Trussed Rafters

In the early 1960s the punched metal connector plate was introduced into the UK from the USA and was to revolutionise the construction of domestic roofs even more than the TDA truss designs described. There are now four main plate manufacturers in the UK, the first in 1967 being Gang-Nail whose name has come to be used to describe all punched metal connector trusses, in the same way that 'Hoover' seems to describe a vacuum cleaner.

Trussed rafters are generally prefabricated in a factory and transported to site, although with certain types of plate, fabrication can take place on site. In the case of metal plates, the manufacturer sells plates backed up to varying degrees with design aids to approved manufacturers, many of whom are also timber merchants. The timber used is both graded for strength and machined on all surfaces to give accuracy to the finished product. Trussed rafters can also be assembled using plywood gussets, the plywood being either nailed to a defined pattern or nailed and glued to the truss members to form the joint. Ply gusseted trusses are not as popular as metal plated trusses, but do offer a method of manufacture not requiring specialist equipment. Similarly the galvanised steel plates punched with a pattern of holes to receive nails can also be used to form truss joints and these too can be fabricated on site.

The punched metal nail plates used in factory trussed rafter production are mechanically pressed into the timbers on both sides of each joint to form a trussed rafter. This trussed rafter is then placed on the roof at approximately 600 mm centres taking the place of the common rafter. Hence its term 'trussed rafter', as distinct from the TRADA type principal truss, although it will be seen later in Chapter 6 that trussed rafters themselves can be used to form principal or girder trusses. A typical Fink trussed rafter is illustrated in Fig. 1.12.

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