What soil conditions require a raft slab?

A raft slab is generally suited to sites with more stable, less reactive soil — where a single thickened, well-reinforced slab can adequately spread the building’s load without needing the more complex ribbed system a waffle slab uses to handle reactive soil movement.

Your soil report’s classification (under AS 2870) is what actually determines this — your engineer weighs the classification against your specific building design to decide whether a raft slab is appropriate, or whether a waffle slab or deep foundation system is genuinely needed instead.

Raft slabs aren’t automatically cheaper or simpler just because they’re a single pour — the right choice depends entirely on what your site’s actual soil conditions call for, not a general preference.

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