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Introduction
In recent pubücations on the history of structures, the term "Belfast Truss" has come to mean any wooden bowstring girder with latticework cross-bracing, regardless of the bracing arrangement. Strictly speaking, the term refers to a wooden bowstring girder in which the diagonal bracings connecting the upper curved bow and the lower straight cord are arranged always to meet in a right angle at the regularly spaced purlins on the bow.
It was probably the development of large medieval churches and associated tithe bams which provided the impetus to the development of timber structural roof forms without intermediate support. These consisted of timber, load carrying elements or trusses supporting a slate or sheet-copper or lead roof. As the industrial revolution took hold, manufacturing processes became concentrated into large units and a need for bigger industrial buUdings, with clear floor areas, arose. Initially the medieval roof forms were utiUsed but, as spans increased, so did the size of timbers required for the individual members making up the trusses. Some reUef came from the use of more complex truss forms but there was a clear need for a Ughter and cheaper form of construction capable of bridging large spans.
In the first half of the nineteenth century, the development of the gas industry resulted in the production of high quality roofing-felts which could be laid on timber decking to produce a waterproof roof. This form of construction reduced significantly the dead load of the roof compared with slates or similar materials. The barrel roof-form, first recorded in the 1860s and later known as the Belfast truss roof, was widely used for industrial buUdings up to the First World War (Fig. 1 ). During this period, its use held up weU against other materials, but with steel trusses becoming more widely available, for example in aircraft hangers from 1918, its use declined. However, it continued to be used for moderate spans throughout the 1930s.
The use of the truss form was recommended during the Second World War but the wide availability of curved corrugated-lion for buildings such as the Nissen hut meant that few examples were constructed. Developments in laminated and gang-nailed trusses from the 1960s saw limited use as, by then, methods of covering large areas...