Preparing a brick wall properly before applying render is arguably the most important part of the whole job — skip it, and even the best render mix will crack, bubble, or fall away within a few years. The good news is that most of the preparation work is well within a capable DIYer's reach, as long as you take your time and follow the right steps.
Why Preparation Matters
Render needs a clean, sound, and slightly absorbent surface to bond correctly. Brick walls that are dirty, painted, crumbling, or too smooth will prevent the render from keying in properly. In New Zealand's variable climate — with its freeze-thaw cycles in the South Island and high humidity in the North — a poorly bonded render will fail faster than you might expect. Taking the time to prepare thoroughly will save you significant money and frustration down the track.
Tools and Materials You'll Need
- Stiff-bristle brush and wire brush
- Garden hose or pressure washer
- Cold chisel and hammer
- Angle grinder (for stubborn material)
- Safety glasses, gloves, and dust mask (P2 rated)
- Bucket and sponge
- Bonding agent or render primer (e.g. products from Sika or Ardex)
- Mortar mix for repointing
- Masking tape and plastic sheeting to protect windows and trims
Step 1 — Inspect the Wall Thoroughly
Before you touch a brush, give the entire wall a close inspection. Look for:
- Cracked or crumbling mortar joints — these must be raked out and repointed before rendering
- Spalling or loose bricks — any loose masonry needs to be re-laid or secured
- Efflorescence (white salt deposits) — a sign of moisture movement that must be addressed at the source
- Mould, algae, or lichen — common in shaded, damp NZ conditions
- Old paint or previous render — this needs to be fully removed or the new render won't bond
If you find significant structural issues — such as bowing walls, large cracks, or signs of subsidence — stop and consult a Licensed Building Practitioner (LBP). You can find qualified professionals at tradedirectory.co.nz or check credentials at LBP.
Step 2 — Clean the Surface
This is where most of the hard work happens.
Remove loose material
Use a cold chisel and hammer (or angle grinder) to chip away any old render, loose mortar, or flaking paint. Work carefully to avoid damaging the brick face.
Treat mould and algae
Apply a suitable fungicidal wash and allow it to dwell as per the manufacturer's instructions, then scrub and rinse thoroughly. Don't skip this — mould left under render will continue to grow.
Tackle efflorescence
Brush off the white deposits with a dry stiff brush first, then wash the area. If it keeps returning, you have a moisture ingress problem that needs to be fixed before rendering — otherwise the render will just push the salts out again.
Pressure wash or hose down
Give the entire wall a thorough wash to remove dust, dirt, and any remaining loose particles. Allow the wall to dry, but note that brick should be damp — not saturated — when you apply render, so timing matters.
Step 3 — Repoint Mortar Joints
Rake out any cracked or crumbling mortar to a depth of at least 15–20 mm using a plugging chisel or angle grinder with a raking disc. Blow or brush out the dust, dampen the joint, and pack in fresh mortar. Allow repointing to cure fully (typically 3–7 days depending on conditions) before applying render.
Step 4 — Check Surface Profile
Render needs something to grip. Standard brickwork with recessed mortar joints usually provides enough mechanical key. However, if the bricks are very smooth (e.g. glazed or engineering bricks), you may need to:
- Scratch the surface with an angle grinder or SDS drill and chisel to create a rougher profile
- Apply a bonding agent — products from Sika or Ardex are widely used in NZ and available from Bunnings, Mitre 10, or PlaceMakers
Always follow the bonding agent manufacturer's instructions for application and open time before rendering.
Step 5 — Apply a Scratch Coat (If Required)
For thicker render systems or very uneven walls, a scratch coat (a thin base layer of render that is scored while still wet) is applied first. This is scratched with a notched tool to create a key for the finish coat. Allow it to cure before applying subsequent coats — typically 24–48 hours minimum.
Step 6 — Protect Surrounding Areas
Mask off windows, doors, sills, and any surfaces you don't want render on. Use good-quality masking tape and plastic sheeting. Render is much easier to keep off surfaces than to remove once it sets.
NZ Building Code Considerations
If your rendering project involves an exterior wall and forms part of the building's weathertightness system, it may need to comply with NZ Building Code Clause E2 (External Moisture). Depending on the scope of work, a building consent may be required — always check with your local council before starting. MBIE Building Performance has guidance on what work requires consent and what falls under Schedule 1 exemptions.
When to Call a Professional
While preparation is DIY-friendly, consider hiring a qualified renderer if:
- The wall is large, high, or requires scaffolding (check WorkSafe NZ guidelines for working at height)
- There is significant structural damage or moisture ingress
- You need a specific finish (textured, coloured, or acrylic render systems)
- The project may require a building consent
A professional renderer will also know which render system suits your specific NZ climate zone and substrate. Find a qualified local renderer on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
