The best mesh to embed in render to reduce cracking is alkali-resistant (AR) fibreglass mesh. Standard fibreglass or ordinary woven mesh will degrade quickly in the high-pH environment of cement-based render, so the alkali-resistant coating on AR mesh is essential for long-term performance. Here's what you need to know before you start.
Why Mesh Matters in Render
Render is a rigid coating applied over a substrate that can move — whether that's timber framing, fibre cement sheet, or masonry. Temperature changes, moisture cycling, and minor structural movement all create stress in the render coat. Without reinforcement, that stress concentrates at weak points and causes map cracking, shrinkage cracks, or delamination. Embedding mesh distributes the stress across a wider area, dramatically reducing visible cracking.
This is particularly important in New Zealand's varied climate, where coastal humidity, UV exposure, and freeze-thaw cycles in colder regions all put render systems under pressure.
The Right Mesh: Alkali-Resistant Fibreglass
AR fibreglass mesh (sometimes called render mesh or base coat mesh) is the industry standard for modern render systems. Key things to look for:
- Alkali-resistant coating — essential; standard fibreglass will break down within months in cement render
- Weight/density — typically 145–165 g/m² for standard wall render; heavier mesh (up to 300 g/m²) is used in high-impact or commercial applications
- Aperture size — usually around 4–5 mm × 4–5 mm grid openings, which allows the render to key through the mesh properly
- Tensile strength — look for mesh with adequate tensile strength in both directions for standard residential work — check the manufacturer's technical data sheet for specific ratings
| Mesh Type | Suitable for Render? | Notes |
|---|---|---|
| AR fibreglass mesh | ✅ Yes | Industry standard; alkali-resistant |
| Standard fibreglass mesh | ❌ No | Degrades in cement's alkaline environment |
| Galvanised metal lath | ✅ Yes (specific uses) | Used under thick-coat render on timber; can rust if coating fails |
| Stainless steel lath | ✅ Yes | Premium option; corrosion-proof; higher cost |
| Polypropylene mesh | Sometimes | Check manufacturer specs; not universal |
Where to Embed the Mesh
Mesh should sit in the middle to upper third of the render coat — not flat against the substrate and not at the surface. The general rule is to:
- Apply a scratch coat (first layer) of render to the substrate
- While still wet (or slightly green), press the AR mesh firmly into the surface, overlapping sheets by at least 100 mm at joins
- Apply the second coat immediately over the top, fully encapsulating the mesh
The mesh must be fully embedded with no dry spots or bridging. Any mesh that sits proud of the surface or isn't fully covered will telegraph through the finish coat and can cause delamination.
Substrate-Specific Considerations
Fibre Cement Sheet (e.g. James Hardie products)
James Hardie systems such as Harditex and Hardiflex have specific approved render and mesh systems. Always follow the manufacturer's technical data sheet — using an unapproved mesh or render product can void the warranty and may not comply with NZ Building Code Clause E2 (External Moisture).
Masonry and Concrete Block
On solid masonry, mesh is often used at stress points — around window and door corners, control joints, and changes in substrate — rather than across the entire wall. A 45-degree diagonal strip of mesh at each opening corner is standard practice to resist corner cracking.
Timber-Framed Walls
On timber framing, a building wrap and metal or fibreglass lath are typically required before render is applied, in accordance with BRANZ guidance and NZS 3604 framing standards. The render system must manage moisture appropriately under E2.
Control Joints and Mesh Together
Mesh alone won't eliminate all cracking on large surfaces. Control joints (deliberate breaks in the render) should be installed at regular intervals — typically every 4–5 metres on large walls and at all substrate sheet joins. Use mesh up to (but not across) control joints; bridging a control joint with mesh defeats its purpose.
Mixing and Application Tips
- Always follow the render manufacturer's mix ratios — a mix that's too rich in cement increases shrinkage cracking
- Keep render coats to the recommended thickness (typically 6–10 mm per coat for most systems)
- Cure render slowly; avoid applying in direct hot sun or strong wind, which causes rapid drying and cracking
- Sika and Ardex both supply polymer-modified render systems with specific mesh recommendations — check their technical data sheets
Sourcing Mesh in New Zealand
AR fibreglass render mesh is widely available from Mitre 10, Bunnings, and PlaceMakers, as well as specialist render and plastering suppliers. Buy from a reputable supplier and confirm the product is genuinely alkali-resistant — check the product data sheet if you're unsure.
When to Call a Professional
While embedding mesh in render is not legally restricted work in the same way as electrical or plumbing, render application is a skilled trade. Poor technique leads to delamination, cracking, and moisture ingress — which can be costly to remediate and may breach the NZ Building Code. If your project involves:
- A weathertight cladding system on a new or renovated home
- Fibre cement sheet systems with manufacturer warranties
- Large wall areas or multi-storey buildings
- Any doubt about substrate preparation or system compatibility
…it's strongly advisable to engage a qualified renderer. You can find experienced local renderers on tradedirectory.co.nz. For guidance on what work requires building consent, check with your local council or visit MBIE Building Performance.
Licensed Building Practitioner (LBP)
