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Guide to Setting Out & Dimensioning

Setting out and Eaves details

Although often employed as the principle truss type in association with appropriate architectural features of a building, the bobtail is most often needed to accommodate re-entrant areas in perimeter walls as shown in figure 5. The horizontal 'A' dimension indicated in figure 6 therefore, is conveniently used to specify the shape for duo-pitch trusses, while double bobtails and bobtailed mono-pitched trusses which more often are principle trusses are more conveniently specified by a vertical 'A' dimension.

Figure 7a shows typical end details when the outer leaf is of masonry, arrangement (b) is best confined to timber frame construction as separate columns of masonry between trusses could be rather unstable. If the end verticals are to be tile clad one of the
arrangements figure 7c or d is suitable. In (c) a specially wide timber is used as the end vertical of the truss so that the tile battens clear the outer leaf of
the wall; the inside of the end vertical must not be located to the right of the centre-line of the wall plate. In some cases the arrangement is impractical
owing to the large width required for the end vertical.  In many cases the diagonal in the cantilevered part (figure d) can be omitted if there is little load from the cladding.

A special bobtail can be designed to suit practically any requirement.
Bobtailed trusses must never be formed through doit-yourself site modifications of standard truss types with which they align.