Suspended Timber Flooring

A suspended flooring system generally comprises a series of joists closely spaced, being either simply supported at their ends or continuous over load-bearing partition walls. The floor boarding or decking is applied on the top of the joists and underneath ceiling linings are fixed. A typical suspended floor arrangement is shown in Figure 1.18a.

The distance between the centres of the joists is normally governed by the size of the decking and ceiling boards, which are normally available in dimensions of 1200 mm wide x 2400 mm long. The size of the decking and ceiling boards allows convenient joist spacings of 300 mm, 400 mm or 600 mm centre to centre. In addition, the choice of joist spacing may also be affected by the spanning capacity of the flooring material, joist span and other geometrical constraints such as an opening for a stairwell.

The most common floor decking in domestic dwellings and timber-framed buildings uses some form of wood-based panel products, for example chipboard, OSB or plywood. Solid timber decking such as softwood tongued and grooved (T&G) decking is often used in roof constructions, in conjunction with glued-laminated members, to produce a pleasant, natural timber ceiling with clear spans between the main structural members. The solid timber T&G boards are normally machined from 150-mm-wide sections with 38 to 75 mm basic thicknesses; see Figure 1.18b.

The supports for joists are provided in various forms depending on the type of construction. Timber wall plates are normally used to support joists on top of masonry walls and foundations; see Figure 1.18e. In situations where joists are to be supported on load-bearing timber-framed walls or internal partitions, header beams or spreader members are provided to evenly distribute the vertical loads. Joist hangers are often used to attach and support joists onto the main timber beams, trimmer members, or masonry walls; see Figure 1.18c.

Timber trimmer joists are frequently used within timber floors of all types of domestic buildings; see Figure 1.18a. There are two main reasons for which trimmer joists may be provided. First is to trim around an opening such as a stairwell or loft access (trimmer joists A), and to support incoming joists (trimmer joists B), and second is to reduce the span of floor joists over long open spans (trimmer joists C), as shown in Figure 1.18a.

Trimming around openings can usually be achieved by using two or more joists nailed together to form a trimmer beam, as shown in Figure 1.18c, or by using a single

Fig. 1.17. SIPs during construction.

but larger timber section, if construction geometry permits. Alternatively, trimmers can be of hardwood or glued-laminated timber, boxed ply-webbed beams, or as shown in Figure 1.18d, composite timber and steel flitched beams.

All flooring systems are required to have fire resistance from the floor below and this is achieved by the ceiling linings, the joists and the floor boarding acting

Table 1.19 Summary of the current engineered wood products and their structural applications




Common sizes



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Beams, columns, trusses, bridges, portal frames, post and beam systems Industrial, commercial, recreational, residential and institutional

No theoretical limits to size, length or shape

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