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Last edited 28 May 2019
The term ‘floor slab’ usually refers to a floor that has been formed using concrete (and generally steel reinforcement) and may form part of the structure of a building. It may form the floor of a basement, at ground level or at upper levels. It will be typically reinforced, either by rebar or steel fibres, and can be either formed on site or prefabricated. A floor that is made of timber or other material is not referred to as a floor slab.
Concrete slabs that form the ground floor of a building may be either supported on beams (called a suspended slab) or supported directly on the subsoil, (via hardcore, for example) called a ‘ground-bearing slab'.
The term 'floor slab' refers to the finished result, but the ways of achieving it are numerous, which means there are different types of floor slab construction. The important point is that it may be referred to as a 'floor slab' irrespective of its formation and its inclusion of other materials.
A floor slab may be:
- A horizontal, flat thickness of concrete (usually 100mm minimal thickness), supported on two or more sides by concrete or steel beams. The underside of the set concrete (if an upper floor) will be visible when viewed from the floor below and may bear the marks of the formwork (or shuttering) that was used to provide temporary support.
- A horizontal, flat thickness of concrete that is supported by profiled steel-ribbed sheeting (decking). The steel provides reinforcement and both temporary and permanent support for the concrete. When viewed from the underside, a steel ceiling (or soffit) is observed which may have recesses for hangars to support ducts and other services. Such slabs are often referred to as ‘composite’, where the resulting structural efficiency of the floor slab is greater than the sum efficiency of the steel and concrete.
- A waffle slab is formed when the concrete is poured over a series of plastic formers to result in an egg-crate-style (like a waffle) formation on the slab’s underside. Although such constructions may typically have thicknesses of 500mm or more, the hollow areas make the construction lighter, more economical and more structurally efficient
- A horizontal, flat thickness of concrete topping that has been poured in-situ over a beam and block substructure or a hollow-rib arrangement that incorporates rebar.
- A pre-cast concrete slab supported by a steel frame (especially in a high-rise building).
For further information on the various floor slab types, read the article on concrete slabs in buildings.
 Related articles on Designing Buildings Wiki
- Building foundations.
- Cast-in-place concrete.
- Cellular raft foundation.
- Concrete slabs in buildings.
- Domestic floors: Part 1: Construction, insulation and damp proofing.
- Finished floor level.
- Floating floors in buildings.
- Pad foundation.
- Strip foundation.
- Types of floor.
- Types of raft foundation.
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