Checking a Room Plaster Estimate and the Contractor's Four Bags
You are preparing an internal plaster estimate for a 4.2 m × 3.6 m room with a clear height of 3.0 m and 250 mm thick walls. There is one door of 1.0 m × 2.1 m and two windows of 1.2 m × 1.2 m each. Specification: 12 mm thick cement plaster in 1:6 mortar on the internal wall faces and on the ceiling; door and window reveals are plastered; curing for 7 days. Local rates are cement ₹420 per 50 kg bag, sand ₹1,400 per cubic metre, a mason ₹900 per day and a helper ₹600 per day, with 8 m² per mason-day achievable for 12 mm internal plaster. A contractor offers to do the whole job as a lump sum of ₹12,000 and asks you to issue only four bags of cement as the entire material supply for the plaster.
1. Measure the plastered area, stating every deduction and addition and the rule behind it.
Read the worked answer
Wall girth: 2 × (4.2 + 3.6) = 15.6 m. Gross wall area = 15.6 × 3.0 = 46.80 m².
Deductions - openings larger than 0.5 m² are deducted; smaller ones are not, because the labour of plastering around them offsets the saving:
- Door: 1.0 × 2.1 = 2.10 m²
- Two windows: 2 × (1.2 × 1.2) = 2.88 m²
- Total deduction = 4.98 m²; net wall face = 41.82 m²
Additions for reveals (measured and paid, since they are plastered surfaces):
- Door: two jambs 2 × 2.1 × 0.25 = 1.05 m², plus soffit 1.0 × 0.25 = 0.25 m² → 1.30 m²
- Each window: two jambs 2 × 1.2 × 0.25 = 0.60, top 1.2 × 0.25 = 0.30, sill 1.2 × 0.25 = 0.30 → 1.20 m²; two windows = 2.40 m²
- Total reveals = 3.70 m²
Net plastered walls = 41.82 + 3.70 = 45.52 m².
Ceiling = 4.2 × 3.6 = 15.12 m², measured separately because it is a different item (overhead work, different rate) and has no openings to deduct.
Total plaster area = 60.64 m², described as 12 mm thick, since thickness is priced within the rate and the measurement is of area.
2. Compute the cement and sand required, showing the conversion factors you use.
Read the worked answer
Wet volume of mortar = 60.64 m² × 0.012 m = 0.728 m³.
Dry volume = wet volume × 1.33, the standard allowance for voids in sand, bulking and the fact that dry cement and sand bulk more than the set mortar: 0.728 × 1.33 = 0.968 m³. Some estimators use 1.30; the difference is small, but the factor must be stated in the rate analysis so that it can be audited.
For a 1:6 mix, the total is 7 parts by volume:
- Cement = 0.968 ÷ 7 = 0.138 m³. At a bulk density of 1,440 kg/m³, that is 0.138 × 1,440 = 199 kg, i.e. almost exactly 4 bags of 50 kg.
- Sand = 0.968 × 6 ÷ 7 = 0.830 m³ (about 29 cft).
Allow 5% wastage for spillage, mortar dropped in handling, and mortar left in the gauge box and pans: cement 199 × 1.05 = 209 kg → order 5 bags; sand 0.830 × 1.05 = 0.87 m³.
Note that the theoretical 4 bags is a bare arithmetic figure for a perfectly flat 12 mm surface. Real walls are uneven, corners and reveals consume extra mortar, and suction takes its share. Issue 5 bags, keep the empty bags on site for reconciliation, and never allow the mortar to be re-tempered with water after it has begun to set.
3. Test the contractor's ₹12,000 lump sum and the four-bag supply against your own rate analysis, and list the risks you would flag.
Read the worked answer
My estimate:
- Cement 5 bags × ₹420 = ₹2,100
- Sand 0.87 m³ × ₹1,400 = ₹1,218
- Curing water, scaffolding/planks for the ceiling, miscellaneous = ₹800
- Labour: 60.64 m² ÷ 8 m² per mason-day = 7.6, say 8 days; mason 8 × ₹900 = ₹7,200, helper 8 × ₹600 = ₹4,800 → ₹12,000
- Subtotal = ₹16,118; add 10% for overheads and profit ≈ ₹1,612 → ≈ ₹17,730, or about ₹292/m²
Judgement: ₹12,000 works out at ₹198/m², roughly a third below cost. The only ways to deliver at that price are: reduce the thickness below 12 mm, apply a single coat instead of the specified build, use a 1:8 mix by stretching the cement, skip curing, or skimp on the ceiling scaffolding. The four-bag supply is the arithmetic figure with no allowance for wastage or uneven surfaces; with a 1:8 mix and a thin coat it will just cover, and the result will be a weak, dusty, hollow-sounding plaster that cracks in the first summer.
Risks to flag in writing: hollow and drummy patches from thin cover over the brick face; shrinkage cracking from a rich mix applied too thickly in one coat; dusting from an over-sanded mix; and no come-back because a lump sum with no material schedule leaves no measurable basis for a defect claim. Recommend converting to an item rate per square metre with a stated mix, thickness and curing period, measured jointly, with 10% retention - and issue cement against measured consumption rather than as a free issue.