Civil Engineering Reference
In-Depth Information
Connections
All timbers on this roof construction would normally be simply nailed together using 75
mm and 100 mm long galvanised wire nails. Whilst a simple 'tosh' or 'skew' nail (i.e. a
nail driven at an angle through one piece of timber into its supporting timber) (see Fig.
8.6) will be adequate on the rafter to plate, purlin and ridge connection, the collar to rafter
and ceiling tie to rafter connection should be made with three or five nails depending on
the size of the individual members and taking care not to nail too near the ends of the tim-
bers, thus avoiding splitting.
The Hip Roof
A simple hip roof is illustrated in Fig. 3.7. Whilst the wall plate is the main support for
such a roof, the main problem of support arises from the lack of a gable end from which to
support the purlin in the hip area. The mechanics of load distribution within the hip area
seems to be open to debate. It is quite clear however, that the majority of the load is trans-
mitted directly to the wall plate with the symmetry of the jack rafter leaning against each
other either side of the hip rafter, tending to provide a self supporting structure. Certainly
on small span roofs where no purlin is required this would be the case. On larger spans
however, where a purlin is required on the longer jack rafters, a more sophisticated solu-
tion must be found.
The Plate.
The wall plate need only be a perfectly standard timber section, but with thrust from the
hip rafter being resolved at the external angle of the wall plate, it is common to fit a tie
across the corner. A more sophisticated corner joint used on some older buildings is illus-
trated in Fig. 3.8, but it would not generally be necessary for the size of the structure nor-
mally encountered on dwellings. It does however illustrate what was found necessary to
contain the thrust from larger hip rafters.
The Purlin
