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Limits of use for Trussed Rafters
 
Trussed rafters provide a flexible method of framing many required roof profiles. However, due to the commercial limits of available timber sections, transport limitations for length and height
and manufacturing limitations of the pressing machinery, the following section provides some ideas as to the types of truss available in the UK and Eire at present.
Physical Dimensions
Trussed rafters can be manufactured in spans up to approximately 20 metres and heights up to approximately 5 metres, although the more normal range is 15 metres span and 3.5 metres high.
Trusses outside the above ranges may be manufactured in two or more sections and siteassembled to the required profile (see section 3.4 on two-tier construction).
Timber Sections
Trussed rafters up to 11 metres in span will generally be fabricated from minimum 35 mm thick timbers.  For trusses over 11 metres and up to 16 metres in span, thicker timber sections up to 47mm wide will be used.
Above 16 metres in span trusses will consist of multiple trussed rafters permanently fastened together by the manufacturer in the factory, or a greater width than 47mm may be used.
Profile
Within the above physical limits, many profiles of roof truss are possible, depending on the requirements of the roofscape. The creation of cantilevers over supports, the cutting back of a profile to form a recessed 'bobtail' area, the introduction of a pitched ceiling to form a 'scissor' truss, the creation of hip end and corner framing and many more common and not so common roof shapes are easily achieved by specification of trussed rafters.

It should also be remembered that, to avoid problems with both manufacturing and deflection of the roof structure, the trussed rafter profile should be of sufficient depth overall.
The recommended minimum depth for manufacturing purposes is approximately 600mm.  The recommendation for structural depth is that the span of the trussed rafter divided by its overall depth should not be greater than 6.67. (This is known as the span to depth ratio).  Cantilevered hip ends and corners can create problems due to a pivoting effect if the cantilever distance is very large and will also require special propping arrangements to be made for loose timber hipboards and jack rafters.

Careful geometry checks should be made if a cantilevered area and an area with standard bearing abut each other to avoid any problems with roof alignment.