Refer to Fig. 6.23I.
The extended rafter sections clearly carry the whole of one side of the truss, necessitating a larger section of timber for the sloping ceiling length. All plate producers have three alternative solutions:
(1) Increase the cross section of the top chord over the whole of the top chord length; if this can be done by using 36 mm stock sections then there is no major cost implication. If, however, it necessitates the use of 47 mm timber then clearly all the truss members must now be 47 mm, and there is a significant cost implication brought about purely by the need to overcome a structural problem occurring in the sloping ceiling area of the truss.
(2) Add scabs (i.e. additional pieces of timber) nailed or bolted to each side of the rafter from the wallplate to the first node joint. This at least leaves the truss itself using relatively lightweight timbers, but it does mean that the bird's mouth becomes a difficult joint to cut on site.
(3) Use the plated chord discussed in 'Innovation', above (see Fig. 7.13), this time taken from Wolf's technical manual. This particular detail allows a greater length of sloping rafter, i.e. a greater raise to the tie than is generally practical with methods 1 and 2, above.
Fig. 7.13 Raised tie bracing.
The raised tie roof truss imposes a horizontal load onto the wall plate and wall when loaded. Unlike the conventional truss which is fully triangulated by virtue of its bottom chord fixing the top chords together, the raised tie roof completes its triangulation at the flat ceiling to sloping ceiling intersection. The extended rafters then act as a beam which deflects to give an element of lateral movement. We are now back in effect to a collar roof (see Fig. 1.2).
To overcome at least the horizontal thrust from the roof structure and its imposed loads, a sliding truss clip or 'glide shoe' can be used. These are referred to in Fig. 7.13 and illustrated in Fig. 7.14. The shoe consists of a flat galvanised steel plate which is fixed to the underside of the rafter seating where it bears on the wall plate. A special 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
Fig. 7.14 Truss glide shoe.
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 Scandinavian countries where it is known as a sarking (see Fig. 6.22).
(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 7.15, again from Gang-Nail's Technical Bulletin 126, shows the use of plywood in this situation.
Option 3 is preferred by the author because the ply can perform two functions: first it stabilises the roof and uses the solid blocking as support; second it provides the ventilation void in the sloping section. This is basically the same technique as that used for stabilising attic trusses (see Fig. 8.22).
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