Render cracking at internal corners of exterior walls is one of the most common complaints NZ homeowners face — and it's almost always preventable. The corner is a stress concentration point where two planes of render meet, and without the right reinforcement and technique, cracking is almost inevitable over time. The good news is that with proper preparation, the correct materials, and a methodical approach, you can achieve a durable, crack-free finish.
Why Internal Corners Crack
Before fixing the problem, it helps to understand why it happens. Internal corners (also called re-entrant corners) are particularly vulnerable because:
- Differential movement — the two adjoining wall planes can move independently due to thermal expansion, moisture cycling, or substrate flex.
- Render shrinkage — all cement-based renders shrink slightly as they cure. At a corner, shrinkage pulls in two directions at once, creating tensile stress right at the junction.
- Poor substrate preparation — dust, paint, or a smooth surface behind the render reduces adhesion and allows cracking to propagate.
- Inadequate reinforcement — skipping corner mesh or using the wrong type leaves the render unsupported at its most vulnerable point.
- Thick single coats — applying render too thick in one pass increases shrinkage stress.
In New Zealand's variable climate — with UV exposure, coastal salt air, and significant temperature swings — these stresses are amplified, making good practice even more important.
Step-by-Step: How to Prevent Cracking at Internal Corners
1. Prepare the Substrate Thoroughly
Clean the substrate of all dust, grease, loose material, and any existing flaking render. If you're rendering over a masonry or fibre-cement substrate (such as James Hardie sheet), check the manufacturer's system requirements — some products require a specific primer or bonding agent before render is applied. A well-keyed, absorbent, and clean surface is the foundation of a crack-free result.
2. Apply a Bonding Coat or Scratch Coat First
Never apply render in one thick coat. A scratch coat (also called a backing coat) of 6–10 mm should be applied first, then scratched horizontally to provide a mechanical key for the finish coat. Allow it to cure sufficiently — typically 24–48 hours depending on conditions — before applying the next coat. This staged approach dramatically reduces shrinkage cracking.
3. Use Alkali-Resistant (AR) Mesh at Every Internal Corner
This is the single most important step. Alkali-resistant fibreglass mesh embedded in the render at internal corners bridges the junction between the two wall planes and distributes stress across a wider area. Standard fibreglass mesh is not suitable — it will degrade in the alkaline environment of cement render. Always specify AR mesh.
- Cut a strip of AR mesh approximately 200–300 mm wide.
- Fold it lengthways so it sits neatly into the corner with equal coverage on each face.
- Embed it into the wet scratch coat, pressing it firmly so it is fully encapsulated — not sitting on the surface.
- Overlap mesh joins by at least 100 mm.
Products from Sika and Ardex include render systems with compatible mesh and bonding agents — always follow the manufacturer's system recommendations rather than mixing products from different systems.
4. Consider a Control Joint
In some situations — particularly on long wall runs or where two different substrates meet — a control joint (also called an expansion joint) at or near the internal corner may be more appropriate than mesh alone. A control joint deliberately creates a neat, sealed break that accommodates movement without random cracking. Consult your render system manufacturer or a qualified renderer about whether this applies to your situation.
5. Apply the Finish Coat Correctly
- Keep individual coats within the thickness recommended by your render system (typically 6–10 mm per coat for cement-based renders).
- Work the finish coat into the corner carefully, maintaining consistent thickness.
- Avoid overworking the surface, which can draw water to the face and cause surface crazing.
- Protect fresh render from direct sun, wind, and rain for at least 48 hours. In hot or windy NZ conditions, mist-curing (lightly dampening the surface) can help slow drying and reduce shrinkage cracking.
6. Seal and Paint with a Flexible Coating
Once the render has fully cured (allow at least 28 days for cement-based renders before painting), apply a flexible exterior paint or texture coat. Products from Resene, Dulux, and Wattyl all offer exterior systems with elastomeric or flexible formulations that can bridge hairline cracks and provide an additional layer of protection. Check the product's suitability for render substrates and follow the recommended primer system.
NZ Building Code Considerations
Exterior render is part of your home's weathertightness system, which falls under NZ Building Code Clause E2 (External Moisture). Poor render detailing — including cracked corners — can allow water ingress, leading to serious structural damage over time. BRANZ publishes guidance on render systems and weathertightness that is well worth reviewing. If your home has had significant weathertightness issues, remediation work may require a building consent — always check with your local council before undertaking substantial re-rendering work.
When to Call a Professional
While an experienced DIYer can tackle small render repairs, internal corner cracking that keeps returning — or cracking that is widespread across your exterior — may indicate a deeper problem such as substrate movement, structural issues, or systemic weathertightness failure. In these cases, it's strongly advisable to engage a qualified renderer or plasterer. Persistent cracking near windows, doors, or roof junctions should be assessed promptly, as these are high-risk areas for water ingress under Clause E2.
You can find experienced, local rendering professionals on tradedirectory.co.nz to get the job done right the first time.
Licensed Building Practitioner (LBP)
