Technical Drawing, Estimation, Measurement and Costing
Drawing conventions
Learn plan, elevation and section, plus the isometric sketch for pipe routes. Line types: continuous thick for visible outlines, dashed for hidden work, chain thin for centre lines; conventional symbols for gate valve, float valve, bib cock, WC, wash basin, floor trap and manhole. Common scales are 1:50 and 1:100 - note the scale on every take-off.
Measurement units
- Pipe: running metre along the centre line, not the developed length over fittings.
- Fittings and fixtures: numbers (each).
- Excavation, concrete, sand bedding: cubic metre.
- Painting, waterproofing, formwork: square metre.
- Testing and commissioning: per system or lump sum.
Estimating
Rate = material + labour + overheads + profit. Add wastage: about 5% on pipe, 2–3% on fittings, 5% on sundries (tape, cement, solder, screws). Labour on small plumbing jobs commonly runs 25–35% of material value. Add contingency of 3–5%, then GST at the applicable rate (18% on most plumbing services).
Worked example. Estimate 20 m of 25 mm GI supply line with 10 elbows and 4 unions.
- Pipe: 20 m × ₹180 = ₹3,600; wastage 5% = ₹180 → ₹3,780
- Elbows: 10 × ₹45 = ₹450; unions: 4 × ₹90 = ₹360
- Material subtotal: ₹4,590
- Labour at 30%: ₹1,377
- Total before tax: ₹5,967; GST 18%: ₹1,074
- Quote: ₹7,041, rounded to ₹7,050. Reasoning: wastage applies to the pipe before labour is calculated, and tax is applied last on the whole taxable value.
Traps that cost marks
Measuring pipe along the wall instead of the centre line; leaving fittings out of the BOQ; mixing running metres with numbers; omitting testing and making good after chasing; quoting a lump sum with no measurement sheet behind it.