General requirements are:
- a sheet material
- a minimum sheet size in the body of the diaphragm of 1800 x 900 mm. Use full sheets where possible – offcuts can be used at the edges to complete the diaphragm.
Material options for ceiling diaphragms given in NZS 3604:2011 Timber-framed buildings are:
- plasterboard with a thickness of not less than 10 mm and a density of not less than 600 kg/m³
- plywood not less than 6 mm thick with a minimum of three plies
- wood-based sheet products not less than 6 mm thick with a minimum density of 600 kg/m³ or not less than 4.5 mm thick and a minimum density of 880 kg/m³
- fibre-cement sheet not less than 6 mm thick with a minimum density of 600 kg/m³ or not less than 4.5 mm thick and a minimum density of 880 kg/m³
- fastenings not less than 10 mm from the edge and at 150 mm centres around the edges of all sheets and at 300 mm centres to intermediate supports.
Parameters specific to plasterboard diaphragms are:
- a maximum length of 7.5 metres
- a maximum ceiling slope of 15°
- specific parameters for plywood, fibre-cement or other wood-based sheets are:
- a maximum length of 12 metres and a maximum ceiling slope of 25° or
- a maximum length of 7.5 metres and a maximum ceiling slope of 45°.
A ceiling diaphragm must have a length of not more than twice its width (length and width are measured between supporting bracing elements), and the basic shape is to be rectangular (cut-outs are not permitted).