In short
- Hairline crazing usually comes from drying shrinkage and is mostly cosmetic.
- Diagonal cracks from window and door corners usually mean mesh reinforcement was missing at those stress points.
- Wide, stepped, tapering or recurring cracks may indicate structural movement and need investigating before repair.
- Cracks let water in; even cosmetic cracks should be repaired before they cause hollow render.
Common causes
- Drying shrinkage: render shrinks as it cures. Fast drying in sun or wind makes it worse, causing fine crazing.
- Over-strong mix: cement-rich render is more rigid than the wall and cracks as the wall moves.
- Missing mesh: corners of openings concentrate stress. Diagonal patch mesh prevents most cracks there.
- Thermal movement: dark colours and long elevations expand and contract more. Movement joints relieve this.
- Mixed substrates: where brick meets block or timber, different movement causes cracks unless mesh bridges the joint.
- Structural movement: subsidence, heave, failed lintels or roof spread.
Reading the crack
| Crack | Likely cause | Action |
|---|---|---|
| Fine crazing across the surface | Shrinkage | Monitor; repaint or re-coat if unsightly |
| Diagonal from window corners | No stress mesh | Cut out, mesh and repair |
| Horizontal along a lintel | Lintel movement or corrosion | Inspect lintel |
| Stepped or tapering, wider at one end | Possible structural movement | Get a surveyor or engineer |
| Crack that reopens after repair | Ongoing movement | Investigate before re-repairing |
Repairing cracked render
Repair means cutting out the crack, treating the cause, filling with compatible material and reinforcing with mesh where needed. For through-coloured render, finish to a natural break to hide the patch. See render repairs.
Preventing cracks in new render
- Use mesh in the base coat, with extra diagonal pieces at the corners of openings.
- Include movement joints on long elevations and where different materials meet.
- Choose a render no stronger than the wall behind it.
- Protect fresh render from sun, wind, rain and frost.
- Allow each coat its full curing time.
When to call a structural engineer
Get professional advice before rendering if cracks are wider than a few millimetres, are wider at one end, step through brickwork joints, line up with cracks inside, or are accompanied by sticking doors and windows. Rendering over active structural movement will not stop it, and the new render will crack along the same lines.
Questions people ask
Are hairline cracks in new render normal?
Some fine crazing can occur, but a well-installed system should not crack significantly. Report cracks in new render to the installer under the guarantee.
Can I fill render cracks myself?
Small cracks in painted render can be filled with a flexible exterior filler and repainted. Through-coloured render and larger cracks are best repaired professionally.