Procedure 01

Checking plaster flatness

A plasterer hands over a room and calls the walls flat. Flat is a measurement taken with a stated instrument over a stated length, not an opinion, so the handover starts with a 2 m straight edge and ends with numbers written against wall references.

01.What is the difference between plaster flatness and plaster plumb?

Plaster flatness measures deviation of the surface from a straight line laid across the surface. Plaster plumb measures the same surface against true vertical. A wall leaning 8 mm out of plumb over a 2.4 m storey height still passes a flatness check, because a 2 m straight edge touches the plaster along its whole length.

The reverse case fails just as often. A wall standing dead plumb at every plumb dot still fails a flatness check when the surface dips 6 mm between the dots, because the straight edge bridges the two high points and the feeler gauge drops into the hollow. Keep two columns on the inspection sheet, one headed gap under a 2 m edge in millimetres and one headed plumb deviation over 2.4 m in millimetres, and record the two properties separately.

02.How do you check plaster flatness with a 2 m straight edge?

The straight edge method holds a 2 m aluminium edge against the plaster with light pressure at one end only, then reads the widest gap with feeler gauges covering 0.05 mm to 5 mm. A wall 4 m long and 2.4 m high produces 12 readings on a fixed pattern, and the sheet records the largest gap and its position.

Check the straight edge before checking any wall. Lay the aluminium edge on a surface known to be flat, such as a machined bench or a second edge held face to face, and sight along the contact line. A straight edge bent by 1.5 mm over its 2 m length produces false readings on every wall for the rest of the day. Mark the edge with the date of the last check and repeat the check every 30 days.

Pressing both ends of the straight edge bridges a high point and hides the hollow behind that high point, so light pressure at one end is the only valid hold. Slide feeler blades into the widest gap and record the largest blade that enters without force. The fixed reading pattern runs horizontal at 300 mm above floor level, horizontal at mid height, horizontal at 300 mm below the ceiling, vertical at 600 mm centres, and one diagonal across each internal corner. Two supervisors using the same pattern produce sheets that can be compared line by line.

03.How much plaster deviation is allowed under a 2 m straight edge?

Allowable plaster deviation depends on the finish that follows: 5 mm under a 2 m straight edge for matt emulsion in a low light corridor, 3 mm for satin paint or thin wallpaper, and 2 mm for a wall carrying a full height mirror. Sheen and reflection multiply the shadow cast by any wave.

Agree the row that applies to each room before the plasterer starts, and copy the agreed row into the room data sheet. A plasterer told after the work that the corridor row of 5 mm has become the mirror row of 2 mm will price a re skim, and the re skim adds 3 mm to 5 mm of build up plus 5 to 7 drying days to the programme.

Table 1. Plaster deviation allowed by the finish that follows

Figures used as acceptance limits on fit out jobs, written here without reference to a standard clause. Where the project specification names a standard, the named standard governs the work.

04.What does raking light show on a plastered wall?

Raking light turns a 1 mm trowel ridge into a shadow roughly 200 mm long. A work lamp placed within 300 mm of the plaster and aimed along the wall at 10 to 20 degrees exposes ridges, dubbing edges and board joints that a ceiling luminaire 2.4 m above the floor leaves completely invisible.

Walk the length of the wall with the lamp while a second person marks every shadow with low tack tape. Raking light finds defects and does not measure defects, so every marked shadow gets a straight edge and a feeler gauge before the shadow becomes a register entry. Two shadows of equal darkness can measure 1 mm and 4 mm, and only one of the two justifies a rejection against a 3 mm limit.

Warn the client early about any wall that faces a window sill below 900 mm on a south elevation. Winter sun crosses that wall at 15 to 25 degrees every afternoon between November and February and shows ridges measuring 2 mm, which pass a 3 mm limit and still produce a complaint. The answer agreed before work starts is a tighter row in the room data sheet, not an argument in January.

05.How long must plaster dry before paint, wallpaper or tiles?

Gypsum finish coat needs 5 to 7 days at 20 degrees Celsius and 50 percent relative humidity before a mist coat, 14 days before wallpaper, and 28 days before thin bed wall tiling. Cement and sand render needs 28 days under the same conditions, counted from the day the second coat went on.

Confirm the wait with a meter rather than a calendar. A pin type moisture meter pressed into the plaster surface reads in percent wood moisture equivalent, and decorators on fit out jobs commonly hold the mist coat until the reading falls below 16 percent WME, and hold wallpaper until the reading falls below 12 percent WME. Take three readings per wall, at 300 mm above floor level, at mid height, and at 300 mm below the ceiling, because the bottom of a wall dries last.

Table 2. Plaster coat thickness and the wait before the next layer

Drying days assume 20 degrees Celsius, 50 percent relative humidity and air movement across the wall. A room at 10 degrees with the windows shut can double every figure in the days column. Pin meter limits are decorator practice rather than a quoted standard.

Tile weight also depends on the background, and a gypsum plaster wall carries less tiling than a cement render wall. Fit out practice holds tiling on gypsum plaster to about 20 kg per square metre, tiling on plasterboard to about 32 kg per square metre, and allows heavier tiling on sand and cement render. Weigh one tile, divide by the tile face area in square metres, add 5 kg per square metre for adhesive and grout, and compare the total against the background limit before the tiler orders material.

06.How should a plaster rejection be written?

A plaster rejection records six items: room number, wall reference, height of the reading above floor level, straight edge length in millimetres, measured gap in millimetres, and the agreed limit for the finish specified in that room. The phrase wall not flat carries no dimension, so no plasterer can price a repair against that phrase.

Good wording reads as a repair instruction: room 04, wall B, reading taken 1.2 m above floor level, 7 mm gap under a 2 m straight edge against an agreed limit of 3 mm for satin paint, re skim the full wall face. The plasterer reads the area, the cause and the fix in one sentence, and the same sentence supports a deduction from the valuation if the re skim never happens.

Attach one photograph per entry, taken with the feeler blade still in the gap and a numbered marker card showing the wall reference. A rejection travels between a supervisor, a contracts manager and a plasterer who never stood in the room together, so the photograph and the six recorded items carry the whole argument.

Plaster deviation does not stay on the wall. Wall tiles bedded on a background outside 3 mm under a 2 m edge produce lippage at every joint, covered on the tiling checks page, and the same measuring discipline applies to floors on the screed tolerances page.