine surface cracks (sometimes called crazing): usually shrinkage as the render cured. Rockcote says its acrylic finishes can bridge micro and hairline shrinkage cracking, but not structural cracking.
• Diagonal cracks from the corners of windows and doors: these openings concentrate movement in a wall. If they are wider than hairline, or growing, have the cause looked at.
• Stepped cracks following the brick joints under the render: usually movement in the masonry or the footings.
• A straight crack where two materials meet, such as brick and a timber or lightweight extension: different materials move differently. That joint needs a movement joint, not more render. Rockcote's article notes that cement-based renders and most texture coatings shouldn't be run over expansion, control or articulation joints.
• Hollow (drummy) patches with or without cracks: render that has lost its bond with the wall. Tap the wall: a dull, hollow sound means the render has come away.

How do you watch a crack?

Photograph it with a ruler or tape measure beside it, and mark the ends with a pencil and the date. Check it again after a few months, and after a wet season and a dry one. A crack that has grown, or a crack that opens and closes with the seasons, tells you the wall is moving. One that hasn't changed is far more likely to be cosmetic.

NSW Fair Trading's guide to standards and tolerances lists landscaping, the type and closeness of trees, and heavy, localised garden watering among the causes of soil moisture changes that move footings and crack walls. Look at what is growing and being watered near the crack, too.

When should you call an engineer before a renderer?

• When cracks are wider than about 5 mm, or there are several cracks of 3 mm or more together.
• When a crack keeps growing.
• When doors and windows near the crack have started sticking or frames look out of square.
• When a wall leans or bulges.

In those cases, the render is the last thing to fix. A renderer can patch a crack, but only fixing the cause stops it coming back.

What do we do when you send us photos of a crack?

Send a wide photo of the wall and a close-up of the crack with a ruler beside it. We tell you whether it looks like render shrinkage we can repair, or a crack to have checked first. If it is a repair, we visit to tap the wall for hollow render and quote the repair, and whether the wall should be recoated so the patch doesn't show.