Cracking render is one of the most common frustrations for DIYers and tradespeople alike — but in most cases it's preventable. The main culprits are drying too quickly, incorrect mix ratios, inadequate surface preparation, and applying coats that are too thick. Follow the steps below and you'll give your render the best chance of curing hard, smooth, and crack-free.
Why Render Cracks in the First Place
Render is a cement-based (or lime-based) coating that needs to cure slowly and evenly. When moisture escapes too fast — due to hot sun, wind, or a thirsty substrate — the surface shrinks faster than the body of the render, causing shrinkage cracks. Other common causes include:
- Too much cement in the mix (a rich mix shrinks more)
- Too much water added on-site to make the mix workable
- Applying render too thick in a single coat
- Skipping a bonding agent or failing to dampen the substrate
- No control joints on large areas
- Applying render in direct sun, high wind, or temperatures above about 30 °C or below 10 °C
Get Your Mix Right
A standard sand-and-cement scratch coat is typically mixed at around 1 part cement : 4–5 parts plastering sand by volume. Finish coats are usually leaner (more sand, less cement) to reduce shrinkage. If you're using a bagged pre-mixed render product — which is often easier for DIYers — follow the manufacturer's water ratio exactly and resist the urge to add extra water.
Lime is sometimes added (a 1:1:6 cement:lime:sand mix is a traditional option) to improve workability and flexibility, which can reduce cracking. Check with your render supplier about what suits your substrate and NZ climate zone.
Tip: Never re-temper render that has started to stiffen by adding more water. Discard it and mix a fresh batch.
Prepare the Surface Properly
Render needs something to grip. On masonry or concrete block, dampen the surface with clean water before applying — but don't soak it. A wet substrate slows moisture loss from the render, giving it time to cure properly. On dry or porous surfaces, a bonding agent (such as a product from Sika or Ardex) can dramatically improve adhesion and reduce the risk of delamination cracking.
For fibre-cement substrates like James Hardie products, always check the manufacturer's system specifications — some require specific primers or mesh reinforcement before rendering.
Apply in Thin, Even Coats
This is where many DIYers go wrong. Applying render too thick in one go is a leading cause of cracking. As a general guide:
| Coat | Typical Thickness |
|---|---|
| Scratch coat (first coat) | 6–10 mm |
| Brown/float coat (second coat) | 6–10 mm |
| Finish coat (final coat) | 3–6 mm |
Allow each coat to firm up and scratch the surface before applying the next. This gives the following coat something to key into. Never apply a stronger (richer) coat over a weaker one — this is a common cause of map cracking.
Slow Down the Curing Process
Curing is about keeping moisture in the render long enough for the cement to hydrate fully. In New Zealand's variable climate — especially in hot, dry Canterbury summers or breezy coastal areas — this can be a real challenge.
- Shade the work from direct sun using shade cloth or hessian if needed
- Mist the render lightly with clean water 2–3 times a day for at least 3 days after application
- Avoid rendering in strong wind — wind strips moisture just as fast as sun
- Don't render when overnight temperatures are forecast to drop below 10 °C, as cold slows hydration and can cause frost damage
- On hot days, render early in the morning and stop by mid-afternoon
Use Control Joints on Large Areas
On any rendered area larger than about 4–5 m² (and especially on long, unbroken runs), control joints (also called movement joints) allow the render to expand and contract without cracking randomly. These are typically formed with a purpose-made PVC or aluminium bead pressed into the scratch coat before it sets. BRANZ has good guidance on movement joint placement for external cladding systems.
Use Reinforcing Mesh Where Required
On substrates prone to movement — such as fibre-cement sheet joins, around window and door corners, or over mixed substrates — fibreglass or alkaline-resistant (AR) mesh embedded in the scratch coat adds tensile strength and helps prevent cracking at stress points. This is standard practice on modern plaster cladding systems and is often required under the NZ Building Code (specifically Clause E2 — External Moisture) for weathertight performance.
When to Call a Professional
Rendering is a skilled trade, and large or complex jobs — particularly external cladding on new builds or re-cladding work — may require a building consent depending on scope. Always check with your local council before starting, as requirements vary. Some re-cladding work is exempt under Schedule 1 of the Building Act, but confirm this with your council rather than assuming.
If your existing render has widespread cracking, delamination, or signs of moisture ingress behind it, this could indicate a deeper weathertightness issue. In that case, it's important to get a qualified renderer or Licensed Building Practitioner (LBP) to assess the situation before patching over the top. You can find experienced local renderers on tradedirectory.co.nz.
Sources & Further Reading
- BRANZ – External Plastering and Rendering Guidance
- MBIE Building Performance – NZ Building Code Clause E2 (External Moisture)
- Sika NZ – Bonding Agents and Render Products
- Ardex NZ – Surface Preparation Products
- James Hardie NZ – Substrate and Cladding Systems
- Licensed Building Practitioners – Find an LBP
Licensed Building Practitioner (LBP)
