The job
Modelled, not recorded. Every figure on this page is re-run through the calculator it names whenever the site is built; the estimate is worked, but no job was carried out, so the last section gives the mechanism of each likely error rather than a measured overrun.
Plastering is a room-sized job with a bag-sized order, and almost everything that goes wrong with the order goes wrong before any plaster is mixed. The area is easy. What is hard is the two questions underneath it: what the plaster is going onto, and how thick it has to be.
The job: a 4 × 3 m room with 2.4 m walls and a plasterboard ceiling. Three walls are boarded; the fourth, a 3 m end wall with a chimney breast 1.2 m wide projecting 250 mm, is the original masonry, stripped back. That mixture is ordinary in a refurbishment and it is the reason this example exists — the two backgrounds take different products in different quantities, and a single figure for "plaster" cannot serve both.
THE MEASUREMENT THAT CONTROLS THE ORDER IS THE FINISHED THICKNESS, and on a refurbishment it is not the nominal figure. A skim over flat, true board is about 2 mm. A wall being brought level takes more, sometimes much more, and the extra is invisible until a straightedge goes on it. On this room the difference between 2 mm and 3 mm is two bags of finish: five become seven.
THE SECOND IS THAT BEADS ARE BOUGHT IN WHOLE LENGTHS. A bead cannot be joined mid-run, so a 2.4 m corner takes a 3 m bead and wastes 600 mm, and this room's eight runs take eight lengths: 24 m bought to cover 17.4 m. Ordering beads by total length is the commonest way a plastering order comes up short, and it comes up short by exactly the offcut.
What was measured, and how
The surfaces, taken one at a time rather than as a room
Boarded walls 26.4 m² (11 m of wall at 2.4 m); masonry wall 8.4 m² (3.5 m of faces, the chimney breast included); ceiling 11.7 m² (12 m² less the breast's 0.3 m²)
Walls and ceiling measured separately, because they may not share a background and on a refurbishment usually do not. Openings deducted only where they are large — a single door is inside the waste allowance on a 2 mm skim, and is not on an 11 mm undercoat.
How true the masonry wall is, before deciding anything
Out by about 14 mm over its length; average undercoat taken as 13 mm rather than 11
A straightedge or a line pulled across the wall at several heights, and the worst point recorded rather than the average. The undercoat is what levels the wall, so the average thickness it needs is what to order against — and the average of the worst and the best point is a better figure than the nominal 11 mm.
Every external corner and every stop, counted as runs
2 external corners at 2.4 m, the chimney breast's; 6 stop runs at 2.1 m
Counted as RUNS rather than metres, because each run consumes a whole bead. External corners are the ones that stick into the room; stops are every edge where the plaster ends — both sides of each door lining, each window reveal.
What the board joints and the angles need before plaster
About 47.6 m (156 ft) of flat joint across walls and ceiling, plus 28.9 m (95 ft) of internal angle: the 14.5 m (48 ft) wall-to-ceiling line and six 2.4 m (7.9 ft) corners, four of the room and two inside the chimney breast
Every board joint and every internal angle is scrimmed, and the two are counted apart: the flat joints follow the BOARD layout, the angles follow the ROOM — where each wall meets the ceiling, and each vertical corner. Folded into one rate per square metre, the angles are the part that goes missing.
The takeoff, in order
Each step needs something from the one before it, which is why the order is part of the answer.
Protect the room before anything is mixed
- Needs
- The floor area and the room's perimeter from the setting out.
- Produces
- Dust sheets from the floor area, and tape from the EDGES — which is the point: tape follows the perimeter and the openings, not the area, so a long narrow room takes more tape than a square one of the same size.
Result3 items: 2 dust sheets for the 11.7 m² (126 sq ft) floor and 1 roll of tape for the 14.5 m (48 ft) perimeter and the openings
Masking and Protection Materials CalculatorScrim the board joints and the internal angles
- Needs
- The boarded area, the board format and the internal angles, from the measurements above.
- Produces
- The tape for the flat joints and the angles. This is done before any plaster and it is the step that decides whether the joints crack later, not whether the plaster covers.
Result84.2 m (276 ft) of scrim: 47.6 m (156 ft) of flat joint from 38.1 m² (410 sq ft) of 2400 × 1200 board and 28.9 m (95 ft) of internal angle, plus a tenth
Drywall Joint Tape CalculatorCount the beads as whole lengths
- Needs
- The corner and stop RUNS counted above, their lengths, and the stock length the merchant sells.
- Produces
- Angle and stop beads as separate counts, and the offcut as a real figure. On this room 8 lengths are bought to run 17.4 m (57 ft), so more than a quarter of what is bought is offcut — which is what the stock length costs you, and the reason to ask whether a shorter one is stocked.
Result8 lengths of 3 m (9.8 ft) bead — 2 angle and 6 stop — running 17.4 m (57 ft) of the 24 m (79 ft) bought
Plaster Angle and Stop Bead CalculatorFloat and set the masonry wall
- Needs
- The masonry area, and the AVERAGE undercoat thickness measured against the straightedge — 13 mm here, not the nominal 11.
- Produces
- Two separate bag counts, because an undercoat and a finish are different products at very different densities. The undercoat is roughly four times the finish by weight and is the figure the extra 2 mm of levelling lands on.
Result130.9 kg (289 lb): 106.7 kg (235 lb) of undercoat in five 25 kg bags and 24.2 kg (53 lb) of finish in one
Float and Set Plaster CalculatorSkim the boarded walls and the ceiling
- Needs
- The boarded wall area plus the ceiling, and the finished thickness the board actually needs.
- Produces
- Bags of finish plaster. At 2 mm, before any allowance, the coat weighs 2.5 kg a square metre, which puts a 25 kg bag at 10 m²: the bag's own published figure, and the check that the arithmetic is the manufacturer's and not this site's.
Result109.5 kg (241 lb) of finish plaster — five 25 kg bags — over 38.1 m² (410 sq ft) at 2 mm
Skim Coat Plaster CalculatorSeal the new plaster before it is decorated
- Needs
- The total plastered area, once it is dry right through — which is a week or more, not dry to the touch.
- Produces
- The paint and the water separately. Only the paint is a purchase; ordering to the mixed volume over-orders by the dilution, which on a 20% thinning is a sixth of the tins.
Result3.9 L (1.0 US gal) of emulsion, thinned with 0.78 L (0.2 US gal) of water — one 5 L tin for 46.5 m² (501 sq ft)
Mist Coat Calculator
The figures
| Step | Calculator | Figure |
|---|---|---|
| Protect the room before anything is mixed | Masking and Protection Materials Calculator | 3 items: 2 dust sheets for the 11.7 m² (126 sq ft) floor and 1 roll of tape for the 14.5 m (48 ft) perimeter and the openings |
| Scrim the board joints and the internal angles | Drywall Joint Tape Calculator | 84.2 m (276 ft) of scrim: 47.6 m (156 ft) of flat joint from 38.1 m² (410 sq ft) of 2400 × 1200 board and 28.9 m (95 ft) of internal angle, plus a tenth |
| Count the beads as whole lengths | Plaster Angle and Stop Bead Calculator | 8 lengths of 3 m (9.8 ft) bead — 2 angle and 6 stop — running 17.4 m (57 ft) of the 24 m (79 ft) bought |
| Float and set the masonry wall | Float and Set Plaster Calculator | 130.9 kg (289 lb): 106.7 kg (235 lb) of undercoat in five 25 kg bags and 24.2 kg (53 lb) of finish in one |
| Skim the boarded walls and the ceiling | Skim Coat Plaster Calculator | 109.5 kg (241 lb) of finish plaster — five 25 kg bags — over 38.1 m² (410 sq ft) at 2 mm |
| Seal the new plaster before it is decorated | Mist Coat Calculator | 3.9 L (1.0 US gal) of emulsion, thinned with 0.78 L (0.2 US gal) of water — one 5 L tin for 46.5 m² (501 sq ft) |
The waste factors, and why these ends of the ranges
| Material | Applied | Why |
|---|---|---|
| Finish plaster | The ordinary allowance for a room, raised for a small one | Finish plaster has a working life of roughly 45 minutes once mixed, so whatever is in the bucket when it goes off is lost. Fifteen per cent suits a room mixed in sensible batches; a small job wants more, because one bucket too many is a larger share of a small order. |
| Undercoat plaster | The same allowance, on a thickness that is itself a measurement | The same mixing loss, but the honest allowance here is in the THICKNESS rather than the percentage. An undercoat ordered at the nominal 11 mm for a wall that needs 13 is nearly a fifth short before any waste is considered, and no allowance recovers that. |
| Beads | None added — whole lengths, with the offcut reported | A bead cannot be joined mid-run, so the count is runs rather than metres and it already rounds up to whole lengths. Adding a percentage on top of that counts the same margin twice. |
| Scrim tape | The ordinary allowance for something bought on a roll and cut repeatedly | Tape is cut at every joint end and at every angle, and the offcuts are short. Ten per cent is the ordinary allowance for something bought on a roll and cut repeatedly. |
Where this estimate is most likely to be wrong
Undercoat on the masonry wall
usually underBecause the thickness is taken from the nominal figure rather than from the wall. An undercoat is the coat doing the levelling, so its average thickness is a property of how bad the wall is — and a wall is always worse than it looks until a straightedge is on it.
Narrow it by: Pull a line across the wall at three heights before ordering and take the average of the worst and the best point. It costs five minutes and it is the only figure that can turn this from a guess into a measurement.
Finish plaster over patched or bonded areas
usually underA skim is quoted at 2 mm over flat board. Anywhere the wall has been made good, bonded or patched is being brought level by the finish coat instead, and that area takes 3 mm or more without anybody deciding it should.
Narrow it by: Measure the made-good area separately and run it through the skim page at the thickness it actually needs, rather than averaging it into the flat board around it.
Stop beads
usually underExternal corners are obvious and get counted. Stops are every edge where plaster simply ends, and most rooms have more of them than corners — both sides of each lining, each reveal, every junction with a different finish.
Narrow it by: Walk the room with the door and window schedule and count the runs out loud. Two per lining, one per reveal edge, one at every junction with a finish that is not plaster.
Mist coat paint
either wayThe spread rate on fresh plaster is a genuine unknown until the first wall is coated: a thirsty skim drinks it and a well-sealed one does not. The honest method is to treat the figure as a starting point and correct it from what that first wall really drank.
Narrow it by: Coat one wall, note what went on, and divide by that wall’s area. That is your spread rate for this plaster, and it is worth more than any published figure.
Tools this job needs
Frequently asked questions
- How much plaster does it take to skim a 4 × 3 m room?
- The three boarded walls and the ceiling come to 38.1 m² (410 sq ft). Skimmed at 2 mm with a 15% mixing allowance, that is 109.5 kg (241 lb) of finish plaster, or five 25 kg bags. Where the board needs bringing level and the skim goes to 3 mm, it becomes 164.3 kg (362 lb) and seven bags. The masonry wall is not skimmed at all: it takes a float and set, ordered separately.
- How much undercoat does an uneven masonry wall need?
- Order against the average thickness the wall needs, not the nominal. The 8.4 m² (90 sq ft) masonry wall here is out by about 14 mm, so the undercoat is taken at 13 mm (about 1/2 in) rather than 11 mm (7/16 in). That is 106.7 kg (235 lb), five 25 kg bags, where the nominal figure gives 90.3 kg (199 lb) and four. The finish coat over it adds one more bag: 24.2 kg (53 lb).
- How much mist coat do I need for new plaster?
- Size it on the whole plastered area, walls and ceiling: 46.5 m² (501 sq ft) here. At 10 m² per litre of mix, about 407 sq ft per US gallon, and a 20% thinning, that is 4.65 L (1.2 US gal) of mix, of which 3.9 L (1.0 US gal) is emulsion and 0.78 L (0.2 US gal) is water. One 5 L tin covers it — buy to the paint figure, not the mixed one.