---
title: "Deduction rules for plastering openings"
subject: "Estimating and Costing"
chapter: 2
key: "semester_7/estimating_and_costing:deduction-plaster-openings"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/deduction-plaster-openings
site: BCT Engineering
---

# Deduction rules for plastering openings

Estimating and Costing · Chapter 2: Method of Estimating

- Asked in: 2076 Chaitra, 2 marks; 2076 Asoj, 2 marks; 2080 Bhadra, 4 marks (asked 3 times)
- Video: [Rules of deduction as per IS 1200](https://www.youtube.com/watch?v=rl8lTwj7bf0) — Nitin Bhopale
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/estimating_and_costing/deduction-plaster-openings
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-2

## Question

Explain the rules for deductions from plastering for openings in brick surfaces (as per IS 1200).

## 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.

## Answer

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Opening up to 0.5 m²: deduction?
   A. Deduct one face
   B. No deduction
   C. Deduct both faces
   D. Deduct half
   Answer: B. Small openings are ignored. (Key point: Small openings)

2. Opening between 0.5 m² and 3 m²: deduction?
   A. No deduction
   B. Deduct both faces
   C. Deduct on one face only
   D. Deduct 50%
   Answer: C. One face only, reveals not added. (Key point: Medium openings)

3. Opening above 3 m²: deduction?
   A. No deduction
   B. One face only
   C. Only reveals deducted
   D. Both faces, with reveals added separately
   Answer: D. Full deduction on each face. (Key point: Large openings)

4. Wall 4 m x 3 m, window 1.44 m², plaster on both faces. Net plaster?
   A. 22.56 m²
   B. 21.12 m²
   C. 24 m²
   D. 20.00 m²
   Answer: A. 2 x 12 - 1.44 = 22.56. (Key point: Example)

## Related in Estimating and Costing

- [Deduction rules: brickwork, plastering, painting](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Adeduction-brick-plaster-painting) — Estimating and Costing, chapter 2

## Related in other subjects

- [Wall with openings: stress, brick, mortar](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awall-with-openings-single-storey) — Concrete Technology and Masonry Structure, chapter 8