Civil Engineering Reference
In-Depth Information
Chapter 4
Bolted Truss Roof Construction
Roofs of all shapes and sizes can be structurally designed using bolt and connector jointed
truss forms. Individually engineered designs for bolted trusses are used for public build-
ings where they are often left exposed as a feature of the design of the building. The
bolted truss designs most frequently found in domestic dwellings are those prepared by
TRADA, and it is these designs which will be dealt with in this chapter.
The bolted trusses to be found in many houses built prior to the trussed rafter era were
based on designs by TRADA. Regrettably these are no longer available in the new stress
grades in existence today. However, these designs may still be copied when extending a
roof, converting to current timber grades and will continue to be a useful form of DIY roof
construction.
The Joints
The bolt and connector roof structure is comprised of principal trusses spaced at centres
dictated by the structural design. The principals in turn support purlins and common
rafters, with binders supported by the principals, carrying the ceiling joists. The strength
of the principal is quite naturally in its joints between the timber members, and therefore
careful assembly of the principal truss is essential if the roof is to perform satisfactorily.
Many such roof forms have been constructed on dwellings since the advent of the
TRADA designs, and it is the sturdiness of the design, rather than high standards of work-
manship, which has led to the method's success.
A simple bolt and connector joint is to be seen in Fig. 1.11. This illustrates a double-sided
toothed plate connector which is held in position (having first been embedded in the meet-
ing timber surfaces) by a bolt with large load spreading washers. The alternative to the
double-sided toothed plate, is the 'split-ring' connector and this will be dealt with later. It
is essential that the teeth of the timber connector be fully bedded into the timber surfaces
if full design strength is to be developed. It is the connector and not the bolt which is the
principal strength of the joint, the bolt mainly being there to hold
