The Roof Construction

The bolt and connector truss is generally used as a principal truss with common rafters and ceiling ties supported from it and on the wall plate. The designs referred to above are spaced at 1.8 m centres with the design of the purlin, binders, plate and ridge being given on each individual sheet. A study should be made of the design sheet illustrated in Fig. 5.3.

Fig. 5.3 Typical TRADA standard roof design.

Fig. 5.3 Typical TRADA standard roof design.

The construction of the roof itself, once the principal trusses have been produced, is quite straightforward, with many of the joints being nailed as with the 'traditional' roof. The exception to this in some designs is the joint between ceiling joist and rafter, and on all designs where the ceiling tie cannot be obtained in one length, also the splice joint between the two lengths, which again uses a combination of connectors and nails. An alternative for the ceiling joist allows for it to be joined over a plate on a partition; this is not to be recommended where the construction process requires non-load bearing partitions to be installed after the roof is constructed. The roof should therefore be constructed wherever possible as an independent clear spanning structure between the two wall plates of the external walls. To avoid trusses inadvertently bearing on internal non-load bearing partitions, it is good practice to have the roof tiled and therefore under normal working load conditions to allow any deflection in the truss to take place before the partitions are fitted.

The trusses are not designed to carry water storage tank loads and thus, wherever possible, these should be directly supported from partitions below. If this is not practical then the advice of TRADA or the truss designer must be sought in order that the truss spacing may be reduced to carry the additional load. One final point on the standard roof assembly is that some difficulty may be experienced in nailing the ceiling joist tightly to its binder. A more effective connection is to use one of the readily available light galvanised metal cleats. These give a much more positive and stronger connection than traditional skew nailing. Figure 5.4 illustrates the roof construction described above.

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