Rendering over a concrete block fence is a satisfying DIY project that can transform a plain masonry wall into a smooth, attractive finish. Done correctly, it will last for decades — but skipping steps or using the wrong mix can lead to cracking, delamination, and costly repairs. Here's how to get it right.
Assess the Surface First
Before you mix a single bucket of render, take a close look at the blocks. Check for:
- Loose or crumbling mortar joints — rake these out at least 10–15 mm deep and repoint before rendering
- Efflorescence (white salt deposits) — scrub off with a stiff brush and treat with a proprietary efflorescence remover
- Oil, paint, or form-release agents — these must be removed completely, as they will prevent adhesion
- Cracks — fill structural cracks and allow to cure before proceeding
A solid, clean, and sound substrate is the single most important factor in a successful render job.
Gather Your Materials and Tools
For a standard cement render on a concrete block fence you will need:
- Portland cement (GP cement)
- Plastering sand (washed, well-graded)
- Lime (optional but recommended for workability and crack resistance)
- Water
- Bonding agent (such as a product from Sika or Ardex)
- Hawk and trowel, scratch comb, spirit level, straight edge (screed rail), mixing drill or cement mixer, bucket, hose
Mix Ratios
A standard two-coat render system uses the following mixes:
| Coat | Cement | Lime | Sand | Thickness |
|---|---|---|---|---|
| Scratch coat (first) | 1 part | 0.5 part | 4–4.5 parts | 8–12 mm |
| Finish coat (second) | 1 part | 1 part | 5–6 parts | 6–8 mm |
The scratch coat is slightly stronger to bond to the block, while the finish coat is leaner and more flexible. Never apply a stronger mix over a weaker one — this is a common cause of delamination.
Alternatively, pre-mixed bagged renders are available from Bunnings or Mitre 10 and take the guesswork out of mixing. Always follow the manufacturer's instructions for water ratios.
Step-by-Step Application
1. Dampen the Surface
Wet the block wall thoroughly with a hose the evening before and again just before applying render. The surface should be damp but not dripping — this prevents the dry blocks from sucking moisture out of the render too quickly, which causes cracking.
2. Apply a Bonding Agent
For smooth or dense blocks, apply a bonding agent or a slurry coat (a thin mix of cement and water brushed on) to improve adhesion. Allow it to become tacky before applying the scratch coat.
3. Apply the Scratch Coat
Using a hawk and trowel, apply the scratch coat firmly, pressing it into the joints and working from the bottom up. Aim for an even thickness of 8–12 mm. Once it begins to stiffen (but before it fully sets), use a scratch comb or notched trowel to score horizontal lines across the surface. These keys help the finish coat bond. Allow the scratch coat to cure for at least 24–48 hours, keeping it damp by misting with water — this is called curing and is critical in hot or windy NZ conditions.
4. Apply the Finish Coat
Once the scratch coat has cured, dampen it lightly and apply the finish coat. Work in manageable sections (around 1–2 m²) to keep a wet edge and avoid visible joins. Use a straight edge to screed the surface flat, then finish with a wooden or sponge float for a textured look, or a steel trowel for a smoother finish.
5. Cure the Finish Coat
Keep the finished render damp for at least 3–7 days by misting regularly, especially in summer or on north-facing walls. Avoid rendering in direct hot sun, strong wind, or when frost is forecast — all of these will cause premature drying and cracking.
Control Joints and Cracking
For long fence runs (generally over 4–5 m), consider installing control joints to manage shrinkage cracking. These are vertical grooves cut or formed into the render at regular intervals. BRANZ provides useful guidance on render systems and crack management for NZ conditions.
Painting the Render
Allow new render to cure for at least 28 days before painting. Use an alkali-resistant primer followed by a quality exterior paint. Resene, Dulux, and Wattyl all offer exterior masonry paint systems suitable for rendered surfaces — check their websites for the correct primer and topcoat recommendations for new render.
When to Call a Professional
While rendering a fence is generally within reach of a capable DIYer, there are situations where you should bring in a qualified renderer:
- Large or complex fence runs where consistent thickness and flatness are critical
- Retaining walls — these may require a building consent depending on height and soil conditions; always check with your local council before starting work. The MBIE Building Performance website has guidance on when consent is needed.
- Heritage or boundary walls where getting it wrong could be costly
- If the existing render is failing and you're unsure of the cause
A skilled renderer will also be faster and produce a more consistent finish. You can find qualified local renderers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
