Openings For Dormers Roof Windows And Stairwells In Attic Trussed Rafter Roofs

Openings in attic trussed rafter roofs can be formed in a similar manner to those shown in the traditionally constructed roof section, Chapter 3. Using two or more trussed rafters as trimmers, both infilled rafters and floor joists need to be supported from those trimming trusses. Short purlins are used to pick up the ends of the trimmed rafters, the purlins themselves being supported on girder trusses. The purlins should be supported on purlin posts built-in to the attic girders at the lower purlin level, and supported on the ceiling tiles as close to the joint between top chord and ceiling tie as possible for the upper purlin. The purlin should be notched under the top chords to give an adequate birdsmouth for the oncoming infill rafters. The purlins will of course carry the infill rafters, leaving the floor joists to be infilled. This can be done using short lengths of floor joists of a matching depth to the bottom chord of the trussed rafter fixed across the opening, the infill joists being supported in joist hangers fixed to the sides of the bottom chord.

Denotes areas of infill timbers

Denotes areas of infill timbers

Fig. 6.20a Trussed rafter attic - poor design.

^ 27 trusses

M II

113 31112 2113

Fig. 6.20b Trussed rafter attic - good design.

Stairwell openings can be similarly constructed, but in this case the whole of both rafter slopes will need supporting on purlins as described above. Figures 6.17 and 6.18 illustrate typical trimmed openings in two types of trussed attic construction. If the sloping ceiling in the infill area between the girder trusses need not align with the main roof, then the purlin can be lifted, and lighter smaller sectioned infill rafters used. On narrow openings between girder trusses it is practical to use joist hangers as purlin supports, fixing these directly to the sides of the top chords of the girder trusses.

As can be seen in Figs 6.20a and 6.20b, the positioning and size of openings can significantly affect the cost of constructing trussed rafter attic roofs. Attention is therefore drawn to the design in Fig. 6.17, which, because it is based on large purlins, allows great freedom of creating openings of any shape or size between the upper and lower purlin without structural complications. If a large number of openings, or alternatively extremely large individual openings are required, then this option is strongly recommended.

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