Civil Engineering Reference
In-Depth Information
Under no circumstances may a trussed rafter be cut or altered in any way on site
without
a design drawing and specification provided by the trussed rafter designer.
The practice of 'skew' or 'tosh' nailing the trussed rafter to the plate is not recommended,
as this can easily damage both the heel joint timber and nail plates. Truss clips made from
galvanised steel are available from most truss suppliers and builders' merchants and these
should be fixed to the wall plate at the trussed rafter setting out marks. The truss clip is il-
lustrated in Fig. 7.10. Under certain design conditions truss straps may also be required,
but these could be fitted just prior to roof covering.
The object at this stage of construction is to erect a portion of the roof that will be stable,
from which the remainder of the roof can be constructed. Diagonal braces should be used
for stabilising the roof,
not the gable end
even if at this stage it has been built. The method
now described will also work for the centre section of a hip roof. Reference should be
made to Fig. 5.5.
The first trussed rafter, A, should be positioned a few trusses away from the gable or hip
peak, such that its peak coincides with the top diagonal brace F. Holding this truss truly
vertical, temporary diagonal brace B should be fitted on both sides of the roof. The brace
should be well nailed to the trussed rafter as close to the web joint as possible but not in it,
and to the wall plate. Temporary fixings should be made using double-headed shuttering
nails to avoid damaging timbers when they are moved.
Prepare a temporary batten for both sides of the roof, long enough to reach from trussed
rafter A to the gable. The battens should be marked with the truss centres by reference to
the wall plate marks and shuttering nails driven in until the points just show through. Be-
fore standing truss C in place, ensure that it is the correct way round to match truss A.
This can be done by checking that the bottom chord splice plates will be in-line down the
length of the roof. If no splice plate is fitted, then check the truss for similarity whilst still
in stack.
Do not hand the trusses
; failure to carry out this check could result in poor roof
alignment caused by the peak being slightly off true centre. Stand trussed rafter C in posi-
tion and fix to wall plate with truss clip. Fit the temporary battens to trussed rafters A and
C on both sides of the roof; the prefixed nails now easily locate the trusses and provide a
free hand to help stabilise the structure. The remainder of the trussed rafters in this first
section can now be fixed, each nailed to the temporary batten on both sides of the roof.
Before proceeding further, check that the trussed rafters are truly vertical. Acceptable tol-
erances for plumb are set out in BS 5268: Part 3: 1998, section 9, paragraph 9.3.1.
