Know the rules for trusses
While the use of trusses is covered under NZS 3604:2011 Timber-framed buildings, there are a number of design limitations applied. In summary, these are:
- the truss must be specifically designed in accordance with B1/VM1 and manufactured by an accredited fabricator
- a maximum truss span of 12 m
- a maximum truss spacing of 900 mm for a heavy roof and 1200 mm for a light roof
- the load on the ends of the trusses to be no more than 16 kN in both directions - up and down
- a maximum snow load of 2 kPa.
In addition, clause 10.2.2.3 requires (amongst other things) that:
- loadbearing elements supporting the trusses are identified - it is not uncommon for truss loads to occur over a wall that has been designated as non-loadbearing
- the truss connections (type and capacity) need to be specified for:
- the connection to the loadbearing (supporting) structure
- connections between trusses such as girder and truncated trusses
- the required lateral support to give stability to the trusses is specified.
All of this information should be provided in the design statement issued by the accredited fabricator.
Summarised from BRANZ Guideline, July 2015