Cracks in Walls: Cosmetic vs Structural — How to Tell the Difference

Why Do Walls Crack in Melbourne Homes?

Melbourne’s reactive clay soils — particularly prevalent in the south-eastern suburbs including Berwick, Cranbourne, Officer, Pakenham, and Frankston — are the primary driver of wall cracking in the region. Reactive clay expands when wet and contracts when dry. A dry summer followed by a wet autumn causes the soil to move, and the house footings and slab move with it. This is normal — it’s why Melbourne homes are built on reinforced slabs designed to flex. But over time, or when soil conditions change (tree removal, pool installation, changing drainage), movement can become excessive.

Other causes include:

• Normal thermal movement: Brick and plaster expand in summer heat and contract in winter. Fine hairline cracks at ceiling-wall junctions and cornice joins are normal seasonal movement.
• Plaster shrinkage: In newly built or renovated homes, new plaster cracks as it cures over 12–18 months. These are cosmetic.
• Settlement after construction: Homes typically settle in the first 2–5 years. Uniform settlement produces cosmetic cracks; differential settlement (one part sinking more than another) produces structural cracks.
• Lintel failure: Steel lintels above doors and windows can corrode and expand, cracking the brickwork above openings in a distinctive pattern.
• Tree root intrusion: Eucalyptus and other large trees within 5 metres of the foundation can extract significant moisture from reactive clay, causing localised subsidence and cracking.

The 5-Question Test: Cosmetic or Structural?

Ask these five questions about any crack you’re concerned about. More than one “yes” warrants a structural engineer’s assessment.

1. Is the crack wider than 3mm at its widest point? Use a credit card (0.76mm) or a 2mm gap gauge from a hardware store. A crack you can insert a credit card into is 1mm+; a coin (2mm) indicates a more serious width.
2. Is there displacement — is one side of the crack higher or lower than the other? Run your finger across the crack. If you feel a step, the wall has moved in three dimensions, not just laterally — a structural concern.
3. Is the crack diagonal, particularly running through brick mortar joints? Diagonal stair-step cracking in brickwork indicates differential settlement. Vertical cracks through bricks (not mortar joints) indicate extreme movement or impact.
4. Has a door or window nearby stopped working correctly? A door that has started sticking, won’t latch, or has gaps at one corner that weren’t there before suggests the frame has racked — structural movement.
5. Is the crack getting longer or wider over time? Apply a pencil mark across the crack with the date. Measure again after 4 weeks and 3 months. Growing cracks — even slow ones — require professional assessment.

Crack Classification Guide

How to Monitor a Crack Safely

Before spending money on a structural engineer, monitor the crack for 6–12 weeks:

1. Draw a pencil line across the crack at each end and write the date next to each line mark.
2. Measure the width at the widest point with a small gap gauge or ruler and record it.
3. Photograph the crack with a coin alongside it for scale.
4. Check again after 4 weeks, 8 weeks, and 12 weeks — note any change in length or width.
5. Note whether cracking is worse after dry spells (soil shrinkage) or wet periods (soil expansion) — this helps a structural engineer understand the cause.

DIY Repair: Filling Cosmetic Cracks

Confirmed cosmetic cracks on plaster, plasterboard, or rendered surfaces can be filled DIY with appropriate products:

• Hairline cracks (<1mm): Sand lightly, apply Dulux Precision Interior Filler or similar acrylic filler with a flexible putty knife, sand flush when dry, prime and paint.
• Wider cracks (1–5mm) in plaster: Rake out any loose material, apply Selleys No More Gaps or flexible filler in layers no more than 5mm deep each pass, allow to dry between coats, sand flush.
• Cracks at ceiling-wall junctions and cornices: Use a flexible acrylic caulk (paintable) — not a rigid filler. These junctions move seasonally and a rigid fill will re-crack within months.
• Brick mortar joints (repointing): Rake out deteriorated mortar to 15–20mm depth with a cold chisel, dampen the joint, press in mortar mix (3:1 sand:cement with plasticiser), finish to match existing profile. This is DIY-possible for isolated joints on single-storey homes.

When to Call a Structural Engineer

A structural engineer assessment is warranted when:

• Any crack with displacement (a step you can feel across the crack)
• Diagonal stair-step cracking through brick mortar joints
• A crack wider than 3mm that is also growing
• Doors or windows that have recently started sticking or racking
• Multiple cracks appearing simultaneously in different parts of the home
• You are buying a property and a pre-purchase inspection has flagged structural cracks

A structural engineer inspection in Melbourne costs $300–$600 for a residential assessment with a written report. This is money well spent before either panicking about a cosmetic issue or ignoring a genuine structural problem. The VBA’s practitioner register lists licensed structural engineers and builders in Victoria.

Tips and Gotchas

1. Never paint over a crack without filling it. Paint bridges hairline cracks temporarily but they re-appear within one season. Fill, prime, then paint.
2. Clay soil needs consistent moisture. If you remove a large tree, the soil may heave as it rehydrates over 2–3 years, causing new cracking even in a home that previously had none. Plan tree removals with a geotechnical consultant if the tree is close to foundations.
3. Expansion joints are not cracks. Many brick veneer homes have deliberate vertical expansion joints (a thin gap filled with mastic) that allow thermal movement. These look alarming but are designed features — do not fill them with rigid mortar.
4. Pre-purchase inspections miss internal structural cracks. A standard pest and building inspection report cannot assess subsurface conditions or foundation performance — only a structural engineer can. For homes in high-reactive-clay areas (most of SE Melbourne), a structural engineering pre-purchase assessment is worth the $400–$600 cost.
5. Home insurance rarely covers subsidence damage. Check your policy carefully — most standard home insurance policies exclude gradual subsidence and movement. Sudden, unexpected events may be covered. Document everything with photographs and dates.

Local Resources

• Victorian Building Authority — Find a licensed structural engineer or builder in Melbourne
• Bunnings Warehouse — Selleys No More Gaps, flexible caulk, crack fillers, repointing mortar
• Mitre 10 — Crack monitors, masonry repair products, pointing tools
• Consumer Affairs Victoria — Building dispute resolution and structural defect claims
• Engineers Australia — Find a structural engineer in Victoria

Frequently Asked Questions