• Use kiln dried softwood from C16 to C24
• Check if SS grade is needed for large, load-bearing sections
• Always check grade and strength before using timber for structural purposes
• Tiling battens should comply with BS 5534: 2003 and be treated.

The simplest form of pitched roof construction is the couple roof, which consists of rafters bearing against each other at the ridge board, with the load and the thrust at the eaves being resisted by the lateral strength of the wall.

Except for the very smallest spans, modern lightweight cavity wall construction is unlikely to be adequate and this form is therefore little used today.

By adding a collar to the roof construction it is possible to relieve the walls of thrust. The nearer the collar is to the eaves the greater will be the effect.

However the collar imposes a bending moment on the rafter; the higher the collar the greater the bending moment. Rafter sizes may have to be increased to handle the stresses, for example, a small roof, tied at the supporting wall plate level requiring 38 x 100mm rafters will require 50 x 150mm rafters when the collar is located at the mid height.

Structurally, the most efficient solution is to use ceiling joists to tie the rafter feet together, thereby relieving the walls of horizontal thrust. Hangers and binders are normally added when the span exceeds about 2.5 metres to prevent the ceiling joist having to be increased to an uneconomical size. This simple, three-member form of a close couple roof can be used for spans up to approximately 5.5 metres. Rafter sizes can be calculated or, depending upon the roof dead load, be ascertained from tables.

In cut-roof construction the common rafters traditionally are birdsmouthed over the wall plate at eaves level and fixed to a ridge board at the apex of the roof. Accurate cutting and assembly are vital to ensure stability of the structure.

For roofs with a span in excess of 6m, additional support for the rafters will be necessary to keep them to an economic size. This can be achieved by fixing purlins to the underside of the rafters midway through their length. The purlins run at right-angles to the rafters and are supported by struts spaced approximately 1.5m apart and bearing upon load bearing partitions or beams.

Purlins are of relatively substantial size and should be set at 90° to the roof pitch. Purlin struts should be notched to the purlin to give maximum support. A strut, in the form of a collar, is often placed above the purlin to minimise the deflection that may take place in the rafters and purlins. A section through a roof of this type, suitable for a span of up to approximately 8m.

The hip and valley rafters have a number of constructional uses. They make convenient jointing pieces enabling the jack rafters to be fixed together in pairs. Similarly they facilitate the location and jointing of purlins. They support the ends of the tile battens, the hip tiles, and the construction of the valleys.

It may be assumed that hip rafters in roofs pitched lower than about 30° are subject to bending stress and need to be propped or designed to resist such bending. However hipped ends are less common on low pitched roofs than on steeper ones.

When the roof pitch is over 30° the hip rafter is carrying virtually no load. The thickness of the hip rafter should provide for nail holding and is usually the same as the common rafter thickness.

Its depth should normally be at least 50mm greater than the common rafter depth to make full contact with the cut end of the jack rafters and deeper where it is required as the jointing member between the mitred ends of purlins.

Further information is available in TRADA Wood Information Sheet 1-10 Principles of pitched roof construction and WIS 1-29 Trussed rafters.

Trussed rafters are factory fabricated from strength-graded timber, which is typically regularised to a constant thickness of 35mm (although thicker sections can be used if specified). The connector plates are normally punched from pre-galvanised steel plate of approximately 1.3mm thickness although, again, thicker versions are used for heavy-duty applications.

Trussed rafters are normally installed at 400 or 600mm centres and typically span up to 10-12m although larger spans have been produced, the limitation being as much controlled by transportation and handling difficulties as by the structural requirements.

Each truss is comparatively flimsy until it is fixed in place and has its requisite bracing, therefore handling, storage and bracing must be correct to avoid damage to the joints by distortion.

Recommendations for site storage, installation and bracing are given in BS 5268 Part 3.

Symmetrical, asymmetrical, monopitched and many other shapes of truss are produced and manufacturers' catalogues should be examined to find the 'normal' range. However, it is common for each trussed rafter to be individually designed to take account of the span, pitch, installed centres and loading requirements.

The software to perform these designs is usually owned by the manufacturers of the metal plates (the 'system owners' such as Mitek, Wolf and Gangnail) who license its use to fabricators through franchise agreements. Complex roof designs incorp-orating hips and valleys, or designs that permit the attic space to be used as a room in the roof, can be readily accommodated but design advice should be sought from the supplier.