Timber pitched roof structures may be divided into two broad classifications. The first is constructed entirely in situ from basic timber sections using simple jointing methods. The second utilises trusses, trussed rafters and trussed purlins which are assembled on site before being lifted into position, or are completely fabricated at works. The first category is based upon traditional methods with modern refinements; the second upon calculated and tested designs produced by the Timber Research and Development Association (TRADA), by the makers of connectors and nail plates, and by timber engineering companies, who market roof components and use new jointing techniques and rationalized production methods. These manufacturers can supply trussed rafters for many combinations of pitch, profile and span, from single house contracts upwards. Cost per unit is, of course, lowest where large scale repetition is required. The traditional timber pitched roof is based upon the utilization of standard sawn or commercially surfaced (planed) rectangular softwood sections, of sizes to suit load and span conditions. These are ordered and delivered to site in appropriate lengths, cut and assembled in position, located and fixed mainly by nailing. All cutting, jointing and assembly has to be done by carpenters at roof level. Covering in of the building interior must await completion of roof carcassing, as there is no prefabrication to shorten construction time. Trussed rafters, however, may be assembled at ground level, or supplied complete from the factory, thus reducing site work time before covering.
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