Perth is a double-brick city. The vast majority of houses built in the Perth metro area from the 1950s through to the 2010s used cavity double-brick construction — two leaves of brickwork separated by an air gap, tied together with metal wall ties. It's durable, termite-resistant in the walls themselves, and well-suited to Perth's hot, dry climate. But double-brick construction has a fundamental limitation: it's rigid. When the ground beneath a double-brick home moves — and in Perth, it moves — the walls crack rather than flex.
This guide explains why Perth double-brick homes crack, which types of cracking are cosmetic, which are structural, and what our inspectors look for when assessing cracked walls.
Why Double-Brick Homes Crack
Cracking in double-brick homes is almost always caused by ground movement beneath the footings. Perth's soils range from stable coastal sands (Quindalup and Spearwood Dunes) to highly reactive Guildford Formation clay in the south-east. When reactive clay soils absorb moisture, they expand; when they dry out, they shrink. This seasonal cycle puts stress on rigid brick walls and concrete footings that can't flex to accommodate the movement.
The Main Causes
- Reactive clay soils: The Guildford Formation clay underlying suburbs like Canning Vale, Harrisdale, Southern River, Thornlie, and Gosnells is Perth's most reactive soil type. Seasonal moisture changes cause the clay to swell and contract, creating differential movement across the slab.
- Tree root moisture extraction: Large trees — particularly eucalyptus — draw enormous volumes of moisture from the soil. A mature tree near a footing can cause localised drying and shrinkage, pulling the footing down on one side. This is one of the most common causes of cracking we see in Perth's established suburbs.
- Plumbing leaks: A slow leak from a water pipe or drain under or near the slab introduces moisture to one area, causing localised swelling. The resulting differential movement can produce significant cracking within months.
- Poor drainage: Water pooling against the house — from garden beds built up against walls, blocked stormwater drains, or inadequate site grading — causes one side of the building to sit on wetter ground than the other.
- Original construction quality: Homes built on poorly compacted fill, or with footings that were undersized for the soil conditions, are more susceptible to movement from the outset.
Types of Cracking — Cosmetic vs Structural
Not all cracking is serious. Understanding the type and pattern of cracking is the key to knowing when to worry.
Cosmetic Cracking (Usually Not a Concern)
- Hairline render cracks: Fine cracks in cement render (less than 0.5mm wide) are extremely common and usually caused by shrinkage of the render itself, not structural movement. These appear on almost every rendered Perth home.
- Straight cracks at joints: Cracks that follow a straight line along a single mortar course or at the junction between two different materials (e.g., brick and render) are usually movement joints doing their job — absorbing minor expansion and contraction.
- Corner cracks in plasterboard: Fine diagonal cracks at the corners of internal door and window openings are common in new homes as the building "settles" in the first 1-2 years. If they don't grow wider over time, they're typically cosmetic.
Structural Cracking (Needs Assessment)
- Stair-step cracking: Cracks that follow the mortar joints in a stair-step pattern through brickwork are the classic sign of differential foundation movement. The wider the crack and the more courses it spans, the more significant the movement.
- Horizontal cracking: A horizontal crack running along a single mortar course, particularly below the damp-proof course (DPC) or at the base of a wall, can indicate lateral pressure from expanding soils or retaining wall failure.
- Vertical cracking through bricks: When a crack runs vertically through the bricks themselves (not just the mortar joints), it indicates higher stress — the force has exceeded the bond strength of the mortar and is breaking the bricks. This is more serious than stair-step cracking.
- Cracks wider than 5mm: Any crack wider than 5mm warrants professional assessment. Cracks this size indicate significant movement and may affect the structural integrity of the wall.
- Cracks that are wider at the top than the bottom (or vice versa): Tapered cracks indicate rotation — one part of the building is tilting relative to another. This pattern suggests ongoing active movement.
Where Cracking Is Most Common in Perth
South-East Growth Belt
The suburbs built on Guildford Formation reactive clay have the highest prevalence of structural cracking in the Perth metro. Canning Vale, Southern River, Harrisdale, Thornlie, Gosnells, and Forrestdale all sit on this soil type. Homes built in the 1990s and 2000s — before the most current footing standards were adopted — are particularly affected.
Established Inner Suburbs
Older homes in suburbs like Doubleview, Tuart Hill, Nollamara, and Dianella were often built with shallower footings than modern standards require. Combined with decades of tree growth, garden irrigation, and ageing plumbing, these homes accumulate movement over time. The cracking is often historical (it happened years ago and has stabilised), but it still needs assessment to confirm it's not active.
Coastal Suburbs
Coastal homes built on limestone-based soils generally experience less cracking from ground movement — these soils are more stable. However, salt crystallisation in the brickwork (salt damp) can cause surface spalling that is sometimes confused with structural cracking. Our inspectors differentiate between movement cracking and salt-related deterioration.
What Our Inspectors Assess
When we inspect a Perth double-brick home with visible cracking, we assess:
- Crack pattern and distribution: Where are the cracks? Do they follow a pattern consistent with foundation movement, or are they isolated and likely cosmetic?
- Crack width and profile: Is the crack uniform width, or tapered? Has it been previously repaired (indicating it has been an ongoing issue)?
- Door and window operation: Doors and windows that stick, bind, or won't close properly are often the first sign of structural movement — sometimes more obvious than the cracks themselves.
- Floor levels: Visible falls or unevenness in floor surfaces can indicate slab movement. We check for gaps between skirting boards and floors, which indicate differential settlement.
- External factors: Large trees near footings, garden beds built up against walls, blocked drains, and evidence of plumbing leaks — all potential causes of the movement that produced the cracking.
- Moisture readings: We take moisture readings in walls and around the base of the building to identify active moisture sources that may be driving ongoing movement.
When to Get a Structural Engineer
A building inspection identifies and documents cracking, but if we find evidence of significant structural movement — particularly active movement — we'll recommend a structural engineer's assessment. A structural engineer can determine the cause of the movement, whether it's ongoing, and what remediation is needed (which may include underpinning, resin injection, or drainage correction).
Our inspection report gives you the documented evidence you need to negotiate with the seller or make an informed purchase decision — whether that means renegotiating the price, requesting repairs, or walking away.
Book a Building Inspection
If you're buying a double-brick home anywhere in Perth, cracking should be assessed by an inspector who understands Perth's soil conditions and construction methods. Our combined building and pest inspections start from $422, with detailed reports delivered within 24 hours. As a 5-star rated building and pest inspection team across Perth, we inspect homes from Two Rocks to Mandurah — and we know what to look for in every suburb.
Call 0481 575 747 for a free quote or book online.
