A floor of a building generally provides a wearing surface on top of a flat support structure. Its form and materials are chosen for architectural, structural, and cost reasons. A ground-supported floor may be of almost any firm material, ranging from compacted soil to reinforced concrete. It is supported directly by the subsoil below.
An elevated floor spans between, and is supported by, beams, columns, or bearing walls. It is designed to be strong and stiff enough to support its design loading without excessive deflection; to provide for an appropriate degree of fire resistance; and to supply diaphragm strength to maintain the shape of the building as a whole, if necessary. A ceiling may be hung from the underside of the floor assembly as a finish surface for the room below. The optimum floor design meets these criteria while being as thin as possible and economical to construct. See Beam, Column, Loads, transverse
Wooden floors are generally used in light residential construction. Such flooring generally consists of a finish floor installed on a subfloor of tongue-and-groove planking or plywood, spanning between wooden beams that are commonly called joists. Slabs fabricated of reinforced concrete are a common type of floor for heavier loading. The concrete is cast on forms, and reinforced with properly placed and shaped steel bars (rebars), so as to span between steel or reinforced concrete beams or between bearing walls. Composite floors are commonly used in modern office building construction. Concrete is cast on, and made structurally integral with, corrugated metal deck, which spans between steel joists of either solid-beam or open-web types, generally spaced between about 16–48 in. (40–120 cm) on center. Prestressed concrete is used for long span slabs. Highly prestressed high-tension steel wires within the high-strength concrete slab produce a thin, stiff, and strong floor deck. A lift slab is used for economy and efficiency. A concrete slab is first formed at ground level, reinforced and cured to adequate strength, and then carefully jacked up into its final position on supporting columns. See Composite beam, Concrete slab, Prestressed concrete, Structural design