FUNCTIONS OF DIFFERENT COATS OF RENDERING [OUTSIDE PLASTERING]

Importance of Rendering or Outside Plastering

‘Rendering’ denotes generally external plaster and related finishes (plain or textured), applied for the purpose of protection and decoration.

Rendered finish can be designed for performance suited to the different situations of use like,

  • whether it is severe or moderate exposure to rain.
  • whether it is dusty, clear, or corrosive atmosphere.
  • whether it is subject to large variations in temperature or other climatic cycles.

The binder in the rendering mix can be varied by adjusting proportions of cement, lime, or other ingredients.

The types of aggregates can be chosen based on durability and finishing requirements.

If required necessary adjustments can be made in their sizes and grading.

Different textures and finishes can be adopted, such as scraped, roughcast, pebbledash, etc.

The techniques of application can be varied to obtain special effects, durability, and better performance.

Undercoats

Undercoats serve various purposes.

They provide a means of straightening or levelling an uneven surface.

They seal the surface of the wall and are often the most important part of the treatment in preventing rain penetration.

They provide a surface of even suction and satisfactory adhesion for the finishing coat, and they prevent ‘grinning’ of joints or of areas of differing suction through the finishing coat.

Also read: 7 Types of Defect in Plastering

Grinning
Grinning

An undercoat in any part shall have a thickness of not less than 8 mm, nor more than 12 mm.

If greater thickness is required to build up an even surface, additional undercoats must be used.

For colored cement renderings, the undercoat must be admixed with waterproofing compound to provide a waterproofing base.

Final or Finishing Coat

The thickness of the final or finishing coat will be governed to some extent by the texture required.

Normally 3 to 8 mm is considered satisfactory.

Some of the fine-textured machine applied finishes may be as thin as 3 mm.

Single Coat Rendering

In the case of dense background, such as of cement concrete, or where surface of the background is very even (as for example, walls built with hollow concrete blocks) or buildings subjected to moderate, or light exposure conditions, wood-float; rendering can be applied in one coat.

It must not be thicker than 18 mm or thinner than 6 mm in any portion.

Extreme care needs to be taken in preparing the surface of the background to ensure adequate bond between the rendering and the background.

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