Pitched Roof Structures
Often used in conjunction with a post and beam system for efficient construction. Can work equally well with conventional stud framing support. It should be noted that where the rafters are not ‘tied’ together via conventional ceiling joists, an alternative must be considered to resist the spreading of the rafters under roof loads. To prevent this, a collar tie must be included, and rafter sizes are generally increased to retain rigidity in the structure. If this increase in size is undesirable, then an extra tie can be located near the base of the rafters, and this can be made of timber, wire or steel rod.

Simple Pitched Roof
The most common method of achieving a so-called ‘cathedral ceiling’ uses rafters simply spanning between ridge and intermediate beams and external walls. Ceiling linings can be fixed to either the top or bottom face of the rafters.
 

Figure 3: Ceiling above rafters

Ceiling above rafters
Has the advantage of concelaing joins in ceiling materials - commonly lining boards, plywood or cement based panel products. If using lining boards, shorter lengths can be utlised. Progressive cover needs to be maintained to prevent water damage and staining.

Fixing of rafters to beams also needs careful detailing, as common methods such as the use of nail plate connectors are not appropriate. The use of long (up to 150mm) type 17 screws and coach screws are comon methods of achieving required tied down. AS1684 should bve utlisied to calculate the laods to be resisted. Assistance can be sought from structural engineers for specific requirements.

The depths of battens is determined largely by insulation requirements. A 70 x 45mm on edge is common. Alternatively, counter battens over rafters with standards battens over can be used. Refer to Figure 3.
 

Figure 4: Ceiling below rafters

Ceiling Below Rafters
As rafters are concealed within the ceiling lining and supporting beams, trusses or other primary structural members are expressed. The depth of the rafters is available for insulation, and ceiling installation is done after the roof covering. Concealing of rafter/support connection is still required for best appearance. Refer to Figure 4.

Connections for these members are simple, and utilise nail requirements akin to site-pitched roof construction. Structural detailing may make use of the previously mentioned post and beam construction. As shown in Figure 2, this involves a pitching beam and ridge beam to support the rafters. In addition, intermediate beams may be necessary to reduce the span of long rafters.