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Design Method
Another important criterion in the design of trussed rafters, which must be considered, is the amount of deflection, or movement of the truss when loading is applied to it. (See figure 22).
BS.5268-3 section 6.5.7 clearly defines how to calculate deflection and the permissible limits on rafters, ceiling ties and on overhangs and cantilevers.
This therefore defines the amount of movement under the differing load conditions permitted. Additionally, the Trussed Rafter Designer should be aware of the problems which may arise due to DIFFERENTIAL DEFLECTION.
Differential deflection may occur between two adjacent trusses within a roof when either the support conditions or the loading conditions change. For example, in a hip end or corner condition (see sections 2.8 and 3.5) the heavily loaded girder truss may show more anticipated deflection than the truss immediately behind it in the hip sequence. Or, where a bobtail (stub) truss is used adjacent to a full span  truss, the deflection of the standard truss may be anticipated to be greater than that shown for the bobtail.
In this situation, the Designer should ensure that the difference in anticipated deflection between the two trusses is kept within limits, to avoid problems in producing a smooth line for the ceiling constructions underneath. This problem of differential deflection between adjacent units is one of the most common  causes of site problems and, once the roof is erected, one of the most difficult to rectify. The remedy is for the Designer to be fully aware of the potential problem at the design stage.