Civil Engineering Reference
In-Depth Information
Rafter diagonal bracing is not generally required in a hip roof, the hip itself being a rigid
diagonal structure. Longitudinal binders G must be continued from the main roof into the
hip construction, plus additional binders H fixed flat on top of the hip trusses' top chords.
Diagonal K should be installed (see Fig. 5.5).
Most trussed rafter hip constructions involve a compound girder assembled from two,
three or in some cases four individual trussed rafters. The nailing together of these indi-
vidual components to form a girder is critical, and should be carried out by the trussed
rafter manufacturer at works. If girders are not nailed by the manufacturer then a nailing
and bolting detail must be provided by the roof designer. When fastened together on site,
bolts must be used for the bottom chord members, in positions marked by the truss manu-
facturer. In all cases the nails and bolts must be positioned strictly in accordance with the
manufacturer's instructions. See a typical nailing and bolting pattern given in Fig. 5.10a.
The setting out of the girder and intermediate trussed rafters is also critical and dimen-
sions given on drawings must be carefully followed.
Valleys and Intersecting Roofs
The construction of valleys on trussed rafter roofs varies little between plate systems,
most using a set of reducing trusses imposed upon the main roof structure. The designs are
however dealt with separately under each system heading in Chapter 6.
Complex intersections normally have special design drawings supplied for them by the
trussed rafter manufacturer. The solutions follow a pattern and typical details are given be-
low. Firstly, Fig. 5.8 illustrates the construction of a roof intersection on which the main
roof cannot be supported on a wall or beam, but is instead supported on a girder or com-
pound truss. Secondly, in Fig. 5.11, a further complication is caused by a hip occurring at
the intersection.
The location of the girder truss must be such that it will carry the main roof standard truss,
the overhang of the standard trussed rafters simply being removed on site if special trusses
have not been provided. If the overhang is removed on site, care must be taken not to cut
too near the heel joint plate: a minimum distance of 10 mm is recommended. See Fig. 5.9
for the girder to standard truss detail.
