Most floorcoverings, including woodblock flooring, can be bonded directly to a dry, smooth, screeded floor, but floorboards cannot be directly bonded and so must be fixed by other means.
The boards are nailed down to 50 x 50mm (2 x 2in) softwood battens, or bearers. These battens are embedded in the concrete while it is wet or are fixed to metal clips which are already implanted in the concrete. In either case the timber must be treated with a wood preserver. A damp- proof membrane (DPM) must be incorporated, usually in the form of a continuous coat of bituminous material sandwiched within the slab.
This means of fixing requires the slab to be level and relatively smooth. The flanges of the clips are pressed into the surface of the concrete before it sets, while a marked guide batten is used to space the clips and align them in rows. The rows are normally set 400mm (1ft 4in) apart to centres, starting 50mm
These are splayed in section so as to key into the concrete slab. Again, the slab is built up in two layers with the DPM sandwiched between them. Before the top layer or screed is laid, the treated battens are positioned at 400mm (1ft 4in) centres and levelled on dabs of concrete. Strips of wood are nailed across them temporarily to hold them in position. When the dabs of concrete are set and the battens firmly held, the wood strips are removed and the top layer of concrete is poured and compacted. It is levelled with a rule that is notched to fit over the battens. As the rule is drawn along the battens, it finishes the concrete 12mm (1/2in) below their top edges. When the concrete layer is fully dry the boards are nailed on the battens in the conventional way.
Flooring-grade chipboard is a relatively recent innovation as a material for boarding over a solid floor. It is quicker and cheaper to lay than a floor made of boards. Chipboard flooring is also more stable and it can be laid without being fixed to the concrete slab.
This technique produces a floor of the type known as a ‘floating floor’. The simplest floor of this kind is laid with 18mm (3/4in) tongue-and-groove chipboard, either the standard grade or the moisture-resistant type.
First a sheet of insulating material such as rigid polystyrene or fibreboard is laid on the concrete slab; then a vapour barrier of polyethylene sheet is laid above the polystyrene. The vapour barrier must be a continuous sheet, with its edges turned up and trapped behind the skirting boards. The chipboard, glued edge to edge, is then laid on the vapour barrier.
Battens can be incorporated in a floating floor. Lengths of 50 x 50mm (2 x 2in) softwood, treated with a preserver, are spaced at 400mm (lft 4in) intervals for 18mm (3/4in) chipboard; for heavy-gauge 22mm (7/8in) material they are spaced 600mm (2ft) apart. A quilt- type sheet of insulating material is laid on the concrete slab, then covered with a polyethylene vapour barrier. The battens are positioned on the insulation, held together temporarily with strips of wood nailed across them. Tongue-and groove chipboard is laid at right angles to the battens and glued at the edges before it is nailed down.
The chipboard flooring is held in place by its own weight and by the skirting boards, which are nailed to the walls round its edges. The skirting boards also cover a 9mm gap between the chipboard and the walls, allowing for expansion across the floor.