IS 2402 : 1963Code of Practice for External Rendered Finishes

IS 2402:1963 is the code of practice for external rendered finishes — the workmanship code for *external* plaster/render on masonry and concrete: the rain-screen finish that protects the wall and the building's weather performance depends on. It is the external-render companion to IS 1661 (cement & cement-lime plaster) and the rendering provisions of masonry practice.

It is read with the finishes stack:

• IS 1661 — application of cement & cement-lime plaster (the general plastering code)
• IS 1542 — sand for plaster · IS 269 — OPC · IS 712 — building lime
• IS 456 — RCC background preparation · NBC 2016 Part 6 — finishes / weatherproofing

External render does two jobs — finish and rain protection — so IS 2402 emphasises crack control and weather resistance, not just appearance:

• Coats & thickness: typically a two- (or three-) coat system — keyed base/scratch coat + finishing coat, ~15–20 mm total — built up, not one thick coat (which slumps and crazes)
• Lean / lime-gauged mixes: the leanest mix that performs (cement-lime gauging, e.g. 1:1:6, markedly reduces shrinkage cracking and improves rain resistance) — rich mixes craze and let water in
• Background preparation: rake masonry joints, hack/key dense concrete, dampen (not soak); reinforce (mesh) over RCC–masonry junctions and chases — the crack initiators
• Crack/movement control: weather grooves/drips at plinth, openings and bands; movement-joint continuity through the render
• Curing: mandatory moist curing (~7 days) — uncured external render crazes and becomes permeable
• Finish & weathering: texture/float finish that sheds water; over-rich smooth float traps and crazes

The key point: external render fails as a weather screen (cracking → water ingress → damp, efflorescence, render debonding) far more than as a 'finish' — the code is about keeping water out.

Scenario: external render on brick masonry, exposed elevation.

Step 1 — background: rake brick joints ~10 mm, remove dust, evenly dampen; fix mesh over RCC bands, beams and service chases (the dissimilar-material crack lines).

Step 2 — base coat: ~12 mm in a lean / lime-gauged mix (e.g. CM 1:5 or 1:1:6 cement:lime:sand), scratched/keyed, allowed to firm and cured.

Step 3 — finishing coat: ~6–8 mm, leaner-or-equal, floated to a water-shedding texture; total ≈ 18–20 mm.

Step 4 — weather details: form drip grooves / throating at plinth, sills, lintels and bands so runoff is thrown clear, not run back into the wall; continue movement joints through the render.

Step 5 — curing: moist-cure ~7 days. The recurring failure — map cracking, damp patches, blown render — is almost always a rich mix, no curing, single thick coat, or unmeshed junctions, all of which IS 2402 explicitly guards against.

1. Rich smooth mix for a 'good finish'. Cement-rich render crazes; the cracks admit water → damp, efflorescence, debonding. Lean / lime-gauged is the correct external practice.

2. No curing. Uncured external render crazes and is permeable from day one — 7-day moist curing is non-negotiable for a weather screen.

3. Single thick coat. Slumps, debonds and cracks; external render must be built up in coats with a keyed base.

4. No mesh / grooves at junctions & openings. RCC–masonry interfaces, bands and chases crack and leak unless meshed; missing drip grooves run water back into the wall.

5. Plastering green/dirty masonry or un-keyed concrete. Guarantees debonding regardless of mix quality.

• IS 1661 — application of cement & cement-lime plaster finishes
• IS 1542 — sand for plaster · IS 269 — OPC · IS 712 — building lime · IS 1635 — lime mortar
• IS 456 — RCC surface preparation/keying
• NBC 2016 Part 6 — building finishes / weatherproofing · IS 2645 — integral waterproofing (mix additive)
• [IS 2402] companion external-finish references (textured/decorative renders)

IS 2402 is old (1963) and reaffirmed; the principles are timeless because external render is fundamentally a rain screen, and almost every façade damp/efflorescence/blown-render complaint traces to the same handful of violations: a rich mix that crazes, no curing, a single thick coat, unmeshed RCC–masonry junctions, or missing drip details. Ready-mix and polymer-modified renders are increasingly used (lower cracking, better rain resistance) but the workmanship discipline IS 2402 codifies still governs site-batched external plaster.

The practitioner contract: lean / lime-gauged mix, build up in coats with a keyed base, mesh the junctions, form drip/weather grooves, and cure for 7 days — and treat external render as part of the building's *weatherproofing*, coordinated with NBC Part 6, not as a cosmetic afterthought. The single most under-used provision is lime gauging; a 1:1:6 render is far more crack-resistant and weather-tight than a straight cement-rich mix, yet is rarely specified.