Quick Answer
Most cracked render on Melbourne homes can be patched for $20–80 in materials using ready-mix render filler or acrylic gap filler. Hairline cracks are cosmetic and DIY-able. Cracks wider than 3 mm, cracks that run diagonally across corners, or cracks with moisture staining behind them may indicate structural movement and need a builder or structural engineer.
Understanding Render Cracks: Cosmetic vs Structural
Rendered walls are common across Melbourne’s SE suburbs, particularly in the 1960s–1990s brick veneer homes of Berwick, Dandenong, and Frankston. Render expands and contracts with temperature — Melbourne’s swings from 10°C winter nights to 40°C summer days put real stress on external render, making cracking almost inevitable over 15–20 years.
The critical question is: is this crack cosmetic or structural?
| Crack Type | Description | Action |
|---|---|---|
| Hairline crack (<1 mm) | Fine surface crack, no depth, no moisture | DIY fill and paint — purely cosmetic |
| Surface crack (1–3 mm) | Visible gap, may follow render joints | DIY repair with flexible render filler |
| Wide crack (3–10 mm) | Gap wide enough to fit a coin | Professional render repair — may need investigation |
| Diagonal / stair-step crack | Cracks at 45° across corners or brickwork | Structural engineer assessment required |
| Crack with moisture / efflorescence | White salt deposits or damp patches behind crack | Builder or waterproofer required |
| Bulging or hollow render | Render sounds hollow when tapped, may bulge | Professional re-render required — fall risk |
What You’ll Need for DIY Render Repair
| Material | Cost | Where to Buy |
|---|---|---|
| Acrylic gap filler / flexible render filler | $8–20 per tube | Bunnings, Mitre 10, hardware stores |
| Ready-mix render (for larger patches) | $15–40 per bag | Bunnings, Boral, concrete suppliers |
| Bonding agent (PVA or acrylic primer) | $10–25 | Bunnings, Mitre 10 |
| Wire brush or angle grinder (disc) | $0 (if owned) / $30–80 rental | Total Tools, Kennards Hire |
| Pointing trowel and hawk | $15–40 | Bunnings, trade suppliers |
| Exterior acrylic paint (to match) | $30–80 per litre | Dulux, Taubmans, hardware stores |
| Painter’s tape | $5–10 | Any hardware store |
Step-by-Step: Repairing Hairline and Surface Cracks
Step 1: Clean and Prepare the Crack
Use a wire brush or stiff bristle brush to remove loose render, dust, and debris from the crack. For anything over 2 mm wide, use a cold chisel to lightly undercut the crack edges — this creates a wider opening at the base than at the surface, which helps the filler grip. Brush away all dust and blow out any remaining debris. Dampen the crack lightly with water before applying filler — dry render absorbs too much moisture from the repair mix, causing it to crack again.
Step 2: Apply Bonding Agent
For patches larger than 50 mm across, brush a coat of PVA bonding agent (diluted 1:1 with water) or acrylic bonding primer into the crack and surrounding area. Allow to dry to a tacky state (15–30 minutes) before applying render. This step dramatically improves adhesion, especially on older sand-cement render where the existing surface is porous.
Step 3: Fill the Crack
For hairline cracks (under 2 mm): Apply a flexible acrylic gap filler directly from the tube using a caulk gun. Push the nozzle into the crack and draw it along smoothly. Wet your finger or a flat tool and smooth the filler flush with the wall surface. Allow to cure per the manufacturer’s instructions (typically 2–24 hours depending on temperature).
For surface cracks (2–10 mm) and patches: Mix ready-mix render or use pre-mixed render filler to a firm, peanut-butter consistency. Pack it firmly into the crack using a pointing trowel, pressing hard to eliminate air pockets. Build up in layers no more than 10 mm thick — thick single applications shrink and crack. Score the surface of each layer before it fully cures to improve bonding for the next layer.
Step 4: Texture Matching
Once the filler is firm (but still slightly soft), texture the surface to match the surrounding render. Common Melbourne render textures include: fine smooth finish (use a wet sponge in circular motions), sand-face (lightly press and remove a damp sponge), and rough dash (flick small render with a brush). Getting this right takes practice — if you’re unsure, do a test patch on a hidden wall section first.
Step 5: Prime and Paint
Allow the repaired area to cure fully — at least 48–72 hours in warm weather, up to 7 days in cold weather. Apply a coat of exterior acrylic primer, then repaint to match the wall colour. Colour matching at Dulux or Taubmans paint centres is possible if you bring a paint chip; expect to pay $40–80 per litre for colour-matched exterior acrylic. Two coats are usually needed to hide the repair patch.
Troubleshooting
| Problem | Cause | Fix |
|---|---|---|
| Repair cracked again within weeks | Applied too thick, no bonding agent, or structural movement | Remove repair, assess for movement, re-apply in thin layers with bonding agent |
| Render won’t stick | Surface too dry or dusty, no bonding agent | Dampen surface, apply PVA bonding agent, re-render |
| Paint won’t match | Original paint faded; new paint a different batch | Take chip to Dulux for colour match; may need to repaint full wall section |
| Crack keeps returning | Structural movement (foundation, lintel, thermal) | Consult structural engineer before further cosmetic repair |
| Hollow-sounding render around crack | Render delaminating from substrate | Cut out hollow section, re-render professionally |
When to Call a Professional
Call a licensed builder or plasterer for: diagonal or stair-step cracks (possible foundation movement), cracks wider than 10 mm, large areas of hollow or delaminating render, any crack associated with moisture intrusion or wall dampness, or cracks that reappear within months of repair. In Victoria, structural assessments should be carried out by a registered building practitioner or structural engineer. The Victorian Building Authority (VBA) maintains a register of licensed practitioners.
Tips and Gotchas
- Hairline cracks in render are normal — Melbourne’s extreme temperature range (10–42°C) expands and contracts render constantly. Most are cosmetic only.
- Flexible fillers beat rigid cement renders for hairline cracks — acrylic-based gap filler moves with the wall and won’t re-crack as quickly.
- Avoid repairing render in full sun — render and fillers dry too quickly and crack. Work in the morning or in shade.
- Don’t over-water the mix — wet render runs, shrinks on drying, and bonds poorly. Mix to a firm consistency that holds its shape.
- Match the render type — old sand-cement render and modern acrylic render behave differently. Using the wrong product causes adhesion failure.
- Photograph the crack before and after — if it reappears in the same place, you’ll have evidence of progressive movement for an insurance claim or builder dispute.
- Check window and door lintels near diagonal cracks — failing lintels are a common cause of cracking in Melbourne’s older brick veneer homes.
- Paint the full wall section if possible — patch-painting almost never matches perfectly on aged or sun-faded render.
Local Resources
- Render repair products at Bunnings
- Boral render products — trade and DIY
- Victorian Building Authority — find a registered builder
- hipages — licensed renderers in Melbourne
FAQ
Is cracked render on a house serious?
Most render cracks are cosmetic and caused by normal thermal movement. Fine hairline cracks (under 1 mm) are never structural. Wide cracks (over 3 mm), diagonal cracks, or cracks associated with moisture are more serious and should be assessed by a builder or structural engineer before repair.
How much does render repair cost in Melbourne?
DIY repair of hairline and surface cracks costs $20–80 in materials. Professional render patching costs $300–800 for small areas, and full re-rendering an external wall can cost $3,000–8,000+ depending on wall size and render type.
What is the best filler for external render cracks?
For fine hairline cracks, a flexible acrylic exterior gap filler is best — it moves with the wall and resists re-cracking. For wider cracks (2–10 mm), use a cement-based render repair compound or acrylic render filler, applied in layers of no more than 10 mm.
Why does my render keep cracking after repair?
Recurring cracks usually indicate either ongoing structural movement (foundation, lintel, or soil heave), applying the repair too thick in one pass, skipping a bonding agent, or using a rigid repair on a flexible crack. If a repair fails twice, call a structural engineer before trying again.
Can I render over cracked render?
Only if the existing render is sound (not hollow or delaminating). Apply a bonding agent first, and don’t apply new render more than 10 mm thick in one layer. If the existing render is hollow or delaminating in large areas, it must be removed before re-rendering.
Final Thoughts
Most render cracks on Melbourne homes are cosmetic and straightforward to fix yourself — the total repair cost is $20–80 and the job takes a Saturday morning. The key is correctly identifying whether the crack is cosmetic or structural before you pick up a trowel. A diagonal crack or a crack that comes back after a good repair is telling you something is moving, and no amount of filler will fix that permanently. In those cases, the best money you can spend is on a structural engineer’s assessment — typically $400–700 — before any further work.
