The single most common shed structural issue I see in Australian backyards is movement caused by reactive clay soil. A shed that sat happily on its slab for the first two summers starts cracking the slab, throwing the doors out of alignment, and showing wall flex by the third or fourth year. The mistake is almost always at the foundation stage.

If you are building or buying a shed in a part of Australia with reactive clay — and that covers most of Melbourne, Adelaide, parts of Sydney, much of regional NSW and Victoria, and large parts of Queensland’s southeast — the foundation choice matters more than the shed choice.

Why clay moves

Reactive clay soils expand when wet and contract when dry. The volume change can be substantial — five to fifteen per cent in highly reactive clays. The shed sitting on the ground above moves with the soil. A typical Australian summer-to-winter cycle is enough to push a foundation up and down by several millimetres. Over multiple years, the movement causes the structural problems.

The state of the soil matters. AS 2870 classifies sites from A (stable) to E (extremely reactive). H1 and H2 sites are the highly reactive class that most clay-heavy parts of Australia fall into. Builders working in these areas know the requirements; backyard shed installers often do not.

The slab options

The standard concrete slab options for sheds on clay are: a thickened-edge waffle slab, a stiffened raft slab, or a slab on piers.

The thickened-edge slab is the cheapest and the most commonly installed for sheds. It works on stable soil. On reactive clay it is the option most likely to crack.

The stiffened raft slab includes internal stiffening ribs running through the slab. The slab is more resistant to differential movement. For sheds in the 6m x 4m to 9m x 6m range on reactive clay, this is the sensible default.

The slab on piers takes the load down to a depth below the seasonal moisture variation. For very reactive sites, sites with poor drainage, or large sheds, this is the durable answer.

The pier-only option

For smaller sheds — garden sheds, small workshops up to about 5m x 4m — a pier foundation without a slab can be appropriate. Concrete piers at the corners and at midpoints take the load down past the reactive layer, and the shed sits on a structural floor frame above the piers.

This works well for sheds that do not need a concrete floor and that can tolerate the structural floor frame. It avoids the cracking issue entirely because there is no rigid slab to crack.

The drainage point

Whatever foundation you use, drainage matters. Reactive clay sites are reactive because the moisture cycles between wet and dry. A site with poor drainage that floods after rain has a more aggressive wet cycle than a well-drained site, and the soil movement is worse.

Get the drainage around the shed sorted before the slab goes in. The shed pad should be slightly elevated, the surrounding ground should drain away, and any gutter or downpipe runoff from the shed should not concentrate water against the slab edge.

The compaction question

The site preparation before the slab pour matters as much as the slab design. Compaction of any fill material, removal of organic matter, attention to existing soil profile — all of this affects how the foundation performs.

The shed installers who do this well take time on the site preparation. The ones who do not take time on site prep are the ones whose sheds crack within five years.

The council permit interaction

Council permit requirements for sheds in 2026 vary significantly across local government areas. The size thresholds for permit-free sheds, the engineering certification requirements, and the inspection regimes are all council-specific. For a shed that crosses the permit threshold, the engineer’s slab specification is usually appropriate for the site, and the inspection process catches the worst foundation mistakes.

For permit-free sheds, the installer’s discretion is what you are relying on. Pick installers who understand reactive clay, ask the engineering question explicitly, and do not accept “she’ll be right” answers on a site you know is reactive.

The honest summary

A shed is a long-term piece of infrastructure. Skimping on the foundation is the most expensive saving a backyard project can make. The few hundred dollars between a thickened-edge slab and a stiffened raft slab on a reactive site is paid back many times over in the cracked slab, the rusted door tracks, and the structural rework you would otherwise face in year five.