Attic Frames can be designed to allow 'clear span' supported at eaves only, (as shown in figure 17a), however for longer spans it may be necessary to
incorporate an intermediate support (shown in figure 17b). This will allow either larger internal room dimensions or reduce the timber sections required. Since attic frames are non-triangulated, the timber content will be considerably greater than that required for a comparable trussed rafter.
Where a more complex attic roof layout is being planned, for example where hipped ends, corners or intersections may occur, it is recommended that a truss designer is contacted to prepare a feasibility study at an early stage of the project.
Dormer Window and Stairwell Locations
The same principles that apply to ordinary roof trusses also apply to attic frames. If a truss is severed or weakened at any point the structural integrity of the whole truss is effected. Therefore, if an opening is planned, the roof must be strengthened by additional frames at smaller than standard spacings or girders at each side of the opening. Guidelines to
these details are given in section 3.3.
Having acknowledged these principles, there is relative freedom in the methods of framing out the actual openings, however there are sensible
economic factors to be considered. Obviously it is of most advantage to locate window openings on different sides of the ridge and directly opposite each other in order that they will lie between the same two trimming trusses. If not, the extent of additional loose infill timber may completely negate the advantages of using prefabricated attic frames. Where possible stairwells should be located parallel to the trusses otherwise, once again, the increase in site infill timber may nullify the benefits of using
attic frames. The following diagram (figure 18) demonstrates the most economic method of incorporating openings to the roof space, whilst figure 19 requires increased loose infill timbers and site work if practical
recommendations are not followed.