A flat ceiling is the same area as the floor under it, which is the useful shortcut and also the reason the figure gets forgotten — people measure the floor and assume. Then there is what interrupts it: a skylight, a hatch, a downlight, a beam, a coved edge that has to be painted as wall rather than ceiling. And on anything other than a flat ceiling the shortcut stops working altogether, because the surface is longer than the floor beneath it. This measures the flat case properly, subtracts what goes through, and lays the plasterboard out so the sheet count is visible.
Room, and what goes through it
The ceiling is the floor area, less whatever hangs through it.
Counted at 1.2 m² each.
Adds surface rather than removing it: each beam is a downstand.
How far the beam hangs below the ceiling line.
Result
The ceiling is the floor, until it is not
For a flat ceiling, the area is exactly the floor area, and that is worth internalising because it is the fastest way to sanity-check a decorating estimate. A five-by-four room has twenty square metres of floor and twenty square metres of ceiling, before anything is subtracted or added.
The shortcut fails in three common ways. A skylight or a loft hatch removes area. An exposed beam adds it, because a beam hanging below the ceiling line has two faces and a soffit, all of which are ceiling surface and none of which appear in the floor area. And a sloped or vaulted ceiling is genuinely larger than the floor beneath it, because the surface is the hypotenuse rather than the base of the triangle.
Beams add surface, they do not remove it
This is the counter-intuitive one. A beam hanging below the ceiling looks like it is in the way, and it is — but it is also more surface to paint or plaster. A beam 200 millimetres deep running across a four-metre room adds 1.6 square metres of face and 0.8 square metres of soffit, so about 2.4 square metres on a twenty square metre ceiling. Three of them and the ceiling is a third bigger than the floor.
Which is why a beamed ceiling is quoted differently from a flat one, and why a per-square-metre rate for a flat ceiling is misleading on a beamed job. The rate is the same; the area is not, and the cutting around the beams is slower per square metre as well.
Everything is overhead: cutting, lifting, screwing, filling. A sheet of 12.5 mm board is about 25 kg and it has to go up and stay up while it is fixed.
Board across the joists, stagger the end joints, and use a lifter or a second pair of hands. A sheet dropped on a corner is a sheet with a hole in it.
Board layout runs across the joists
Ceiling board goes across the joists, with the long dimension perpendicular to them, so that the tapered edges run along the joists and every joint lands on timber. Board the other way and the long edges run between joists with nothing behind them, which is how a ceiling gets a crack along every board joint.
Stagger the end joints as well: adjacent rows should not have their short edges lining up, because four corners meeting at one point is a weak point and it shows. And leave a small gap at the perimeter — board should not be tight into the wall, or it picks up movement from the structure and cracks at the angle.
| Ceiling | Area vs floor | Waste | Note |
|---|---|---|---|
| Flat plasterboard | Same | 8% | The default case |
| Flat, overboarded | Same | 6% | Simpler shape, fewer cuts |
| One skylight | Minus 1.2 m² | 10% | Shaped cuts around the aperture |
| Exposed beams | Plus per beam | 15% | Cutting around downstands |
| Vaulted or sloped | Plus 8 to 25% | 12% | Measure the slope, not the floor |
| Coved or curved edge | Same | 12% | The coved part counts as wall |
A worked example
- Floor5.0 x 4.0 m is 20.0 m², and a flat ceiling is the same.
- SkylightOne skylight at 1.2 m² takes it to 18.8 m².
- By area18.8 m² at 2.88 m² a sheet, plus eight percent, is 8 sheets.
- Laid outAcross the joists it takes 2 rows of 2.4 m and 5 of 1.2 m: 10 sheets.
- CheckTen sheets cover 28.8 m² for 18.8 m² of ceiling — 10.0 m² of offcuts, because a 5 m room does not divide by a 2.4 m sheet.
Eight sheets by area and ten laid out is a twenty-five percent gap, and it is the same lesson as plasterboard on a wall: sheets are rectangular and so are ceilings, and the count follows how the two divide. A 5 m room takes two rows of 2.4 m and a thin strip, and the strip comes out of a whole sheet. Choose the sheet size to suit the room, not the other way round.
What the figure feeds
Ceilings are usually painted in a flatter, whiter emulsion than the walls, and usually in fewer coats, so they are a separate line in the order rather than part of the wall quantity. The paint figure takes this area and turns it into cans with its own spreading rate.
Lighting is the other thing that hangs off it: the number of downlights, and how far apart they go, is a function of the area and the ceiling height rather than of the room's footprint. The lighting figure works that out from the area, and the wall area figure is the matching surface for the same room — the two are normally decorated together.
Do
- Board across the joists, with the tapered edges along them.
- Stagger the end joints so four corners never meet.
- Leave a small gap at the perimeter so the board cannot pick up movement.
- Measure along the slope on a vaulted ceiling, not across the floor.
- Count beam faces and soffits as ceiling surface, because they are.
Avoid
- Assuming a beamed ceiling is the same area as the floor beneath it.
- Boarding parallel to the joists, so the joints land on nothing.
- Lining up end joints between rows.
- Butting board tight into the wall at the perimeter.
- Using the floor area on a sloped ceiling and being twenty percent short.
Where the numbers stop being true
This measures a flat ceiling. A vaulted, sloped, barrel-vaulted or coffered ceiling is a different surface, and the honest way to measure it is along the slope: take the horizontal run, work out the sloping length, and multiply. On anything with a curve, measure the arc rather than the chord, and add for the cuts.
The skylight allowance is a flat 1.2 square metres, which suits a standard roof window. A large roof lantern, or a glazed section of a flat roof, is several square metres and should be measured properly. So should a loft hatch, which is usually smaller than 1.2 but has a surround that is ceiling surface.
And nothing here covers a coved or corniced junction. The cove is wall, not ceiling, and on a room with a deep cornice the junction is a meaningful surface that belongs in the wall quantity rather than this one.
How the figures are produced
Floor area is length times width, and a flat ceiling starts there. Skylights are counted at 1.2 square metres and subtracted. Beams add twice the downstand times the shorter room dimension, which is the two faces, with the soffit allowed for in the same figure. The area-based sheet count divides the net area by 2.88 square metres and adds the cutting allowance. The laid-out count takes the width in 2.4 m rows and the length in 1.2 m rows, each rounded up, and multiplies — and that is the number to buy.
Questions people ask
Is the ceiling the same area as the floor?
For a flat ceiling with nothing through it, yes. Skylights reduce it, beams increase it, and a slope makes it larger than either.
How many sheets for a 5 x 4 m ceiling?
Seven 1200 x 2400 mm sheets with an eight percent allowance, laid out five across and four down.
Do I add or subtract for a beam?
Add. A beam has two faces and a soffit, and all of it is ceiling surface that a flat ceiling does not have.
Which way does ceiling board go?
Across the joists, with the tapered long edges running along them, so every joint lands on timber.
How do I measure a sloped ceiling?
Along the slope, not across the floor. Take the horizontal run and the rise and work out the sloping length.
Does a skylight count as ceiling?
No, it is subtracted. But the aperture surround, if it is plastered, is ceiling surface and should be added back.