History of L/360

During the time the author has worked with trusses, the subject of L/360 and other deflection limitations often came up. Therefore, a search of the literature was made. Thomas Tredgold (1788-1829) appears to have been the first person to perform simple bending experiments with wood beams. He concluded that any deflection, under load, would not be acceptable to carpenters of his time if a beam deflected more than 1/40 of an inch regardless of the span (L/480 for a span of 20 feet). Therefore, L/360 would not have been acceptable to him. However, others following him did use L/360 as a deflection criteria for floor joists and roof trusses, while for specialized types of construction such as floor trusses L/480 is being used. L/180 is being used for roof trusses in mobile home construction.

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