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There is no mandatory requirement to treat timber used in the manufacture of trussed rafters over most of the UK except for an area of the South East where insect attack from the Long Horn Beetle necessitates the use of treated timber.
The majority of trussed rafters manufactured in the UK are fabricated using European Whitewood which has been Kiln Dried to a moisture content of 12% and machine stress graded. The most common grade used is TR26 which is a specific grade for truss manufacture.
Common timber width for trusses is 35mm for truss spans up to 11.0m and 47mm for spans greater although there are other thickness restrictions for very long spans.
No. There is no mandatory regulations that require the use of a metal clip connection although the British Standards and the Trussed Rafter Association strongly recommend the use of a preparatory mechanical connection between the truss and the supporting structure. The main purpose of the connection beyond simple positioning is to resist uplift forces generated by wind loads.
Skew nailing is a method of connection often seen but is not recommended. Firstly it is very difficult to apply proof calculations to a pair of skew nails and secondly there is very often damage caused to the nail plate on the eaves joint while hammering home the skew nails.
Glide shoes fitted at the bearing points of trusses allow a limited amount of horizontal displacement of the truss at the bearing. The raised tie design leads to flexing of the extended rafter members and as the truss tries to deflect this a horizontal force is generated at the bearing points. If the bearings were rigidly fixed to the truss this force would result in damage to the supporting structure and decoration. By introducing a sliding bearing mechanism such as the glide shoe the amount of horizontal force transposed to the wall is significantly reduced and thus minimises the potential for structural damage.
Unless otherwise noted all carpenters metalwork should be fully nailed using the special square twisted nails supplied. Other nails, such as plasterboard nails should not be used and could result in failure of the connection.