The trussed rafter attic roof must be constructed strictly in accordance with the designer's and manufacturer's drawings. It must be fully appreciated that the structure about to be constructed forms almost 50% of the new home and will carry far more load than the conventional roof structure. The trussed rafters impart great rigidity across the building within themselves, but rely purely on the bracing construction between them for their lateral stability, and therefore the lateral stability of the whole building above the wall plate position. It is not correct to consider that brick and block gable ends will impart any support for the roof structure in this lateral direction, indeed the timber roof structure itself must be strong enough to restrain adequately the large gable end areas.
A preliminary consideration of bracing is given below, but the more detailed implications of correct bracing are dealt with in Chapter 8 where they cover both traditional and trussed rafter attic roofs.
Temporary bracing of these larger heavier trussed rafter components is vital. The temporary diagonal brace placed on the top of the top chord should be at least 22 mm x 97 mm in section and well nailed to both plate and the trussed rafters as far up the rafter as practical. On a single-storey building it may be possible to place a temporary support, or prop, from the floor slab to the vertical side wall and top chord junction of the attic trussed rafter for additional security. Erection should proceed basically as for the standard trussed rafter roof, except that temporary diagonal bracing should be added to the underside of the top chord until permanent bracing of the roof has been achieved. This temporary bracing may well have to be fixed on the underside of the top chord or rafter within what will become the room area.
Longitudinal binders should be fitted generally as before, but there will be more of them and these are indicated in Fig. 6.21. This illustration does not show all of the bracing required in an attic roof; further reference should be made to Chapter 8, Fig. 8.22. The floor boarding will of course eventually act as a substantial binder for the bottom chord, therefore only temporary binders need to be installed prior to the laying of the floor itself. These binders are essential if correct spacing of the
trusses is to be maintained, which will of course assist cutting and fitting of the floor boards at a later date.
The problem of the lateral stability of the room remains to be solved on a permanent basis. In Scotland, Scandinavia and the USA where roof sarking is common,
this forms a substantial brace and nothing further is required. The sarking often takes the form of a sheet material nailed to the top of the rafters over the whole roof area (see Fig. 6.22). BS 5268: Part 3: 1998, annex A, section A2, gives guidance on suitable sarking materials and fixing. For information on loft to attic conversion work see Chapters 9 to 12 inclusive.
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