Ceiling joists
The ceiling joists have a double function:
- they provide the structure onto which the ceiling is fixed
- they are the member that connects or ties the lower ends of rafter couples (pairs). Therefore, they prevent the rafters spreading and causing the roof to sag or collapse.
Ceiling joists are normally horizontal members, however, under special circumstances they can be fixed on a slope. They are spaced according to the material they support. Typically the material is plasterboard which can span 450 mm.
For coupled roofs, ceiling joists have to be in single lengths or spliced. They must be at the same spacing and in the same direction as the main rafters so they can be fixed to, and act as ties between the feet of opposing rafter pairs. Where there is no internal wall to support the ceiling joist a hanging beam must be installed.
Any opening, manhole, skylight and similar must be trimmed to provide full support for ceiling linings.
Refer to the Table 21 of AS 1684.2 for details about the required spans for ceiling joists.