Sheet Materials & Plasterboard — Walls, Ceilings & Floors

Plasterboard · Floor sheets · Ceiling boards · Screws, taping and finishing

Sheet materials are the fastest and most widely used system for lining walls, ceilings and subfloors indoors. Choosing the right board type, correct fixing technique and proper surface finishing produces a durable, professional result. This guide covers everything from board selection through to final decoration.

Board Types

Plasterboard

• Standard plasterboard (white/grey face): 12.5 mm, 1,200×2,400 mm — living rooms, hallways, bedrooms
• Moisture-resistant board (green): 12.5 mm — bathrooms, utility rooms and other areas with documented moisture exposure. In kitchens, use this only where required by the manufacturer or system
• Fire-resistant board (pink/red): 12.5 or 15 mm — used in documented systems where fire resistance is required (e.g. garage walls)
• Acoustic plasterboard: 12.5 mm heavier-density board (approx. 13 kg/m²) — can, in documented systems, provide improved acoustic performance compared with standard board
• Double-layer plasterboard: two layers of 12.5 mm can, in documented systems, provide improved acoustic performance — used in fire and acoustic separating walls in accordance with manufacturer guidance

Floor Sheets

• Tongue-and-groove chipboard (P5 grade): 22 mm, 600×2,400 mm — subfloor on joists. CE-marked moisture-resistant grade
• OSB/3 boards: 18–22 mm, 1,200×2,400 mm — load-bearing boards for humid conditions, used as subfloor, sheathing and wind barrier. Generally stiffer and better for racking/bracing than chipboard, while P5 chipboard gives a flatter, smoother floor surface — choose by the application and the manufacturer's rating, not a blanket “stronger” rule
• Gypsum fibreboard (e.g. Fermacell): 12.5–20 mm — dry-screed and floating floor systems

Ceiling Boards

• Plasterboard for ceilings is fixed to the joists or a metal ceiling system. 12.5 mm board can sag over time on ceilings if the supports are too far apart, so use closer support centres or a thicker/ceiling-grade board (e.g. 15 mm) per the manufacturer's ceiling spec — especially where there is insulation loading above
• Where acoustic separation from the floor above is required: double-layer plasterboard (2 × 12.5 mm) or a documented ceiling system is often used
• Timber ceiling cladding (pine/spruce tongue-and-groove) follows the same area calculation principles as plasterboard

Installation

Golden rule: boards always meet on a stud

Standard 1,200 mm wide plasterboard is designed for stud spacing of 600 mm centres. Two boards must meet in the middle of a stud — never in mid-air. This applies to vertical joints. Horizontal joints should be installed in accordance with the manufacturer's guidance and have adequate backing or fixing support where the system requires it. When in doubt, follow a documented system or consult a professional.

Screw Fixing

• Screw spacing: 200 mm along edges, 300 mm at intermediate studs
• Screws must be set at least 12 mm from the board edge (closer risks splitting the plaster core)
• Screw head should be “dimpled” — set 0.5–1 mm below the board face without breaking through the paper
• Screw length: 35 mm for single-layer (12.5 mm); 55 mm for double-layer
• Typical consumption: approx. 20–30 drywall screws per 1,200×2,400 mm board, depending on screw spacing and installation system

Staggering

Vertical joints on opposite faces of a wall should be staggered — never aligned on the same stud. This increases racking strength and improves acoustic performance. Similarly, horizontal joints should be staggered in ceiling installations.

Cutting

• Score the face paper with a sharp knife against a straightedge
• Snap the board away from the cut — the plaster core breaks cleanly along the score line
• Cut through the back paper
• Smooth the cut edge with sandpaper (60–80 grit) or a rasp

Waste & Off-cuts

Waste factor depends on room shape:

• Rectangular surface with no openings: 5–8%
• Wall with windows/doors: 10–15% (off-cuts around openings cannot be reused)
• Ceiling with sloped sections or dormers: 15–20%

Jointing & Finishing

Three-coat jointing

1. Tape coat: bed paper joint tape into joints with a thin layer of joint compound
2. Fill coat: after 24 hours drying — apply a wider, thinner coat over joints and screw heads
3. Finish coat + sanding: thin skim coat, sand with 120 grit. Check with a raking sidelight

Primer before painting

• New plasterboard absorbs paint at different rates over the board face and at joints — priming is essential
• Typical primer coverage: 8–12 m²/litre
• After priming: 2 coats of topcoat (interior acrylic, choose sheen level to suit the room)

Reference Table

Common Mistakes

• ✗Forgetting to deduct openings — ordering too many boards by calculating gross area without subtracting windows and doors
• ✗Insufficient waste allowance — cuts at corners, cut-outs and board splits produce 10–15% waste; always add this on top
• ✗Boards fixed with joints in the same line — all vertical and horizontal joints should be staggered for rigidity and a better finish
• ✗Wrong board / build-up in wet areas — standard plasterboard cannot handle moisture. Moisture-resistant (green) board suits humid rooms, but it is NOT waterproof and is not enough on its own in a shower zone or behind tiles in a wet room: those need a documented waterproofed wet-room system (tile backer board plus a tanking membrane). Follow the system and local wet-room rules
• ✗Screw spacing too large or inconsistent — follow the system (commonly ~200 mm at board edges and ~300 mm in the field on walls, and closer on ceilings). Too few screws and the board is not held flush and tight against the framing
• ✗Screws placed too close to the board edge (under 10–12 mm) — the edge splits and the screw loses its grip

Calculate Your Materials

Use the area calculator for walls, ceilings or floors: