Topping slab

A topping slab is an in-situ concrete layer poured over prestressed hollowcore planks, precast panels, or an existing slab. It ties the precast units together, provides the finished floor level, and, when bonded, creates composite structural action.

Bonded vs unbonded

Bonded topping acts compositely with the planks below. The plank surface must be clean, roughened or broom-finished, and saturated surface-dry (SSD) before the pour. Mesh ties the units together and helps the floor act as a lateral diaphragm for wind and seismic loads. A bonded topping can increase hollowcore plank load capacity significantly (verified 2026-06-11).

Unbonded topping sits on a debonding layer and acts independently, adding level and fire or acoustic mass without composite action. The structural engineer specifies bonded or unbonded; it is not a builder’s choice.

Thickness and reinforcement

Thickness and mesh are design items, not fixed rules. Supplier guides reference 50 mm over hollowcore in some systems and 75 mm for a structural composite topping, but the engineer’s drawings govern (verified 2026-06-11). Mesh covers both shrinkage crack control and diaphragm tie forces.

Not a screed

A screed is a non-structural levelling layer. A topping slab is reinforced concrete sized by an engineer for loads. Wrong mix, missing mesh, or pouring out of sequence compromises the structural floor.

Set services, conduit, and embedments before the pour. Coring or cutting a topping afterwards needs engineer approval.

Also known as: structural topping, composite topping.

Related

• Prestressed concrete
• Reinforced concrete
• Waffle pod slab
• Raft footings

See also

• Screed
• Cold joint

References

• National Precast Concrete Association Australia, Hollowcore Prestressed Flooring (verified 2026-06-11)
• Precast Structural Solutions, Structural Concrete Topping to Precast (verified 2026-06-11)

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Last updated: 2026-06-11. Verified: 2026-06-11. Quarterly review for currency.