Loads and tolerances
Ceiling board and plaster weight, and load per fixing
- where
- dead load of the ceiling lining (kg/m²)
- board weight (kg/m²)
- skim or plaster weight (kg/m²)
- load per screw (N)
- gravitational acceleration (N/kg)
- screw centres along the joist (mm)
- joist centres (mm)
- total lining weight (kg)
- ceiling area (m²)
Explain this equation
Works out how heavy a plasterboard ceiling is once skimmed, the total weight hanging on the joists, and how much each screw carries. Use it when planning a second board layer, a backing coat on board, or extra weight.
wb is the board weight and ws the plaster weight per m², from the data sheets; w is their sum. Each screw supports a rectangle of ceiling equal to the screw spacing ss times the joist spacing sj; dividing each by 1000 converts mm to metres. Multiplying by 9.81 converts kilograms to newtons. M is the total weight over area A.
Load per fixing is simply weight per unit area times the area each fixing looks after, so closer screws or joists reduce the load on each.
Sense check: 12.5 mm board with a 2 mm skim weighs about 10.4 kg/m²; at 230 mm screws and 400 mm joists each screw carries about 9.4 N, roughly 1 kg. A 12 m² ceiling weighs about 125 kg.
Watch out: board edges have lower pull-through resistance, so fixings there are closer together. Follow the system specification.
Works out how heavy a plasterboard ceiling is once skimmed, and how much each screw carries.
Enter your values to see the result.
Good to know
Gyproc WallBoard 12.5 mm weighs at least 7.7 kg/m²; a 2 mm Thistle finish adds about 2.7 kg/m² dry (British Gypsum dry set weights); backing plasters add far more (around 8–12 kg/m² at 11 mm). The British Gypsum White Book fixes single-layer ceiling board at 230 mm screw centres in the field and closer at ends and perimeters. Each screw carries the board within its tributary area (screw spacing × joist spacing). Fixing extra loads (lights, fans, radiators on walls) needs proper fixings into the framing or tested cavity fixings.
Common mistake
Plastering a backing coat onto a board ceiling, or adding a second layer, without re-checking screw centres and joist capacity. Weight can more than double.
Limits
Static dead load only. Ignores the joists' own capacity and deflection, imposed loads, and the reduced pull-through resistance near board edges. Follow the board manufacturer's fixing specification for fire or acoustic systems.
Sources
- British Gypsum product data sheet: Gyproc WallBoard 12.5 mm (minimum 7.7 kg/m², λ = 0.19 W/mK)
- British Gypsum White Book: Thistle plaster finishes (coat thicknesses, coverage per 25 kg bag, dry set weights, minimum temperature 2 °C until dry, plaster must be thoroughly dry before decorating)
- Board fixing and screw spacing guide (summarising British Gypsum White Book: 230 mm centres in the field on ceilings, 300 mm on walls)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.