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In the early 19th century the requirement for clear span industrial buildings brought about the development of a variety of timber truss types. The ‘Belfast’ truss was introduced circa 1860 to meet the demand for efficient wide span industrial buildings. It has essentially a bow-string configuration with a curved top chord, straight horizontal bottom chord and close-spaced lattice web. Several thousand still exist in Ireland, many in buildings of historic significance. This paper sets out to demonstrate the efficiency of the Belfast truss and to show that, by modern structural design criteria, the concept, member sizes and joint details were well chosen. Trusses in historic buildings can be replicated almost exactly as originally fabricated. Results of a theoretical study are compared with the experimental behaviour of two full-scale trusses: one a replacement truss, tested in the laboratory; the other an 80-year-old truss tested on site. In addition, experimental results from a manufacturer’s archive material of full-scale truss tests carried out about 100 years ago are compared with theoretical models. As well as considering their significance in building conservation the paper proposes that Belfast trusses are an attractive sustainable alternative to other roof structures. The analysis, design, fabrication and testing of trusses have resulted in a better understanding of their behaviour which is not only of historic interest and fundamental to the repair/restoration of existing trusses, but also relevant to the design of modern timber trusses and the promotion of a sustainable form of roof construction. J. R. Gilfillan, BSc (Eng), PhD, FIWSc Senior Lecturer, Faculty of Engineering, Queen’s University, Belfast, BT7 1NN, Northern Ireland S. G. Gilbert, BSc(Eng), PhD, CEng, MIStructE, MICE Senior Lecturer, Faculty of Engineering, Queen’s University, Belfast, BT7 1NN, Northern Ireland