Footing and Foundation Defects in Australian Residential Builds

What it is

A footing defect is any failure of the slab, strip footing or pier system that supports the house. It shows up as cracking in walls (usually diagonal at door and window openings), uneven floors, doors and windows that stick, separation between the slab edge and brickwork, or visible step cracking in masonry. By the time the symptoms are visible the cause has often been working for months or years.

The relevant standard is AS 2870 (Residential slabs and footings). The NCC Volume Two Part H1 (H1D4) calls AS 2870 in as the Deemed-to-Satisfy pathway for residential footings. Statutory warranty across most Australian states treats structural defects (including footings) as the most serious category, with a longer warranty period than non-structural defects. In NSW the structural warranty is 6 years from completion. Victoria, Queensland, Western Australia and South Australia have similar but not identical periods.

Common defect patterns

Wrong site classification

A geotechnical investigation is done by a person who is not a registered geotechnical engineer, or the report uses a single test pit on a clearly variable site. The slab is poured to a Class M spec on what is actually a Class H1 site. Within 12 to 36 months the differential movement starts to crack the building.

Slab on uncontrolled fill

The block was filled before sale. The builder did not require a compaction certificate or did not require depth-of-fill data. The slab sits on fill that consolidates over years. Settlement is uneven because the fill depth is uneven.

Trees too close to the footing

A mature tree drawing moisture from the soil under or near the slab causes seasonal heave and shrink. AS 2870 has a tree adjustment for clay sites. It is sometimes missed at design.

Reactive site without articulation

Class H1 and H2 sites need articulation joints in brickwork and slab design that allows for heave. A continuous brick veneer with no articulation will crack at door and window openings as the slab moves.

Poor concrete or reinforcement placement

Reinforcement chairs not used. Mesh laid on the ground. Saw cuts not made within the AS 1379 window. Concrete strength below the spec. Each of these turns a designed slab into a non-designed slab.

Drainage failures around the slab

Roof water discharging to the perimeter of the slab. No agi drain on a cut and fill site. Stormwater connecting into the subsoil drain. All of these saturate the founding soil and drive movement.

Rectification cost magnitude

Cosmetic crack repair is $2,000 to $8,000. It does nothing if the underlying movement continues. Underpinning a single-storey home costs $25,000 to $90,000 depending on the number of piers and access. Underpinning plus rectification of finishes commonly hits $80,000 to $180,000. A complete slab failure where the building has to be demolished is rare but possible on a poorly investigated reactive site.

Insurance treats footing defects in the structural warranty pool, where the financial exposure to the builder is at its highest.

What an auditor inspects

For TradeLens, the audit trail starts with the geotechnical report (date, site address, classification, recommendations) and the engineer footing design that references it. The audit then looks at the as-built records: the slab inspection report from the engineer or surveyor before the pour, the concrete delivery dockets showing strength and slump, the photographic record of reinforcement placement, and the pour record. After the slab, the audit looks at the perimeter drainage as installed, any articulation joints in brickwork at the right intervals, and stormwater discharge well clear of the foundation.

Where any document is missing, the foundation is treated as undocumented and the builder carries the full warranty exposure if a defect emerges.

What builders can do early

Refusing to start a slab on a fill site without compaction data is the single most powerful defence against footing claims. Specifying articulation in brickwork on Class M or higher sites is a low-cost step that prevents most visible cracking. Recording the geotechnical report number on every site instruction and inspection form gives the engineer something to trace.

Footing defects are catchable through documentation. The builds that suffer them are almost always the builds where the documentation was thin to start with.