Rendering a below-ground retaining wall is more demanding than above-ground work because the render must resist constant ground moisture, hydrostatic pressure, and soil movement. Done correctly, it protects the wall structure and extends its lifespan significantly. Done poorly, it can crack, delaminate, and allow water ingress that undermines the wall itself.
Understand What You're Working With
Before you pick up a trowel, identify your wall substrate. Most residential retaining walls in NZ are concrete block (masonry), poured concrete, or timber (treated pine). Render adheres well to masonry and concrete but is generally not suitable for timber retaining walls below ground — timber flexes too much and the render will crack. For timber walls, a waterproof membrane is a better solution.
Also check whether your retaining wall required a building consent. Under the NZ Building Code and Schedule 1 of the Building Act, walls retaining more than 1.5 m of ground (or in certain ground conditions) may require consent — the exact threshold depends on factors such as ground conditions, surcharge, and proximity to boundaries or buildings, so check with your local council. If your wall had consent, any remedial or finishing work should comply with the approved plans. Check with your local council if you're unsure — MBIE Building Performance has useful guidance on consent requirements.
Gather the Right Materials
For below-ground render, you need a waterproof or tanking render system — standard sand-and-cement render is not enough on its own. Key materials include:
- Portland cement-based render (minimum 1:3 cement-to-sand ratio for below-ground use)
- Waterproofing admixture — products from Sika or Ardex are widely used in NZ and are specifically formulated for tanking and waterproofing applications
- SBR bonding agent (styrene-butadiene rubber) to improve adhesion to the substrate
- Fibreglass reinforcing mesh for crack resistance in the scratch coat
- Clean sharp sand, clean water, and a reliable mixer
Always read the manufacturer's data sheet for your chosen waterproofing system — mix ratios and application methods vary between products.
Step-by-Step Application Process
1. Surface Preparation
This is the most critical step. The substrate must be:
- Clean — remove all dirt, loose material, efflorescence, and any existing failed render using a wire brush, angle grinder, or pressure washer
- Sound — tap the surface to find hollow spots and cut them out; patch any voids with a suitable repair mortar
- Dampened — wet the surface thoroughly before applying render so the substrate doesn't draw moisture out of the mix too quickly, causing weak adhesion
For very smooth poured concrete, consider scabbling (mechanical roughening) or applying a bonding agent to give the render something to grip.
2. Apply a Bonding Slurry
Mix your SBR bonding agent with cement and water to a thick slurry consistency, following the product instructions. Scrub this firmly into the surface and allow it to become tacky — not fully dry — before applying your first coat.
3. Scratch Coat (First Coat)
Apply a scratch coat approximately 8–10 mm thick using your waterproof render mix. Press it firmly into the surface to ensure full contact. While still green (partially set but not hard), use a notched scratcher or devil float to score horizontal lines across the surface. This mechanical key is essential for the second coat to bond properly. Allow to cure for at least 24 hours, keeping it damp.
4. Float Coat (Second Coat)
Apply a second coat of 8–10 mm, again using your waterproof mix. Work it to a flat, even finish with a float. For below-ground walls, a steel-trowelled finish is preferable to a textured finish as it is denser and more water-resistant. Embed fibreglass mesh in this coat if additional crack resistance is needed.
5. Waterproof Membrane (Highly Recommended)
For walls in contact with soil, render alone is rarely sufficient as a long-term waterproofing solution. Apply a tanking membrane — either a cementitious slurry system (such as those available from Ardex or Sika) or a sheet membrane — over the cured render before backfilling. This is the belt-and-braces approach that professional renderers and waterproofers use.
6. Curing
Curing is non-negotiable. Keep the render moist for a minimum of 3–7 days by misting with water or covering with damp hessian. Render that dries too fast will be weak and prone to cracking. Do not backfill until the render and membrane system have fully cured per the manufacturer's recommendations.
Drainage Matters Too
Even the best render system will eventually fail if hydrostatic pressure builds up behind the wall. Ensure adequate subsoil drainage — ag pipe at the base of the wall, drainage aggregate, and a geotextile fabric — is in place before backfilling. BRANZ has technical resources on retaining wall drainage best practice.
Common Mistakes to Avoid
- Applying render to a wet or dirty surface
- Using too weak a mix (too much sand, not enough cement)
- Skipping the bonding agent on smooth concrete
- Backfilling before the render has fully cured
- Relying on render alone without a membrane for high-moisture sites
- Ignoring drainage behind the wall
When to Call a Professional
Below-ground waterproofing is a specialist area. If your retaining wall is:
- Over 1.5 m high or required a building consent
- Showing signs of structural movement or cracking
- In a high water table area or on a sloping site with significant water run-off
- Part of a foundation wall for a building
…you should engage a licensed renderer or waterproofing specialist. Errors in below-ground waterproofing can be very expensive to fix once backfilled. You can find qualified local renderers and waterproofing tradespeople on tradedirectory.co.nz.
If any drainage or plumbing work is involved (such as connecting ag pipe to a stormwater system), that may constitute restricted drainlaying work requiring a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board.
Licensed Building Practitioner (LBP)
