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Hatch and Chimney Openings
Where possible, hatches and chimneys should be accommodated in the standard spacing between trusses.

Each member and joint in a truss performs an important role, essential to the effective functioning of all other parts and the component as a whole. Trusses
must never be cut and trimmed except according to details supplied by the Trussed Rafter Designer.

The principle behind the methods and details given in this section is to ensure that no individual trussed rafter is subject to a load significantly greater than that applied, were it at standard spacing.
Figure 77a shows a system suitable for openings greater than 10% over standard and up to twice standard spacing. Battens and plasterboard should be given extra support.

Support of the loose timbers is provided in line with each truss joint by a purlin, binder or ridge board and by trimmers at the actual opening.
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.