The structural design, manufacture and some aspects of construction are dealt with in BS 5268: Part 3: 2002, Code of practice for trussed rafter roofs and reference will be made to this important document throughout this chapter. Whilst timber sizes may be obtained from safe span tables, the design of the joint plate or gusset must be provided by a qualified structural engineer.
Most trussed rafter manufacturers use terminology not yet covered in previous chapters and the reader should refer to Fig. 6.2 for familiarisation with terms to be used throughout this chapter. The illustration shows a Fink truss, the most common configuration in use today. The geometry of this configuration can be found in Fig. 2.5. The standard spacing for trussed rafters is 600 mm, although 400 mm and 450 mm are not uncommon. Timber sizes are standardised throughout the United Kingdom, the timber being machined on all surfaces for accuracy in accordance with the standards set out in BS EN 1313/1. The timber is usually stress graded in accordance with BS 4978 and should be stamped with a grade mark. Whilst so called 'nominal' sizes are often quoted, i.e. 75, 87, 100, 125 and 150 x 38 mm, the finished section will be 72, 84, 97, 122 and 147 x 35 mm. Timber of 47 mm finished thickness is frequently used for attic trussed rafters with depths going up to 222 mm for heavily loaded rafters and floor joists. Trussed rafters in excess of 11 m span must use this thicker timber, or be made of multiple trusses of minimum 35 mm thickness, permanently fixed together by the truss manufacturer at works.
An understanding of the function of the trussed rafter is essential if good roof construction is to be achieved. The trussed rafter is designed to carry only the vertical loads imposed upon it, no lateral loads are catered for. The design assumes that the trussed rafter is maintained in its truly upright position by the various bracing and restraining members. Figure 6.2 illustrates this point. The wall plate, binders, tile battens and diagonal braces are all assumed in the structural design. All of these items have to be site fixed and it is essential therefore that the specification for these fixings is carried out. The load on the top chord is from self weight of the truss, the tiles, felt and battens plus statutory snow loading. On the bottom chord the load is from the self weight, ceiling and insulation plus a nominal loft loading and an
additional load imposed by a man walking on the bottom chord. No other loads are designed for. Wall plates must be a minimum of 75 mm in width (unless structural design specifies otherwise), to avoid the load from the truss crushing the timber on the underside of the bottom chord or the wall plate at the point of contact. As a guide the bearing length should not be less than 0.008 times the span of the trussed rafter. This effectively means that on spans above 9.3 m it is essential to use 100 mm wide wall plates and this indeed is the most common practice, this width matching precisely the width of the inner skin of a conventional cavity wall construction. Refer to section 7.3 of BS 5268: Part 3 for more detailed information.
Was this article helpful?