| Primary value | 20 mm (12 mm rough + 8 mm finish · 1:4 mortar) |
| Applies to | External brick / block walls exposed to rain and sun · Compound walls and parapets · Plinth-band protection plaster |
| Exceptions | Single-coat external (rough finish) → 15 mm @ 1:4 |
| Two-coat external (rough + finish) → 12 + 8 mm = 20 mm | |
| Sand-faced finish (architectural) → 20 + 6 mm = 26 mm | |
| Coastal / monsoon belt → Add waterproofing additive to mortar | |
| Measured as | Total finished thickness between wall face and external paint / texture coat. Measured at right angles to the wall. |
| Source | IS 2402 — Clause 6 ✓ Verified |
4 related items across IS codes, knowledge articles, design rules, maps and tools
External walls take rain, sun and thermal cycling — 20 mm in two coats provides the thermal mass and waterproofness that single 12 mm internal plaster cannot. The first 12 mm rough coat gives mechanical key + bulk; the 8 mm finish coat carries the architectural texture and the paint substrate.
Most Indian projects use 1:4 cement-sand mortar with 1 kg integral waterproofing compound per bag of cement for the external coat. Coastal projects (Mumbai, Chennai) often add a third 5 mm sand-faced finish coat. Bare 12 mm external plaster is a common shortcut that fails within 2 monsoons.