Civil Engineering Reference
In-Depth Information
Particular care is needed with CCA preservatives to ensure that the timber is dried out
thoroughly before the metal plates are fixed at the factory (BS 5268: Part 3: 1998, section
5, paragraph 5.5 deals with preservation treatment of trussed rafters). This significantly re-
duces the risk of plate corrosion by the preservative. It must be said, however, that the
practicalities of carrying out this drying at the factory, bearing in mind the relatively short
notice given to the manufacturer to produce the trusses, is impractical. It is therefore
strongly recommended that CCA preservatives are not used with trussed rafter assemblies.
The specification should therefore call for organic solvent based or the new generation of
waterborne organic wood preservatives.
In situations of high hazard in the roof space, e.g. over a swimming pool, then preserva-
tion should be considered essential. Trussed rafters used in these conditions should use
stainless steel plates, and either type of treatment system would then be acceptable. Other
conditions of high hazard are, in the writer's opinion, those areas of the country immedi-
ately adjacent to coastlines, where the corrosive salt-laden atmosphere is of course encour-
aged to enter the building via the ventilating system.
Preservative Identification and Safe Handling
When preservation has been specified, there is obviously a responsibility on the part of the
contractor to make sure that the specification has been met. Many of the organic solvent
based preservatives carry a faint tracer dye (often light reddish brown) but some manufac-
turers use a clear preservative. With COSHH regulations now in force, much of the telltale
oily smell of organic solvent preservative will have evaporated by the time the trusses ar-
rive on site. These regulations require safe handling of preservative treated timber to pro-
tect operatives from the chemicals contained in the preservatives themselves. This means
a delay of some 48 hours between treatment and handling the timber to manufacture the
trussed rafter. Some manufacturers overcome this delay by pre-treating all timber stock,
but then cut ends exposing untreated timber have to be swabbed with preservative before
assembly. If specifying treated timber trussed rafters the purchaser should be aware of a
slightly longer delivery time required by some manufacturers.
