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Basic design Principles
A trussed rafter is an engineered framework consisting of structural members forming triangles.  The framework derives its inherent strength from this triangulation. 
The members around the perimeter of the trussed rafter are known as Chords (top and bottom, also called rafters and ceiling ties), and the internal members providing the internal
triangles are known as Webs (sometimes also called struts and ties).  A true trussed rafter is formed only when the webs form triangles between the top and bottom chords.  Attic frames and Raised-Tie trusses (see section 1.7 and 3.13), do not provide this triangulation and are therefore technically not trussed rafters.
Principles of design
When loading is applied to a trussed rafter (from tiles, ceiling construction, snow etc), forces are generated in the members forming the truss. 

The magnitude of the bending moment in a particular chord is largely due to the Panel Length (the distance between the joints at each end of the member, usually measured horizontally, also known as the Bay
Length). The general rule is, the longer the panel length the greater the bending moment and hence the larger the section of timber required to safely resist these forces.

Further, BS.5268-3 defines the maximum bay lengths permitted in Table 3 (page 5) a copy of which is given below:
Figure 1

Table 3: Maximum Bay Lengths of Rafters and Ceiling Ties

These lengths are to ensure robustness of the truss during manufacture and handling.  The choice of a different truss type with a smaller panel length (and hence more webs) will usually yield a smaller section of timber required.