Trussed rafters are designed to carry simply the direct load imposed upon them. It is assumed that they are to be kept upright by other members, these members being the binders and diagonal bracing and even the tile batten vital to the overall stability of the roof. Whilst most trussed rafters are used for roofs of housing, their use is increasing for roofs of public buildings, commercial buildings and to a lesser extent for industrial and agricultural buildings. Clear spans in excess of 30 m can be achieved with lightweight roof coverings.
When first introduced into the UK, the designs were limited to those contained in standard design manuals, thus the duo pitch and mono pitched roofs were common but more complex roofs needed individual designs prepared. The advent of the computer both speeded up and dramatically reduced the cost of the design process, and this has been further advanced by the use of microcomputers installed in all trussed rafter manufacturers' offices. There are now almost no limitations to the possible shape of trussed rafters, except those imposed by the practicality of production and
transportation to site. The power of computers enables not only the individual trussed rafter to be designed but also the whole roof as a structural entity. Roof layout drawings can be produced in minutes and then either plotted on to paper or sent via 'email' to the end user.
Trussed rafter roofs use approximately 30% less timber than a traditional roof, and can be built into a roof form in a fraction of the time taken for either a truly traditional common and purlin roof, or a TRADA construction. Factory production keeps the labour cost of trussed rafter manufacture very low compared to that necessary to assemble a bolt and connected jointed truss, thus giving further cost advantages to the trussed rafter. Almost all new housing now uses a trussed rafter form of roof construction.
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