Prefabricated Variations

For many years timber framing was conventionally cut-onsite, meaning long lengths of timber were ordered, delivered to the building site and then cut-to-length by the carpenter/ builder as he required them and according to the working drawings.

Later, the timber supplier offered a cut-to-size service which fulfilled a builder's order for accurately cut studs, for example, and in the case of a simple house design, eliminated much onsite wastage of materials and labour.

The progression to full off-site fabrication in small or large factories is still occurring although many of the large-volume house builders have already converted to use substantial numbers of prefabricated wall frames and mostly prefabricated roof framing in the form of nail-plated trusses.

While these factory made wall frames are not substantially different from old-style or conventional framing, the prefabricated roof framing must be considered more as a complete, separate, engineered (i.e. individually designed) structure.

The prefabricated roof frame now consists of an assembly of trusses designed by the fabricators computer programme which, in effect, controls the design, the schedule for cutting and the assembly of the components. The final design of the "trussed" roof is nominated by the fabricators technical expertise and the builder should follow those detailed instructions, particularly as regards bracing and the hold-down fixings which attach the individual trusses to the top plates.

Generally, the bottom chord of a prefabricated roof truss would act as a ceiling joist and on occasions ceiling materials would be fixed directly to those chords.