Cracking defects in residential construction: causes and risk profile

What it is

A crack is any visible separation in a building element. In residential construction across Australia cracking accounts for a large share of defect claims heard by tribunals. The cause matters more than the appearance. A 0.3 mm hairline in plaster is cosmetic. A 4 mm step crack through brickwork over a footing is a structural failure that triggers the six-year statutory warranty in most states.

Why it fails

Cracking has four main drivers. Reactive clay soils swell and shrink with moisture content. Concrete shrinks as it cures. Thermal movement expands and contracts every element in the building every day. Differential settlement under footings concentrates stress at the weakest section, usually a window head or a corner.

The Australian Standard for residential slabs and footings is AS 2870. It classifies sites from A (stable) to P (problem) based on soil reactivity. A site classed M (moderately reactive) needs a stiffer slab than a site classed S (slightly reactive). When a builder uses the wrong site class, or the engineer never inspects the actual ground conditions before the pour, cracking shows up within two years.

How it is classified

CSIRO Building Technology File 17 is the reference for crack width in masonry walls. Category 0 covers cracks under 0.1 mm (hairline, no action). Category 1 covers 0.1 to 1 mm (fine, redecoration only). Category 2 covers 1 to 5 mm (noticeable, doors and windows may stick). Category 3 covers 5 to 15 mm (significant, repointing needed). Category 4 and 5 cover wider cracks requiring partial or full rebuild.

A defect inspector who measures every crack with a crack-width comparator and photographs it next to a ruler will hold up in any AU tribunal. One who eyeballs it will not.

Who is liable

Under the statutory warranty regimes in NSW, VIC, QLD, WA, SA, TAS and the ACT, structural cracking falls inside the six-year major defect window. Non-structural cracking such as shrinkage in render falls inside the two-year non-structural window. The builder carries the obligation. The engineer who specified the slab carries co-liability if the design was wrong for the site class.

NCAT, VCAT and QCAT decisions consistently award rectification costs that include engineering reports, underpinning where required, and full replacement of affected wall sections. Resin injection is accepted for fine cracks. It is rejected as a permanent fix for anything Category 3 or above.

TradeLens risk flags

A builder is exposed when any of the following appear in the file. Site classification missing or copied from a neighbouring lot. No geotechnical report. Slab poured in temperatures above 32 degrees without curing controls. Articulation joints absent from masonry walls longer than six metres. No control joints in render. Tribunal files show these five gaps drive most successful cracking claims against builders.

Typical rectification cost

Cosmetic crack repair runs 500 to 2000 dollars per wall. Underpinning a corner of a slab is 8000 to 25000 dollars depending on depth. Full slab remediation on a reactive site can exceed 80000 dollars and routinely sits in the top 10 of tribunal award sizes. The cheaper the original site work, the larger the eventual rectification bill.