IS 1661 : 1972Code of practice for the application of cement and cement-lime plaster finishes

IS 1661:1972 is the code of practice for application of cement and cement-lime plaster finishes. It is the workmanship standard behind the 'plastering' item in every building specification — internal smooth plaster, external rendered plaster and ceiling plaster on masonry and concrete backgrounds.

It is read with the materials and allied-finish codes:

• IS 1542 — sand for plaster · IS 269 — OPC (the binder)
• IS 712 — building lime · IS 1635 — field application of lime mortar
• IS 2402 — code of practice for external rendered finishes (the external-plaster companion)
• IS 456 — for keying/preparation of RCC surfaces before plaster

IS 1661 prescribes background preparation, mix proportions, coat thickness and curing:

• Mixes: cement–sand from about 1:4 to 1:6; cement-lime gauged mixes (e.g. 1:1:6 cement:lime:sand) are recommended where workability and crack resistance matter — the lime reduces shrinkage cracking and improves finish
• Thickness/coats: internal single coat ≈ 12 mm; external two-coat ≈ 15–20 mm (rough base + finish); ceiling ≈ 6 mm
• Background: RCC and dense surfaces must be hacked/keyed and dampened; masonry joints raked; surfaces clean and wetted (not soaked)
• Curing: keep plaster moist for about 7 days — uncured plaster is the commonest cause of map cracking and chalky, weak finishes

Provide grooves/bands at dissimilar-material junctions and reinforce (chicken mesh) over RCC–masonry interfaces and chases.

Spec: 20 mm external plaster on brick masonry, two coats.

Step 1 — background: rake masonry joints ~10 mm, remove dust, evenly dampen; fix chicken mesh over any RCC bands and service chases.

Step 2 — base (rough) coat: ~12 mm in CM 1:5, scratched/keyed for the next coat, allowed to firm and cured.

Step 3 — finish coat: ~8 mm in CM 1:4 (or 1:1:6 cement-lime for crack control), floated to the required finish; total ≈ 20 mm.

Step 4 — junction grooves: form a V/weather groove at the plinth, openings and RCC–masonry interfaces to localise shrinkage movement.

Step 5 — curing: moist-cure ~7 days. *Indicative materials per 10 m² of 20 mm work:* on the order of 1.0–1.3 bags of cement + ~0.20 m³ sand, refined by the chosen mix and wastage.

1. Rich mixes (1:3 or richer). More cement = more shrinkage = map cracking. Use the leanest mix that gives the required strength/finish, and gauge with lime to cut cracking.

2. No curing. Plaster cured 'by the afternoon sun' chalks, dusts and crazes — 7 days moist curing is not optional.

3. One thick coat instead of two. Thick single coats slump, debond and crack; build external plaster in base + finish coats.

4. Plastering un-keyed RCC / green or dirty masonry. Smooth dense concrete must be hacked or bonding-agent treated; plastering freshly-built or dusty masonry guarantees debonding.

5. No grooves or mesh at junctions. The RCC–masonry interface and chases crack unless a groove localises the movement and mesh bridges the dissimilar materials.

• IS 1542 — sand for plaster · IS 2116 — sand for masonry mortar
• IS 269 — OPC · IS 455 — Portland slag cement
• IS 712 — building limes · IS 1635 — field application of lime mortar
• IS 2402 — code of practice for external rendered finishes
• IS 4031 (parts) — physical tests for cement (binder quality)
• IS 456 — RCC surface preparation/keying before plaster

IS 1661 is old and reaffirmed; the principles (lean mix, two coats, keying, curing, junction detailing) are timeless and still the reference for site-batched cement plaster. The market has shifted towards ready-mix dry plaster and gypsum plaster for internal work because they crack less, need little or no curing and give a faster, flatter finish — but external work and most government/CPWD specifications are still site-batched cement plaster governed by IS 1661.

The single most under-used provision is lime gauging: a 1:1:6 cement-lime-sand mix is markedly more workable and crack-resistant than a straight 1:4/1:5, yet it is rarely specified. If you remember three things from IS 1661 — lean/gauged mix, mandatory 7-day curing, and junction grooves + mesh at RCC interfaces — you eliminate the large majority of plaster cracking complaints.