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Deduction rules for plastering openings
Short answer
Per IS 1200, for openings up to 0.5 m² there is no deduction and no addition for reveals. For openings 0.5–3 m², deduct the area from one face only and do not add reveals. For openings over 3 m², deduct from both faces and measure and add the reveals separately for jambs, sills, and soffits.
Explain the rules for deductions from plastering for openings in brick surfaces (as per IS 1200).
Plastering is measured over the wall surface in m². For openings in brick surfaces, IS 1200 (Part 12) gives the following rules.
| Size of opening | Deduction | Reveals (jambs, sills, soffits) |
|---|---|---|
| Up to 0.5 m² | No deduction | No addition |
| More than 0.5 m² and up to 3 m² | Deduct on one face only (the area of the opening is deducted from one side) | Not added |
| More than 3 m² | Deduct both faces (full opening on each plastered face) | Measured and added separately |
Other points:
- Where the plaster is on one face only, the opening is deducted from that face as per the same limits.
- For ends of walls, jambs, sills and soffits of larger openings, the plastering of the reveal is measured by width x girth and added.
- No deduction is made for the ends of beams, joists, or rafters, and for small holes.
- Many Nepali offices, for simplicity, deduct the full area of every opening on the plastered face and then add the reveals.
Example: a wall 4 m x 3 m has a window 1.2 m x 1.2 m (1.44 m²) and plaster on both faces. The deduction is 1.44 m² once (one face only), so the net plaster = 2 x 12 - 1.44 = 22.56 m².
✓ Answer checked by Claude Opus 5.5 (high effort)
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