For this roof form it is not technically impossible to provide a series of different shaped trussed rafters simply fanned out around the dog leg. However, this is usually prohibitive on cost as each truss would be a different shape because the span would increase while the overall height would have to remain constant, necessitating a different jig setting for each pair of trusses produced. This latter statement of course assumes both legs of the dog leg are of equal span!
The solution illustrated in Fig. 7.8 assumes the use of a girder truss at the last point into the dog leg where a truss can be placed at true right angles. This is indicated as Girder B on the illustration. A further compound truss (Girder C) is then designed to fit across the intersection of the dog leg. These girders between them carry an infill timber system to complete the two triangular infill areas.
Whilst on small spans, a compound of the Fink of the common roof could be used at Girder B; the heavier load and effectively lower pitch of Girder C may dictate that some different configuration may be necessary. Obviously to gain best support for infill purlins and ceiling joist binders, the configuration of Girder B and Girder C must be identical. This invariably means a Howe girder is used in most domestic size
roof constructions. The purlins and ceiling joist binders are supported at node points on heavy duty joist hangers or truss shoes, nailed or bolted to the girder truss. Similarly, a ridge must be fitted to continue the ridge line into the dog leg. On to these secondary supporting members the common rafters and ceiling joists can be fitted.
A second possible solution is indicated in Fig. 7.8 which uses a trussed purlin on which monopitch trusses can be supported. This option, of course, reduces the amount of site infill and, the author suggests, may be a more economic solution for larger spans or perhaps where the dog leg is a repetitive feature throughout the building site.
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