The roof must be able to support both its own weight and that of imposed loads.
- Imposed loads include non-voluntary such as snow loads and also the possible application of insulation, solar panels or water tanks in the future.
- The force induced upon the roof by wind must also be taken into consideration during the calculations of roof strength. The design of the roof must also resist wind uprooting. This may involve straps to hold down parts of the structure if the weight alone is insufficient.
- Holding straps may also be required in some parts of the UK. Straps are generally steel with a galvanized finish with appropriate fixings.
- Rafters supporting the roof and ceiling joints must not be more than 600 millimeters apart. Ceiling joists are horizontal supporting beams, rafters are the sloped supporting beams.
- Ceiling joists must also be calibrated by the size and thickness of the plasterboard or other material that the ceiling is constructed from. For example, for plasterboard sheets of 9.5 millimeter thickness, joist spacing must be no mote than 450 millimeters apart. For plasterboard sheets of 12.5 and 15 millimeter thickness, this joist spacing increases to 600 millimeters.
- Tile battens are used to provide a strong anchor for nails and clips that secure tiles/slates and also give rigidity to the structure. For natural slate battens, 25x50 sized battens can be used with both 450 and 600 millimeter spacing. Fibre cement or concrete slates must also be 25x50 for 600 millimeter spans, but can be 25x38 for 450 millimeter spacing.
- The size for nails fixing battens should be 10 gauge or 3.35 millimeters by 54 millimeters long. Nails can be either ring shank or galvanized smooth round types. The ring shank type should be used where the maximum basic wind speed is over 26 metres per second.