Plasterboard Ceiling Calculator — Boards, Noggings & Screws

Plasterboard should be fixed perpendicular to the ceiling joists (i.e. the long edge of the board crosses the joists). This ensures maximum support along the board length. The long tapered edges should run perpendicular to the joists, making them easier to tape and joint. Stagger end joints between rows by at least 600mm to avoid a continuous joint line.

British Gypsum sizes the screw as board thickness plus a minimum 25mm penetration into timber, so use 38mm drywall screws for 12.5mm board, 32mm for 9.5mm board, and 40mm for 15mm board (the 10mm minimum penetration only applies to fine-thread screws into metal framing). On a timber-joist ceiling fix coarse-thread (W-type) screws at 150mm centres around board ends and the perimeter, and at a maximum of 230mm centres in the field where boards cross intermediate joists. Set the heads just below the surface without breaking the paper face.

Noggings (timber cross-pieces between joists) are needed wherever board edges meet without a joist beneath them — chiefly the short end joints that run parallel to the joists. With 12.5mm board on joists at 400–450mm centres no extra mid-span noggings are required; at 600mm centres you should either step up to 15mm board or add mid-span noggings. Noggings should be 50×50mm or the same depth as the joists, nailed or screwed between joists to give a fixing for the board edges. The calculator counts one nogging per joist bay along each line of end joints, matching the joist centres you select.

A single layer of 12.5mm plasterboard fixed to joists at 400mm centres can support approximately 25 kg/m² of imposed load (e.g. insulation above). For heavier loads or wider joist spacing, use 15mm board or double-board. Light fittings up to 3kg can be fixed directly to the board. Heavier items must be fixed to joists or use specialist plasterboard fixings rated for the load.

12.5mm is standard for domestic ceilings with joists at 400mm centres. Use 15mm board for joists at 600mm centres, where additional fire resistance is needed, or where heavy insulation will be placed above. 15mm board is heavier (approximately 30-36 kg per 2400×1200mm sheet vs 22-25 kg for 12.5mm) so requires two people to handle on ceilings.