What is the recommended thickness for the first coat (undercoat) of external plaster?

The undercoat should generally be between 10 mm and 15 mm thick.

Can external rendering be applied directly on old painted surfaces?

No, all traces of paint, dirt, oil, and efflorescence must be thoroughly removed, and the surface properly roughened before applying new render.

What is the typical mix proportion for rendering on normal masonry exposed to severe conditions?

A typical recommended mix is 1:1:6 (Cement:Lime:Sand) or 1:3 to 1:4 (Cement:Sand) depending on the background and severity of exposure.

IS 2402

: 1963

Code of Practice for External Rendered Finishes

CurrentFrequently UsedCode of PracticeArchitectural · Flooring, Wall Finishing and Roofing

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This standard lays down the specifications and guidelines for external rendered finishes (plastering) on building facades using cement, lime, and sand. It details background preparation, mortar mix proportioning, and systematic application to ensure long-term durability, weather resistance, and prevention of cracks.

Code of Practice for External Rendered Finishes

Quick Reference Values

Thickness of undercoat10 to 15 mm

Thickness of finishing coat3 to 8 mm

Curing duration per coatMinimum 7 days

Time interval between coatsMinimum 2 days

Practical Notes

Never apply a stronger (richer) plaster mix over a weaker background or a weaker undercoat, as this invariably leads to shrinkage cracks and delamination.

Roughening of dense concrete surfaces (e.g., by hacking) is absolutely crucial before applying the render to ensure proper mechanical keying.

Incorporating hydrated lime into cement plaster significantly improves workability, increases water retention, and reduces the likelihood of shrinkage cracking.