A Belfast truss was a timber roof structure apparently designed and first used for industrial scale sheds in the town of Belfast, Ireland in the 1860's. Such trusses were originally designed for use with tarred roofing felt laid onto timber boarding.
The Belfast Truss consists of a lower horizontal member (tie-beam) and a curved upper member (bow) connecting at each extremity. The two members are also connected repeatedly with light section lattices.
The segmental curve of the upper member commonly has a radius of 1/2 of the span of the truss. The setting out of the radiating lines of the lattice was commonly based on two Setting Out Points placed at a distance of 3/4 of the span below the ends of the tie beam. The lattices are set out on radial lines from each setting out point to meet at the arched upper member at equal centres to support the purlins.
The three basic components of the Belfast truss are assembled as follows:
The resultant shape of a Belfast Truss is that of a bow-strung girder which is a close approximation to the bending moment diagram for evenly distributed load. It is considered an economical type of truss as it makes use of material of small section, in reasonably short lengths with nailed joints. It has been suggested that the roof form developed partly in an attempt to utilize the short off-cuts from the timber trade.
The ends of the bow are anchored to the tie-beam by gusset boards and a metal strap. The purlins spanned between the trusses and supported curved roof covering.
When making the Belfast Truss, the lattice bar bracings are laid and nailed face to face, with spaces at the bottom cord. However each layer is pinched in by half of its thickness so that the braces lay side by side at the top bow. A contractor would easily set out and fabricate such a truss on site.
The speed of construction of the Belfast roof, had great advantages over other forms of roof construction. In America the Belfast roof was also known as the 'Build Fast' roof. The speed of construction was due to the fact that it was nailed and prepared on the ground and then hoisted up into position.
W Martinson, October 2015