Civil Engineering Reference
In-Depth Information
truss clip is then fitted to the wall plate and the truss lowered onto it. The Alpine TW964
glide shoe illustrated can accommodate up to 15 mm of horizontal movement, but Alpine
recommend this is limited to 6 mm on each bearing within the truss design. To allow the
shoe to take up its movement, nails are fixed in the slots but not fully driven home, thus
allowing the truss to slide as the roof is constructed and loaded. When the roof is fully
loaded (i.e. tiled), further nails are added to fix the truss in its deflected working condition
and the first-fix nails are fully driven home. The glide shoe no longer 'glides', acting then
as a conventional truss clip.
Bracing the raised tie in the standard configuration truss area follows normal procedures
but presents its own problems in the sloping ceiling area where special attention must be
paid to lateral stability, i.e. avoiding the domino effect of the trussed rafters. Options 1 and
2, above, produce relatively stiff trusses in this area and the length of the sloping ceiling is
relatively small compared to the plated chord solution. This latter technique needs special
considerations, as follows:
(1) Because the thickness of the plated chord is relatively slim compared to its depth,
buckling must be avoided and solid blocking should be fitted between the trusses in the
raised tie area. This would follow similar practice used for floor joists.
(2) Because the length of the sloping ceiling rafter is probably greater, there is probably a
greater need to diagonally brace this element of the roof.
This additional bracing can be provided in three ways:
(1) Sheath the roof on the top of the rafters, a common practice in Scotland and Scand-
inavian countries where it is known as a sarking (see Fig. 5.22).
(2) Continue the under rafter diagonal bracing down over the underside of the sloping
ceiling rafter. The change of plane presents problems which have to be effectively dealt
with by blocking down from the common diagonal brace to the plated chord diagonal
brace. This also means further packing out for fitting of plasterboard.
(3) Fit ply between rafters utilising the solid blocking mentioned above as support. Figure
6.15, again from Gang-Nail's Technical Bulletin 126, shows the use of plywood in this
situation.
