PLASTER & MORTAR · FIELD TOOL
Ceiling plaster = slab soffit area + beams (length × (width + 2 × depth below the slab)). 120 m² of ceiling with 40 m of 300 × 450 mm beams is 120 + 48 = 168 m². At 6 mm in 1:3 that is 9.2 bags of cement and 0.96 m³ of sand.
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| Flat soffit | 120.0 m² |
| Beam soffits and sides | 48.0 m² |
| Cement (50 kg bags) | 9.2 bags (461 kg) |
| Sand | 0.96 m³ |
| Wet plaster | 1.008 m³ |
Concrete soffits need a bonding treatment (hacking, SBR slurry or a bonding coat) so the plaster does not delaminate.
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Beams = beam length × (width + 2 × depth below slab)
Area = soffit + beams
Cement = area × thickness × 1.27 ÷ (1 + n)120 m² soffit, 40 m of beams 300 mm wide and 450 mm deep below the slab, 6 mm, 1:3:
| Beam width × depth below slab | Plaster per metre |
|---|---|
| 230 × 300 mm | 0.83 m² |
| 300 × 450 mm | 1.20 m² |
| 300 × 600 mm | 1.50 m² |
Usually 6 mm on smooth concrete soffits, up to 10 mm where the soffit is rough. Thick plaster on ceilings is heavy and more likely to fall.
A rich mix, commonly 1:3 (or 1:4), so the thin coat bonds well to the concrete.
Add each beam's soffit and both sides below the slab: length × (width + 2 × depth). A 300 × 450 mm downstand adds 1.2 m² per metre of beam.