The loads (weights) to be supported are from various sources:
- Materials: This would include tiles, battens, felt, insulation etc
- Weather: e.g. wind, snow and rain
- Maintenance: A person needing to access the roof in order to repair items
The roof should be tied down to the structure to stop it from lifting in times of strong winds. This is normally achieved by providing straps that are approximately 1.2m long with a cranked end which is fixed to the wall plate (to which the roof timbers are fixed) and then the inner skin of the wall at every 2.0m centres.
The timbers that make a pitch roof (rafters), will always want to spread apart. Ceiling joists are a way of stopping this as they are fixed to the bottom end of the rafters and stop them from pulling apart. However, if you wish to take away the ceiling and be able to see roof, then another system should be used to tie down the rafters to the walls and stop them from spreading. For a roof that is to be exposed with no ceiling joists, you may wish to seek advice from an engineer.