Why Render Cracks, and Which Cracks Actually Matter

A crack in render is not really the problem. It is the wall reporting a problem, and the shape of the crack tells you which problem.

This matters because the fix is completely different depending on the cause, and filling a crack without understanding it is why so many walls get patched over and over in the same spot.

Here are the five patterns you will actually see, roughly in order from harmless to serious.

A network of very fine cracks spread randomly across the surface, like a dried out riverbed in miniature. You often only notice it in raking light.

This is drying shrinkage. The render lost its moisture too fast, usually because it went on in hot or windy conditions and was not kept damp while it cured. It is cosmetic and it is confined to the surface.

It fills and disappears under a texture coat. No structural concern at all.

Clean vertical lines appearing at fairly consistent spacings along a long wall, often full height.

This is a control joint problem. Long uninterrupted panels of render need somewhere to relieve thermal movement, and if they do not get a joint they will make their own. The spacing gives it away, because the crack will appear roughly where a joint should have been.

The proper fix is to cut a genuine control joint at the crack, seal it correctly, and coat over. Filling it without cutting a joint means it comes back in the same place next summer.

Cracks forming a rectangular grid, matching the layout of the sheets behind. Very common on blueboard, and it usually shows up in the second summer.

This is missing or inadequate mesh reinforcement. Panel products move independently, and without alkali resistant mesh embedded across the full wall to tie adjacent sheets together, the render splits along every joint.

There is no patching fix for this. The affected area has to be cut back, meshed properly and recoated. Anyone offering to just fill them is selling you next year's problem.

A crack that runs diagonally in a stepped pattern, following the mortar joints in the brickwork behind. Often wider at one end than the other, and frequently running from the corner of a window or a door.

This is structural movement. The footing is moving, and the render is faithfully reporting it. On reactive clay sites across western and south west Sydney this is extremely common, and it is usually seasonal.

Render will not fix this and rendering over it will not stop it. If the crack is wider than about two millimetres, is growing, or you can see it change between wet and dry seasons, get the footing assessed by a structural engineer before spending anything on the surface.

Not strictly cracking, but it belongs on this list. A band of blistered, powdery or flaking render along the lower part of a wall, usually the bottom half metre to a metre.

This is rising damp carrying salt. Groundwater wicks up through the masonry, brings dissolved salts with it, and those salts crystallise as the wall dries. The crystals expand and push the render off from behind.

The moisture source has to be dealt with first. Rendering over it traps everything behind a new coating and it comes back worse. This is one of the most common failure modes we see across the Inner West and inner city.

As a rough guide: crazing is cosmetic, control joint cracks and mesh failures are render problems that a renderer can fix properly, and stair step cracking is a building problem that needs assessing before anyone touches the surface.

The one test anybody can do is to sound the wall. Tap systematically across it with your knuckle or the handle of a screwdriver. Solid, dull note means the render is bonded. Hollow note means it has let go and it will need to come off.

If a decent portion of the wall sounds hollow, patching becomes false economy quite quickly, and stripping the elevation and starting again is usually the better answer.