Two Estimates for the Same Room
You must price the brickwork for a single-room store, 6.0 m × 4.0 m measured centre-to-centre of the walls, wall thickness 230 mm, height 3.0 m from floor to roof slab. There is one door of 1.0 m × 2.1 m and two windows of 1.2 m × 1.2 m each. Bricks are traditional modular size and the site norm is 500 bricks per cubic metre of brickwork. Mortar is cement-sand 1:6, and the office assumption is 0.30 m³ of wet mortar per m³ of brickwork, with a dry-volume factor of 1.33; one bag of cement is 50 kg and 1 m³ of cement is taken as 1440 kg. A wastage allowance of 5% is applied to bricks, and cement comes in whole bags. The contractor's quantity surveyor submits 14.8 m³ of brickwork and asks for payment for 'extra corner work'. Your own take-off, done by the centre-line method, must be checked and defended before the bill is passed.
1. Compute the net brickwork volume, the number of bricks to be ordered, and the cement and sand required. Show each step.
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Step 1 - gross brickwork by centre-line method. Centre-line length = 2 × (6.0 + 4.0) = 20.0 m. Volume = 20.0 × 0.23 × 3.0 = 13.80 m³. The centre-line method is used because it counts each corner junction once; the long-wall/short-wall method gives the same total when done properly, so it is a check, not an extra.
Step 2 - deductions for openings. Door: 1.0 × 2.1 = 2.10 m². Windows: 2 × (1.2 × 1.2) = 2.88 m². Total = 4.98 m²; volume = 4.98 × 0.23 = 1.145 m³. (Under standard measurement rules, openings above 0.1 m² are deducted, so both door and windows qualify.)
Step 3 - net brickwork. 13.80 − 1.15 = 12.65 m³.
Step 4 - bricks. 12.65 × 500 = 6,325 bricks; plus 5% wastage = 6,641. Say 6,650 bricks.
Step 5 - mortar. Wet mortar = 12.65 × 0.30 = 3.795 m³. Dry mortar = 3.795 × 1.33 = 5.048 m³. Cement = 5.048 × (1/7) = 0.721 m³ = 0.721 × 1440 = 1,039 kg ≈ 21 bags. Sand = 5.048 × (6/7) = 4.33 m³.
Step 6 - present it in a measurement sheet: description, unit, quantity, rate, amount, with deductions shown separately, so that a checker can follow each line. State your assumptions (centre-line basis, 0.30 m³ mortar, 5% wastage) on the sheet, because those assumptions are what the bill will be argued over.
2. The contractor claims 14.8 m³ and 'extra corner work'. How do you answer that, and what is the principle behind the centre-line method?
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The principle. In the centre-line method, every wall length is measured between the centre lines of the cross walls, so the volume of a corner junction belongs to neither wall twice: it is counted once, inside the length of one of them. The method is a bookkeeping device that avoids double counting. The long-wall/short-wall method reaches the same total by adding corner lengths to the long walls and deducting them from the short walls. Whichever you use, the physical volume of brickwork in the room does not change. There is no such thing as 'extra corner work' as a separate bill item; corner work is included in the rate per cubic metre.
Answering the claim.
- Show the arithmetic: 20.0 m centre-line × 0.23 × 3.0 = 13.80 m³ gross; less 1.15 m³ of openings = 12.65 m³. The 14.8 m³ figure is about 17% higher and cannot be explained by corners.
- Ask the surveyor to produce the measurement sheet with lengths and deductions. Most disputes die at this point, because the claim is usually based on adding wall lengths without deducting the overlaps at the four corners - the very double counting the centre-line method removes.
- Check what else might legitimately be in the 14.8 m³: lintels, sills, plinth, parapet, or a wall the drawing shows but your take-off missed. If the drawing shows them, they should be added as separate items, not as 'corner work'.
- If there is a genuine difference after the review, measure jointly on site against the drawing, and record the agreed quantity in writing before payment.
The professional habit: pay for measured work, not for claims; every extra should be traceable to a drawing dimension and a measurement rule.
3. Your estimate and the contractor's are now close, but the site has used 25 bags of cement instead of 21. What could explain the gap, and what would you do about the 5% wastage and the way the material is handled?
Read the worked answer
Reconcile before accusing. Twenty-five bags against 21 is about 19% more, which is too much to be rounding. Possible causes:
- Different mortar assumption. If the site actually mixes 1:5 instead of 1:6, cement rises by about 17% for the same mortar volume: 5.048 × 1/6 = 0.841 m³ = 1,211 kg = 25 bags. That single change explains almost the whole gap, so check the mortar gauge on the platform first.
- Thicker joints. Joints above the specified 10 mm swallow more mortar; a 12-13 mm joint can add 15-20% mortar.
- Over-ordering and storage loss. Bags stacked on a damp floor, against a wall, or 12 high set solid, go lumpy; the lumpy bags are then thrown away or used as hard-core. That loss is real and should be controlled by a proper cement shed, a timber pallet, 8-10 bags maximum, and use of the oldest stock first.
- Wastage on site. Broken bricks, spilled mortar, and mortar left in the mixer at the end of the day. The 5% brick wastage is a pricing allowance; it is not a licence to waste, and the actual wastage should be measured.
- Unmeasured work. Small items such as a plinth course, a parapet or a repair may genuinely have used cement and should be measured and added.
What to do: agree a measured reconciliation, keep a cement register (date, bags received, bags issued, bags returned, work done), insist on gauge boxes and a measured mortar batch, and record the wastage percentage actually achieved on site. Then update the rate analysis for the next bill, because an estimate based on 0.30 m³ and 1:6 that is quietly executed in 1:5 will always look like a theft when it is really a specification slip.