The two tier truss (figure 78) comprises a flat-topped base truss and a triangular capping truss, fitting alongside the base truss on longitudinal bearers. Each truss may be one of a large selection of types. The base truss will generally be made as high as practical but not so high that the span of the capping truss is less than 2-3m. Although a duo-pitched shape is shown in figure 78, all basic configurations can be constructed by the two tier method.
The bracing of the flat top chord of the base truss is important in ensuring its performance in compression.
The base trusses should be erected, full permanent bracing installed and battens fixed, up to the top position of the capping trusses. The resulting
structure then forms a safe, rigid working platform for the erection of the capping trusses. Tiling or loading of the base trusses should not proceed until
the capping trusses are fully installed and braced.