Estimating, Costing and Quantity Surveying
Estimating and Costing converts a drawing into money. The paper tests types of estimate, IS 1200 measurement rules, the centre-line and long-wall short-wall methods, bar bending schedules, and rate analysis with overheads and contingency.
Types of estimate
- Approximate, by plinth area, cubical content or unit rate, for preliminary work.
- Detailed estimate: quantities from drawings multiplied by rates, plus 3-5 percent contingency and 1-2 percent for work-charged establishment.
- Revised and supplementary estimates when scope or cost changes after approval.
Units of measurement, IS 1200
Earthwork and concrete in m3; plaster, painting, roofing and damp-proofing in m2; cornice and moulding in m; steel reinforcement in kg or quintal; doors and windows by each number or in m2; pipes in m.
Worked example: long wall and short wall
A room 4.0 x 3.0 m inside, walls 230 mm thick, height 3 m, foundation offsets ignored. Long wall centre length = 2 x (4.0 + 0.23) = 8.46 m. Short wall centre length = 2 x 3.0 = 6.00 m. Total centre length = 14.46 m. Volume of masonry = 14.46 x 0.23 x 3 = 9.98 m3.
Bar bending
A standard hook is 9d. A 90 degree bend deducts 2d, a 45 degree bend 1d and a 135 degree bend 3d per bend. Cutting length = clear length + hooks - bend deductions.
Traps
- Deducting a door opening from plaster on both faces but adding back only one jamb.
- Using the external dimension as the centre-line length.
- Omitting wastage of 3-5 percent on steel and the contractor profit in rate analysis.
- Mixing the unit each with square metres for doors and windows.