Estimating Suspended Slab Formwork, Shoring, and Reshoring

Suspended slabs are where formwork estimating gets expensive — you cannot strip early, so you carry multiple floors of shoring at once.

Formwork contact area

Beam soffits and sides that frame into the slab are taken off with the beams.

Shoring density

Props carry the wet concrete, the formwork, and construction live load. Prop grid spacing comes from the prop’s safe working load against the slab load per square metre:

For a typical 150 mm slab, expect roughly one prop per 1.0–1.4 m², plus stringers and joists.

Number of formwork sets

A slab soffit cannot be stripped until the concrete reaches a set strength (often 7–14 days, or per the engineer). If the construction cycle pours a new floor every 7–10 days, you need 2–3 sets of slab formwork and shoring in rotation. This multiplies the formwork quantity — and the hire cost — directly.

Reshoring

When formwork is struck, reshores are installed to share load from the young slab back to the floors below until the structure gains full strength. Typically 1–2 floors of reshoring below the active formwork level. Add reshore props and the labour to strike and reset them.

Deliverable

Formwork area × number of sets, prop and reshore counts × floors carried, joists and bearers by length, plywood by area / reuse, and the crane or hoist time to cycle the material up the building.