When the two trimming trusses at each side of the opening (figure 77b) are actually nominally fixed together with nails, at say 600mm centres along all
members, an opening of up to three standard spacings may be used. Deeper purlins, binders and ridge board of typically 47 x 175mm and trimmers of 47 x 125mm should be installed.
Depending on the design of the chimney flue and stack, appropriate clearance should be allowed between timber and chimney.
Although intended primarily for trussed rafters, the above principles can also be used for framing with Attic Frames. Raised Tie or Extended joist trusses
require careful consideration when framing around hatch or chimney openings, as often reinforcing timbers (Scabs) are already required on the 'standard' unit and it is often not possible to design multiple ply units of this type.