Trussed rafters: Recent developments

Recently, whole roof structure systems have become available. These are mainly intended for use where the roof void is used extensively for habitable space. These ‘roof cassettes’ are similar to typical timber-frame wall panels but are designed to be inclined and supported by prefabricated timber floor and gable or party wall structures. Unlike timber frames, the panels are ‘closed’ – i.e. prefabricated with insulation, a vapour control layer, roof window openings, felt and battens ready for tiling and counter-battened ready to form an internal service void. They are delivered to site ready for site erection by crane.

Roof cassettes are a form of ‘stress skin panel’, where the principal structure consists of a series of prefabricated engineered timber I-beams spaced at 600mm centres. These are braced and fixed together by gluing and fixing a sheathing board (9mm oriented strand board is common) to either side of the I-beams. This creates a very strong and rigid composite panel. The depth of the I-beams is determined by the span and loading required, but it also relates to the required depth and type of insulation. Very low U-values (<0.15Wm2/K) can be achieved and the built-in service void is intended to ensure that the integrated vapour control layer remains uncompromised during service installations.