Cracked Slabs on Darling Downs Clay in Toowoomba

Under the Standard, the Commission says, concrete floors with hairline cracks less than 2 mm wide are acceptable movement and concrete floors with cracks wider than 2 mm are not; a crack wider than that may be a subsidence problem to raise with the builder. A coating goes over the crack, so how wide it is comes before the coat.

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What cracked slab coating in Toowoomba actually involves

The Commission says most Queensland homes are built on clay soils, which dry out rapidly in dry seasons and become wet quickly in wet seasons, causing sudden extreme volume changes and soil movement. The Queensland Government says Vertosols are brown, grey or black soils that crack open when dry and are cracking clay soils of the Darling Downs; the Department of State Development, Infrastructure and Planning lists Toowoomba in its Darling Downs South West region. Neither page says what is under any particular house.

The Toowoomba record

Toowoomba Airport (641 m above sea level) averages 104.3 mm of rain in February against 30.0 mm in August (Bureau of Meteorology, readings from 1996, read 6 October 2026): a wet summer and a drier winter. That is one airport's record, not a reading of any slab. The article on checking a slab sets out the steps.

A distinct crack running across a concrete floor with a masonry chisel lying beside it — cracked slabs on Darling Downs clay, Toowoomba. Illustrative example.

What to ask before a coating goes on

Ask how wide the cracks are, what the contractor will do about them before coating, and whether anything about the slab is a matter for the builder. A crack that keeps moving is the Commission's subsidence question, not a coating question, and this site does not say which a given crack is.

A slab that has been wet is a different starting point from one that has not. The Toowoomba City and Oakey pages set out where the council area has a recorded flood history.

Dark wet soil beside a house slab after summer rain with puddles in the lawn — cracked slabs on clay soil, Toowoomba. Illustrative example.

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