The truss design is initiated by the builder or owner, or his
designate; i.e. the general contractor or architect/engineer. The
documentation must include:
- the size of the building,
- the building occupancy,
- the shape and span of the truss,
- where the truss will be supported,
- the type of roof construction (finish, insulation, sheeting, ceiling type)
- what the loads on the truss will be
- special requirements to be considered in truss design such as special loading and
- placement geometry defining parameters (pitches, heel heights, overhang and cantilever details)
For more details on the design responsibilities of the truss designer and the building designer consult the
TPIC Truss Design Procedures and Specifications for light metal plate
connected wood trusses Section 4
Typically, the building designer or builder will contract with the truss fabricator,
who will supply a truss layout
and a structural design of each significant element of the roof system. The truss plate manufacturer's engineer
usually reviews and seals the individual truss designs on behalf of the truss fabricator.
In Canada, designs are based on structural requirements from Building Codes
using design standards
referenced in Building Codes and approved material properties:
- Structural analogues and methodology have been developed and standardized by the
- Lumber design values for use in Canada are published by the
Canadian Standards Association
(CSA 086-Engineering Design in Wood).
- Truss connector plates are proprietary and therefore come with different structural properties. Design values for
truss plates are developed through tests and analyses in accordance with CSA STANDARD S347; the standard
setting the plate values based on testing is CSA STANDARD 086-01 ENGINEERING DESIGN IN WOOD.
The Canadian Construction Materials Centre (CCMC) assures tests comply to the standard.
Preliminary discussion of a design with a truss fabricator may result in suggestions leading to the most efficient
and the most cost effective design.
Truss design is facilitated by the use of computer software that designs all members, connections and
produces a design drawing for an engineer's approval and also shop drawings and cutting lists for manufacturing
and quality control.
The approval and acceptance process of truss design drawings depends on local requirements. In most
jurisdictions, a professional engineer's stamp on the truss design drawing may be compulsory. In other cases
alternate procedures may be acceptable.