Suspended slab

A suspended slab is a concrete slab that spans between beams, walls, or columns rather than bearing on the ground. An upper floor, a slab over a basement or garage, a podium deck, are all suspended slabs. They are engineered to AS 3600 (concrete structures), not the slab-on-ground standard AS 2870.

The defining difference is how the slab is supported, and it changes everything structurally:

• A slab on ground is supported continuously by the soil beneath it; it is designed under AS 2870 for ground movement and the soil’s reactivity.
• A suspended slab has nothing under it; it has to carry its own weight and the imposed load across a span back to its supports, so it is designed under AS 3600 as a spanning concrete element, with bending reinforcement, deflection control, and (often) more steel top and bottom.

Common forms are a conventionally reinforced slab, a post-tensioned slab, or a composite slab on profiled steel decking (Bondek-type). Each has its own propping, pour, and curing regime, and suspended slabs need backpropping so the formwork load is carried while the concrete gains strength.

For a builder the practical points are that a suspended slab is a fully engineered element, the reinforcement layout, cover, slab thickness, and propping/backpropping all come from the engineer and must be followed exactly. Do not strike formwork or backprops early (early striking is a classic cause of slab deflection and cracking), keep the top steel up on chairs (walking it down kills the slab’s capacity over supports), and cure it properly. It is not a slab-on-ground with the dirt taken away; it is a different design problem.

Also known as: Elevated slab, structural floor slab.

Category: Concrete / Slabs.

Related

• AS 3600 concrete structures
• Concrete grades

See also

• AS 2870 residential slabs and footings

References

• AS 3600 Concrete structures, Standards Australia (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.