How to check a concrete slab for cracks before a floor coating on Darling Downs clay

Why the slab comes first

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, and that this can cause 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 Bureau of Meteorology's table for Toowoomba Airport (641 m above sea level, 30 years of readings from 1996) gives mean rainfall of 104.3 mm in February and 103.5 mm in December, against 30.0 mm in August and 31.2 mm in July. We read it on 6 October 2026. It is one airport's record: it does not say what is under any house.

The checks, in order

1. Find the widest crack

   Walk the floor in raking light, from a doorway or with a torch held low, and find the widest crack. Note whether it runs the length of the slab or stops.

2. Measure it

   Measure its width at the widest point. The Commission lists, under the Standard, concrete floors with hairline cracks less than 2 mm wide as acceptable and concrete floors with cracks wider than 2 mm as not acceptable.

3. Ask the builder if it is wider than 2 mm

   The Commission says unacceptable movement may be a subsidence problem to take up with the builder. A coating is not a repair for that, and this site does not say which a given crack is.

4. Say what is on the floor

   Tell the provider if there is old vinyl, tiles or adhesive on the slab, or if water has been on it. The Queensland Government's asbestos site lists asbestos backed vinyl sheet flooring and vinyl floor tiles among the common locations in a house built in the 1970s.

5. Ask what happens to the cracks before the coat

   Ask what the contractor does about each crack before a coating goes on, and what the quote says if the slab moves afterwards. We state no method, because no non-commercial source gives one.

What this does not tell you

None of these pages says what soil lies under a particular Toowoomba house, or whether a given crack is movement or something else. Read each body's current page before you rely on anything here: the figures were read on 6 October 2026.