What width crack in a wall is considered serious?

As a general guide: hairline cracks (under 0.5mm) are cosmetic; fine cracks (0.5–1mm) are monitor-worthy; moderate cracks (1–5mm) should be assessed by a professional; wide cracks (over 5mm) or cracks with displacement should be assessed urgently.

Why do Melbourne homes crack more than houses in other cities?

Melbourne's clay-rich soils are particularly reactive to moisture changes. The dry summers and wet winters cause significant seasonal swelling and shrinkage — more extreme than coastal cities with more stable soil moisture.

Can I just fill and repaint cracks without getting them checked?

For fine hairline cracks in isolated locations, yes — this is a normal maintenance task. For larger cracks, growing cracks, or cracks that are part of a larger pattern, filling without investigation simply hides the symptom and allows the underlying cause to worsen.

Homeowner symptom guide · Melbourne

Cracks in Walls: How to Tell if They're Structural or Cosmetic

Cracks in Melbourne home walls are extremely common — but they range from entirely harmless to indicators of serious foundation movement. The key is knowing which characteristics separate a cosmetic plaster crack from a structural warning sign. This guide gives you the framework to assess what you're seeing.

Common Causes

• Seasonal soil movement: Melbourne's reactive clay soils expand and contract with moisture changes, creating cyclic movement that stresses footings and walls. Small cracks from this cause typically stabilise when ground moisture stabilises.
• Normal plaster settlement: plaster and render applied over new timber frames naturally shrinks and cracks as the moisture in the substrate dries. These are typically fine hairlines that appear in the first few years of a home's life.
• Thermal expansion: temperature changes cause all building materials to expand and contract. Cracks at material junctions (brick to timber, concrete to render) are common and usually cosmetic.
• Foundation movement: underpinning failure, poor original construction, tree root extraction of soil moisture, or inadequate footings for the soil conditions can cause progressive structural movement — the most serious cause.
• Lintel failure: the steel or concrete beam above windows and doors can rust, crack, or deflect over time, causing characteristic diagonal cracks radiating from the corners of openings.

When to Be Concerned

⚠ Diagonal cracks running from corners of windows or doors, particularly if wider than 3mm
⚠ Horizontal cracks running along a mortar joint in brickwork
⚠ Cracks that you can see have widened since you first noticed them
⚠ Multiple cracks appearing simultaneously with sticking doors or windows
⚠ Step-cracking following the mortar joints in brick (indicates differential settlement)

What to Do

1 Photograph cracks with a coin or ruler for scale, and date the photo — this documents whether cracks are growing.
2 Mark the ends of cracks with pencil and date them — check again in 6–12 weeks to see if they have extended.
3 For fine hairline cracks in plaster not associated with door/window movement, fill and paint — these are usually cosmetic.
4 For cracks wider than 3mm, growing, or associated with diagonal patterns at openings, engage a licensed building inspector or structural engineer.

Frequently Asked Questions

What width crack in a wall is considered serious?

As a general guide: hairline cracks (under 0.5mm) are cosmetic; fine cracks (0.5–1mm) are monitor-worthy; moderate cracks (1–5mm) should be assessed by a professional; wide cracks (over 5mm) or cracks with displacement should be assessed urgently.
Why do Melbourne homes crack more than houses in other cities?

Melbourne's clay-rich soils are particularly reactive to moisture changes. The dry summers and wet winters cause significant seasonal swelling and shrinkage — more extreme than coastal cities with more stable soil moisture.
Can I just fill and repaint cracks without getting them checked?

For fine hairline cracks in isolated locations, yes — this is a normal maintenance task. For larger cracks, growing cracks, or cracks that are part of a larger pattern, filling without investigation simply hides the symptom and allows the underlying cause to worsen.

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