There are four elements in roof truss construction that are vital for a sound end result. The Institute for Timber Construction South Africa (ITC-SA) weighs in:

Structural timber to be used & must comply with the design intent (SATAS or SABS marked)
Structural timber needs to be marked with red ink on the face of the timber at 1m intervals. If it does not bear these structural markings, it will be condemned. One should beware of unmarked timber, as this is not structurally graded timber.

Designer must create an accurate cutting bill
The cutting bill will dictate the exact lengths and angles at which the timber must be cut for proper assembly.

ITC-SA accredited System Suppliers must be used
The metal connector plates used must be supplied by the specific engineering system that is being used. In addition, these plates must be the correct size, and positioned in the right locations as per the standard methods and tolerances.

All connections & bracing details must be in accordance with the design intent
In other words, everything that holds the structure together, such as the number of nails, bolts, washers, brackets and cleats, must be in accordance with the engineering design. All necessary bracing accessories must be stipulated on the design plans.

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Image courtesy: Mitek Industries South Africa.