Cracking at internal corners of exterior walls is a common problem on rendered homes, especially where materials move differently or the original prep was rushed. In Auckland, damp weather, sun exposure and building movement can all make those corner cracks reappear if they are not handled properly.
The key is not just filling the crack and painting over it. You need to understand why it happened, then use the right repair method so the finish can flex with the building instead of splitting again.
Why render cracks at internal corners
Internal corners are natural stress points. Two walls meet there, and even small movement from framing, weather changes or settling can open a crack along the join.
Another common cause is poor plastering detail. If the render was applied too rigidly, without proper mesh reinforcement or correct substrate prep, the corner can crack as soon as the building shifts.
Moisture can also play a part. If water gets behind the coating through failed sealant, hairline gaps or tired paint, the render can soften, lose adhesion and crack more easily.
- building movement at wall junctions
- missing or poorly installed reinforcing mesh
- shrinkage from thick or fast-dried render
- water ingress from failed sealants or cracks
How to repair the crack properly
For small, stable cracks, the first step is to open the crack slightly, clean out loose material and check whether the substrate is sound. A flexible exterior filler may be suitable for very fine cracks, but only if the area is dry and not actively moving.
If the crack runs along the corner or keeps returning, it usually needs a more robust repair. That often means removing loose render, reinstating mesh tape or reinforcing mesh across the corner, and rebuilding the surface with a compatible repair product.
Once repaired, the area should be primed correctly before repainting. A good exterior coating system helps seal the repair and reduces the chance of water getting back in.
- do not just skim over loose or hollow render
- use flexible, exterior-grade repair materials
- reinforce the corner if movement is likely
- prime and repaint with a suitable exterior system
How to prevent it from coming back
Prevention starts with good detailing. Internal corners should be formed cleanly, with suitable reinforcement and enough flexibility in the system to cope with movement.
Keep an eye on sealants around windows, doors and trim. If those fail, water can track into the render and trigger cracking nearby. Regular washing and repainting also help protect the finish from weathering.
If your home has older render, it is worth checking for other signs of movement such as recurring hairline cracks, blistering paint or hollow-sounding areas. Fixing the symptom without dealing with the cause usually leads to repeat repairs.
When to call Steve
If the crack is wider than a hairline, keeps reappearing, or you suspect water is getting behind the render, it is worth getting professional advice before you start patching. Steve is a Kiwi and a trade-qualified painter, and he can assess whether the problem is a simple cosmetic repair or a deeper prep issue that needs proper treatment.
For Auckland homes, especially in exposed coastal suburbs or older plastered houses, the right repair method matters. Steve can help with render crack repair, surface preparation and exterior repainting so the finish lasts and the corner detail looks tidy again.
Frequently Asked Questions
Can I just fill a cracked internal corner and paint over it?
You can for very fine, stable cracks, but if the crack is caused by movement or poor prep it will likely come back. The area should be cleaned, assessed and repaired with the right flexible product or reinforcement.
Is cracking at an internal corner always a structural problem?
Not always. Many corner cracks are due to normal movement, shrinkage or poor detailing rather than serious structural issues. But repeated or widening cracks should be checked properly.
What is the best way to stop render cracking in the future?
Good substrate preparation, correct mesh reinforcement, flexible repair materials, sound sealants and a quality exterior paint system all help reduce cracking and water ingress.
Need a hand with your project?
Happy to tackle some of it yourself? Great — but for anything you’d rather leave to a professional, Steve Lunjevich is a Kiwi and a trade-qualified painter with 20+ years’ experience. Call or email for honest advice or a free, no-obligation quote.