![]() Consequently, the ground floor layout and upper floor layout need no longer harmonise. Trussed rafters enabled Architects the freedom with the layout of the upper floor by avoiding the need for internal load bearing walls. The system owner’s software is very sophisticated and not only designs the trusses, but also produces quotations, manufacturing information and production data. The software used by each manufacturer will be supplied by one of three companies who also make the connector plates used to fasten the members together (often referred to as “the system owners”). Trusses are engineered to suit the particular application requested. Trusses are made by some forty four manufacturing companies dotted around the country. The trusses supported purlins and binders which in turn supported rafters and ceiling joists. The TDA trusses were site built using toothed plate connectors based on standard designs published by the Timber Development Association (TDA), the forerunner of TRADA. Trussed rafters should not be confused with a system, commonplace in the 1950’s, which used “TDA” trusses at 1.8 to 2.4 metre centres. The idea of trussed rafters arrived in this country from the States during the 1960’s and quickly became the standard method of constructing roof structures in domestic construction. Trussed rafters are defined as triangulated timber frameworks positioned at close centres (generally 600mm) to form roof structures. It is hoped that by examining common misunderstandings, costly and awkward modifications can be avoided. ![]() This paper seeks to provide a basic understanding of how trussed rafters work, theirs strengths and their limitations. Such misunderstanding can, in some circumstances, lead to poor installation or worse unsuitable alteration. However, experience has shown that trussed rafters are not always well understood by the people who specify them or who erect them. To date, there has been no known failure of a trussed rafter in its “ex factory” condition, a remarkable safety record. Conservative estimates would suggest there is somewhere in excess of 60 million trussed rafters in service in the UK.
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