Trussed rafters of course are designed to work at 600 mm centres (or some other specified dimension), and these spacings should not be increased without adjusting either the design of the roof truss itself, or the spacing on either side of the opening created. On no account should a trussed rafter be cut.
Fig. 8.27 Trimming for small openings - trussed rafters.
British Standard 5268: Part 3, pages 23-25, gives details of openings for chimneys and hatches. Section 7.6 of the standard sets out details for the maximum spacings between trimming trussed rafters. Using the British Standard's lettering, the standard trussed spacing equals a, the spacing between trimming trusses and the adjacent truss equals b, with the distance between the centres of the trimming trusses being c. This gives a formula of c = 2a - b. The nominal opening c is not that which the designer would need to know, so let the actual opening between the trimming trussed rafters be w. Assuming then a truss thickness of t, the actual opening width between trussed rafters becomes 2a - b - t. To find the maximum opening width permissible for a truss spacing a = 600 mm, with a truss thickness t = 36 mm, we have 2 x 600 - 36
- 36 = 1128 mm. The dimension b must be equal to the truss thickness t, because to give the widest opening the trimming truss must be immediately adjacent to the last standard truss.
Let us now assume that we need an actual opening of 800 mm, and w = 800, a = 600 and t = 36. Substituting in the formula above we have 800 mm = 2 x 600 - b
- 36, b = 1200 - 800 - 36, b = 364 mm. The setting out of this truss would then be as shown in Fig. 8.27.
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