Civil Engineering Reference
In-Depth Information
defects. The cross section should not be less than 2134 mm
2
. This means minimum bra-
cing sizes can be
22
mm × 97 mm or 24 mm ×
89
mm. No other strength grading is re-
quired.
Annex A(h) of the standard also stipulates that the bracing members must be nailed to the
trussed rafter with 2 × 3.35 mm diameter galvanised wire nails on every crossing point.
The minimum nail length should be equal to the bracing thickness plus 32 mm, i.e. 54 mm
for 22 mm thick bracing or 56 mm for 24 mm thick bracing. In practice the nearest readily
available galvanised nail of 3.35 mm diameter is 65 mm long and nail specification has
been general practice for many years. For the purposes of the following description we
will use 22 mm × 97 mm bracing and 3.35 mm diameter galvanised wire nails 65 mm
long.
Diagonal brace F, 22 mm × 97 mm, should now be fitted to the underside of the top
chords of the trusses on both sides of the roof at an angle of approximately 45º to the
rafters. At least four diagonals 'F' should be fitted to each roof. Two 3.35 mm diameter ×
65 mm long galvanised round wire nails should be used at each trussed rafter where the
diagonals cross. The diagonals must extend down the wall plate, and should be let in to
the top of the plate and nailed as above. If the brace has to be lapped in its length, then the
lap should cover two trussed rafters and each piece be nailed to the trussed rafters as de-
scribed. Figure 5.6 illustrates the diagonal brace lap, viewed from beneath the rafters; oth-
er binders are omitted for clarity.
Longitudinal binders G, again 22 mm × 97 mm in section, should now be fitted at peak
and third point. Nailing is the same as for the diagonal, but check the centres of the trusses
and the bottom chord alignment before fixing. Failure to do this could result in ceiling fix-
ing problems see Fig. 5.7, the trusses illustrated, not being located at precisely 600 mm or
400 mm centres, will not readily accept ceiling boards which are generally 1200 mm
wide. This necessitates a batten being nailed to the side of the trussed rafters to correct the
spacing error. The ends of these binders should be tight against the gable wall. On timber
framed housing binders should be nailed to the gable end panel. Blocking pieces will be
required, the depth of the bottom chord sufficient to enable the third point binders to be
fixed to the panel frame.
