Why is my rendered wall cracking?

Render cracks tell you where the problem is. Cracks that are only in the render are a render problem. Cracks that continue through the render and into the wall behind, or that follow the shape of the structure, are the wall talking through the render.

Step by step

1. Look at the pattern before anything else.Fine random crazing across a whole panel is shrinkage. A regular grid or map pattern suggests the render is losing bond. A single straight crack running from a window corner is structural movement. Diagonal stepped cracking is the most serious pattern.
2. Tap the render around the crack.A hollow drumming sound means the render has debonded from the wall. Solid render sounds dull and dead. Map the hollow areas with chalk — this tells you the extent of the repair.
3. Check whether the crack goes through to the wall.Rake a little render out at the crack. If the crack continues into the masonry behind, this is structural movement and needs monitoring and probably professional assessment.
4. Look for the water.Cracks let water in behind the render, which then cannot get out. Look for damp patches, staining, moss and frost damage below the crack — that tells you the crack is already doing harm.
5. Check the details above and below.A missing bellcast bead at the base, a render that runs down past the damp-proof course, a coping or sill that discharges onto the render, or a blocked gutter overflowing — all cause cracking in predictable places.
6. Monitor structural cracks before repairing.Dated pencil marks at the crack ends, or a crack monitor, checked over two to three months. Repairing a moving crack achieves nothing.
7. Understand the render type.Cement render on a solid older wall is a common cause of trouble — it is hard and impermeable, so any water that gets behind is trapped. Lime render on an older building behaves very differently and is usually the correct material.
8. Decide between patch and replace.Isolated cracking in otherwise well-bonded render can be cut out and patched. More than about a quarter of a wall sounding hollow means the render on that elevation is at the end of its life.

Tips

• Photograph cracks with a ruler in shot and date them. The pattern over a year is far more informative than a single look.
• Cracks are usually widest where the movement is greatest, which points towards the cause.
• Vertical cracks at the corners of a bay or extension usually mean two structures moving at different rates — very common and often stable.

Common mistakes

• Filling render cracks with mastic and painting over — It hides the crack without addressing the water getting behind, and masonry paint over a moving crack splits again within months. It also makes proper diagnosis harder later.
• Assuming all cracking is cosmetic — Diagonal stepped cracking and cracks that continue into the masonry are structural signals. They need monitoring and often an engineer, not filler.

If it doesn't work

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