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Practical Roof Solutions
 
Extended Rafters and Extended Joists
Extended rafters and extended joists, as shown in figure 15 require special consideration because the trusses are not fully triangulated to the bearings. As a result of the lack of triangulation, the extended member is subject to exceptionally large bending  moments. In the example shown in figure 16 the
rafter, or the top chord, is subject to a bending moment no less than ten times that which occurs in a conventionally supported truss. 
 
Standard trusses can be adapted and strengthened to withstand the large bending moments and shear force occurring in the extended member at the rafter-tie junction. This may be accomplished by fixing a strengthening piece to each side of the extended member, using bolts or a special nailing arrangement. Another way to strengthen the chord as shown on the right-hand side of figure 16.

Large rafter extensions will produce outward thrust and movement at the bearings. This is often a critical factor in design and is rigorously controlled by
BS.5268-3.
Hatch and Chimney Opening

Where possible, hatches and chimneys should be accommodated in the standard spacing between trusses.  Each member and joint in a truss performs an important role essential to the effective functioning of all other parts and the component as a whole. Trusses must never be cut and trimmed except according to details supplied by the truss designer. The full detailing of the construction of these features is given in section 3.3.