Civil Engineering Reference
In-Depth Information
Chapter 5
The Construction of Trussed Rafter Roofs
The majority of roofs constructed for domestic dwellings in the United Kingdom now use
the punched metal plated trussed rafter construction. Over two million units are produced
each year. The majority of the trussed rafters are produced in factories where capacity to
both design and produce varies enormously from those manufacturers able to design the
trussed rafters only, to those fully competent timber engineering companies employing
their own structural designers. Production capacity ranges from 200 to 5000 trussed rafters
per week, with quality also varying both in the final product and in the service offered to
the customer. The punched metal nail plate connector can only be fixed using the special-
ist equipment needed for pressing the plates into both sides of the timber joint. It is
not
possible to fix them on site. Figure 5. 1a illustrates a typical punched metal plate joint.
There are two alternative systems to the punched metal plate fastener, one being a metal
plate punched with holes which is then fixed to the timber joint with special twisted nails
(Fig. 5. 1b). The other is to use plywood gusseted joints, the plywood being fixed either
by glue with nails to hold it in position whilst the glue cures, or exclusively by nails of de-
signed size and fixed to a specific designed pattern on the joint: Fig. 5.1c illustrates such a
joint. With the exception of the glued option, the latter two methods are suitable for site
assembly.
Performance in Use
Much has been written in trade journals concerning possible problems occurring with the
trussed rafter form of construction, with reference particularly to its long-term durability.
An authoritative paper was prepared in 1983 by the Building Research Establishment en-
titled
Trussed rafter roofs
(IP14/83) in which the results of a nationwide survey were sum-
marised. The survey looked at the manufacture, site use, performance in service, and plate
corrosion with certain types of preservative. Whilst some short-comings were found in the
manufacture, the areas causing greatest concern were site
