Boarding out a 4.8 × 3.6 m room: sheets, screws and compound
One room taken back to the studs and boarded out. The sheet count is straightforward arithmetic. The screw you buy is where most of these jobs go wrong, and almost nobody notices.
The room
- 4.8 m long × 3.6 m wide, 2.4 m floor to ceiling
- Four walls and the ceiling, all being boarded
- One door opening deducted
- Timber studwork at 400 mm centres, joists the same
- Two external corners where the chimney breast returns
- Finish taken to level 4 — the standard for flat emulsion
Step 1 — the boards
Wall area is the perimeter, 2 × (4.8 + 3.6) = 16.8 m, times 2.4 m high, less the door: 39.3 m². The ceiling is 4.8 × 3.6 = 17.3 m². Together, 56.6 m² of board.
A 1200 × 2400 mm sheet covers 2.88 m². With 10% cutting waste that is 22 sheets.
Ten per cent is the right allowance here and it is not generous. Offcuts from around a door opening are rarely large enough to use anywhere else, and a 2400 mm sheet that gets dropped on a corner is scrap. If the room has more openings than this one, push it to 12%.
Step 2 — the screw, which is probably not the one you were going to buy
This is the part worth reading even if you never use the calculator.
British Gypsum specify screw length as the board thickness plus a minimum penetration into the frame: 25 mm into timber, 10 mm into metal. Not a rule of thumb — their own published figure.
Run that for 12.5 mm standard wallboard on timber studs: 12.5 + 25 = 37.5 mm, so you need a 38 mm screw.
The 32 mm drywall screw is the default on every merchant's shelf, and it is too short for this. Under 12.5 mm board it leaves 19.5 mm in the stud, against a specified minimum of 25 mm. It will hold, and thousands of rooms are boarded with it every week. But it is below spec, and on a ceiling — where the fixing is carrying the board's weight rather than just holding it flat — that margin is the one you would rather have.
| Board | Into timber | Into metal |
|---|---|---|
| 9.5 mm — ceilings, overboarding | 38 mm | 25 mm |
| 12.5 mm — standard wallboard | 38 mm | 25 mm |
| 15 mm — fire, acoustic | 42 mm | 25 mm |
Note that 9.5 and 12.5 mm board land on the same 38 mm screw, because 34.5 mm is not a size anyone sells and the next one up is 38. Metal framing is far more forgiving: 10 mm of penetration means a 25 mm screw covers every board thickness in normal use.
Step 3 — how many screws
Screw count follows the studs the sheet crosses, not the sheet's area. At 300 mm along the board ends and 400 mm through the field, a 1200 × 2400 sheet on 400 mm centres takes about 30 screws. Across 22 sheets with a 15% allowance for the ones you strip, overdrive or drop: 759 screws, which is about 1.0 kg of 38 mm.
Buy a 1000-box. You will use most of it, and running out at nine on a Sunday with half a ceiling up is its own kind of misery.
Step 4 — tape and compound
Tape follows the joint length, which on 1200 mm wide board works out at about 1.05 m of joint per square metre of wall once internal corners are counted. Over 56.6 m² that is 59 m — one 75 m roll.
Compound is set by the finish level, and the levels differ by roughly double:
- Level 3 — 0.45 kg/m². Taped and two coats. Fine under texture or a heavy paint.
- Level 4 — 0.60 kg/m². The standard for flat emulsion, and what this room is costed at.
- Level 5 — 0.95 kg/m². A full skim over the whole surface. For gloss, strong side-light, or a wall that will be lit down its length.
At level 4 over 56.6 m²: 34.0 kg, so 2 × 20 kg bags. Choosing level 5 instead would take it to 53.8 kg and a third bag — which is the honest reason to decide the finish level before ordering rather than after.
Step 5 — the full order
| Item | Qty | Why this much |
|---|---|---|
| Plasterboard, 1200×2400×12.5 | 22 sheets | 56.6 m² plus 10% cutting waste |
| Drywall screws, 38 mm | 1 box of 1000 | 759 needed. 38 mm, not 32 — see above. |
| Paper jointing tape, 75 m | 1 roll | 59 m of joint and internal corner |
| Jointing compound, 20 kg | 2 bags | 34.0 kg at level 4 |
| Corner bead, 2.4 m | 2 lengths | Two external corners, full height |
| Taping knives, 150 mm and 250 mm | 1 each | Bed the tape with the small one, feather with the large |
| Mud pan | 1 | Working out of the bag ruins the bag |
| Plasterboard saw and a sharp knife | 1 each | Score-and-snap for straight cuts, saw for openings |
| Board lifter or a second pair of hands | 1 | A 2400 mm ceiling sheet is 22 kg held overhead |
| Dust masks, FFP2 | 1 box | Sanding compound is the dustiest job in the house |
| Sanding pole and 120 grit | 1 | Overhead sanding by hand is slow and hard to keep flat |
What people forget
- That the ceiling goes up first. Board the ceiling, then the walls, so the wall sheets support the ceiling's edges. Doing it the other way leaves the ceiling perimeter unsupported.
- Which way the board runs. Sheets go across the joists, not along them, so each sheet crosses as many as possible. On walls, horizontal is usually right for a 2.4 m height because it puts the joint at waist level where it is easy to finish.
- Noggins at the horizontal joints. Every board edge needs something behind it. A horizontal joint with nothing behind it will crack, whatever you tape it with.
- The screw depth. The head must dimple the paper without breaking it. A screw through the paper holds nothing at all — the board's strength is in the liner. Use a depth-stop bit.
- That 15 mm board exists for a reason. If this were a garage conversion with a habitable room over it, or a wall between two dwellings, the fire rating may require it. Check before ordering 12.5.
What this costs in time
- Day 1 — ceiling. Slowest surface by a distance, and the one you want fresh.
- Day 2 — walls, cutting around the door and the chimney returns.
- Day 3 — tape and first compound coat. Corner beads on.
- Day 4 — second coat, wider. Overnight to dry.
- Day 5 — third coat if going to level 4, then sand.
Each compound coat needs to dry fully before the next, which is what makes this a week rather than a weekend. Rushing a coat is how you get a joint that shrinks back and shows a month later under emulsion.
Run your own version
- Plasterboard calculator — set the thickness and frame and it works the screw length out
- Fixings by substrate — for what goes into the board afterwards
- Painting a replastered room — the next job, and why the mist coat matters
- How to measure a room properly — before anything else
Last reviewed 2026-10-17. Quantities are for the room described. Fire and acoustic requirements are set by building regulations for your specific situation — check them before choosing a board.