Services in 'RiR' Trussed Rafters
The lower void area in 'RiR' trusses is an ideal location for services, allowing lateral runs to be positioned between the bottom chords of the trusses (see Fig. 1). Access to this void area and the service runs can be made via a small hatch in the low level partition.
The floor joists of a RiR trussed rafter should never be notched or drilled to accommodate services without the prior agreement of the trussed rafter manufacturer / roof designer who will advise on maximum sizes and locations of any such modifications. Further information is available in the TRA Product Information sheet on RiR trusses.
Layout of 'RiR' Trussed Rafters & Planning Position of Openings
The application of a few basic principles at the concept stage of a project can often result in substantial cost savings by maximising the use of prefabricated components and minimising loose infill areas. Try to locate opening in the roof to fit in with the normal spacing of 'RiR' trussed rafters (usually 600 mm). This can often result in reducing the number of trussed rafters required (see Fig. 2).
Dormer windows and stairwell openings are formed by placing multiple trusses either side of the openings and framing the resulting space with loose timbers. Placing stairwells parallel to truss spans and ensuring that windows are positioned opposite each other will make the overall roof design simpler and cheaper. Fig. 3 exemplifies the problems associated with misaligned roof features.