Performance In

Much has been written in trade journals concerning possible problems occurring with the trussed rafter form of construction, with reference particularly to its long-term durability. An authoritative paper was prepared in 1983 by the Building Research Establishment entitled Trussed rafter roofs (IP14/83) in which the results of a nationwide survey were summarised. The survey looked at the manufacture, site use, performance in service, and plate corrosion with certain types of preservative. Whilst some shortcomings were found in the manufacture, the areas causing greatest concern were site handling and construction of the roof structure using the trussed

Fig. 6.1a Punched metal plated joint.

rafters on site. Inspection of the trussed rafter components for compliance with the relevant British Standards before assembly into the roof structure should be made, thus overcoming the possibility of faulty structural components being installed. The problem of inadequate design and assembly information for the roof structure as a whole remains, and whilst BS 5268: Part 3 gives some guidance for conventional gable ended roofs and the individual plate manufacturers provide standard details

6.1c Plywood nailed gusset joint.

Fig.

6.1c Plywood nailed gusset joint.

for hips and valleys, the builder is not generally presented with a specific set of drawings for the assembly of the roof on which he is working. Increasingly the larger trussed rafter producers and the plate manufacturers are providing computer programs which generate 'whole roof' designs. These designs not only include the layout of the trussed rafter itself, but all other infill timbers and support metalwork required for the structure. This considerably aids the correct site assembly of the roof structure.

This chapter concentrates on the construction of trussed rafter roofs including hips, valleys, attics and trimmed openings. The storage and handling of trussed rafters and other timber components, and the many ancillary details required to complete the roof are set out in Chapter 8.

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