Stiffened raft slab
A stiffened raft slab is the most common residential slab type in Australia: a flat concrete panel with deepened edge and internal beams designed under AS 2870.
Ask Chalkline about this →A stiffened raft slab is a reinforced concrete slab on ground consisting of a flat panel with deepened edge beams and internal stiffening beams, all cast monolithically. It is the most common residential foundation system in Australia, designed under AS 2870:2011.
The edge and internal beams resist differential ground movement. For Class A and S sites, standard AS 2870 tables provide the beam depths and reinforcement without a full engineering design. For Class M sites and above, a structural engineer specifies beam depths, reinforcement grade, and concrete strength. For Class H1/H2 sites, a full engineering design is typical.
Panel thickness is generally 85 to 100 mm for Class A/S/M sites, with edge beam depths ranging from 300 mm (Class A) to over 600 mm on highly reactive Class H2 sites. Concrete grade is N20 minimum (NCC 2022 Housing Provisions 4.2); N25 or N32 is common on reactive sites.
Also known as: Raft slab, stiffened raft, concrete raft.
Category: Structural.
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Last updated: 2026-05-07. Verified: 2026-05-07. Quarterly review for currency.