Civil Engineering Reference
In-Depth Information
Cost Advantages
Trussed rafters are designed to carry simply the direct load imposed upon them. It is as-
sumed that they are to be kept upright by other members, these members being the binders
and diagonal bracing and even the tile batten vital to the overall stability of the roof.
Whilst most trussed rafters are used for roofs of housing, their use is increasing for roofs
of public buildings, commercial buildings and to a lesser extent for industrial and agricul-
tural buildings. Clear spans in excess of 30 m can be achieved with lightweight roof cov-
erings.
When first introduced into the UK, the designs were limited to those contained in standard
design manuals, thus the duo pitch and mono pitched roofs were common but more com-
plex roofs needed individual designs prepared. The advent of the computer both speeded
up and dramatically reduced the cost of the design process, and this has been further ad-
vanced by the use of microcomputers installed in all trussed rafter manufacturers' offices.
There are now almost no limitations to the possible shape of trussed rafters, except those
imposed by the practicality of production and transportation to site. The power of com-
puters enables not only the individual trussed rafter to be designed but also the whole roof
as a structural entity. Roof layout drawings can be produced in minutes and then either
plotted on to paper or sent via 'e-mail' to the end user.
Trussed rafter roofs use approximately 30% less timber than a traditional roof, and can be
built into a roof form in a fraction of the time taken for either a truly traditional common
and purlin roof, or a TRADA construction. Factory production keeps the labour cost of
trussed rafter manufacture very low compared to that necessary to assemble a bolt and
connected jointed truss, thus giving further cost advantages to the trussed rafter. Almost
all new housing now uses a trussed rafter form of roof construction.
Legislation
Having looked at the development of the roof form, we must take account of the legisla-
tion controlling building construction in the UK. Before this century no controls existed,
and it was not until the introduction of the model byelaws by each local authority area that
some degree of control was placed upon the design of buildings.
