What is Plastering
Plastering covers the rough inner and outer faces of walls, beams, columns and ceilings with a protective layer. This layer is of cement mortar or lime mortar.
The mortar used for plastering is called Plaster.
Purposes of plastering
Smooth Appearance
Plastering provides even, smooth, regular and clean surfaces to walls, columns, beams, etc., to improve appearance.
Durability
Plastering improves the durability of the exposed surfaces of walls, etc.
Concealing Defects
Plastering conceals unevenness and other defects in workmanship. It covers the use of inferior and porous materials used in masonry walls, concrete beams, etc. Also, it conceals the imperfections in the joints of the masonry work.
Effects of Atmospheric Agencies
Plastering prevents damping of the walls, etc., due to atmospheric agencies like rain, sun, wind, etc
Base
Plastered surface forms a satisfactory base for whitewashing, colour washing, distempering or painting.
Requirements or Qualities of a good plaster
Workability
A good plaster should possess good workability properties.
Adherence
A good plaster should adhere to the surface firmly.
Appearance
It should provide the surface with a decorative appearance.
Durability
A good plaster should be durable.
Atmospheric Agencies
A good plaster should resist the effects of atmospheric agencies.
Smooth and Even surface
It should provide a smooth, even and washable surface.
Shrinkage and Cracking
A good plaster should not shrink while drying. Shrinkage may cause cracking of the surface.
Insulation
A good plaster should have good insulation properties against sound and heat.
Types of Mortars used for Plastering
Cement Mortar
Cement Mortar is a mixture of Portland cement and coarse sand. The usual mix for cement mortar for plastering varies from 1:3 for the surfaces in contact with water (bathroom, etc.) to 1:4 to 1:6 for other surfaces.
Cement and sand are thoroughly mixed in a dry state. The mixture is added with the required amount of water to make a plastic mass or paste of desired consistency and workability
Precaution
The wet mortar should be used within 30 minutes because the cement starts setting after about 30 minutes.
Lime Mortar
Lime Mortar used for plastering is prepared as follows: Equal volumes of lime and fine sand are thoroughly mixed. The mixture is ground in a mortar mill by adding the required quantity of water to form a paste of the required consistency and workability.
Water-Proof Mortar
It is prepared by mixing I part of cement with 2 parts of sand and pulverized alum. Soap water is added to this dry mixture to make it water-proof and to obtain the required consistency and workability. This mortar is used for water-retaining structures.
Mortars used for different plastering works
For interior and exterior plastering, we may use the following proportions:
- Cement Mortar ( Cement-Sand plaster ) = 1: 4
- Lime Mortar ( Lime Plaster ) = 1: 1
- Lime Surkhi Plaster = 1: 2
Different Stages in plastering
Number of plaster coats
Plastering may be done in one or two or three coats. One coat of plaster is used in Cheap and Unimportant Buildings. Two coats may be applied for Ordinary Buildings. Three coats are applied for Superior Buildings. Partially-set mortar should not be used for plastering.
Plastering is always started from the top of the surface to be plastered and worked downwards. The following procedure is adopted for plastering:
Mortar Joints
Mortar Joints of the wall to be plastered are racked out to a depth of 20 mm
Background Surface
The background surface is made even and uniform by removing the projections. It is cleaned by brushing and is roughened. The surface is then wetted.
Preparation of Plaster
The plaster is prepared in a suitable proportion depending on the requirement. The ceiling is plastered first and then the walls.
Preparation of Screed
To obtain a uniform thickness of plastering, a Dot of 15 cm x 15 cm square and 10 mm thick is marked using a small quantity of mortar on the wall surface. Another dot is fixed 2 m vertically below.
These two dots are checked by plumb bob. These are joined with a vertical strip of mortar, called a Screed.
Screeds are placed at a horizontal distance of 2 m. Plaster is applied between them to obtain uniform thickness.
Plastering of the first coat
The mortar placed above between successive screeds is called First Coat. The surface is levelled using a trowel and finished. Curing is done for 7 days.
Plastering of the Second coat
For Two Coat Plastering, the surface of the first coat should not be finished smooth but left rough. The second coat is applied after 2 days. Curing is done for 7 days.
Plastering of the Third coat
Depending on the appearance, the Third Coat is as follows: Rough Coat Finish: The finishing mortar of 1: 3 mixed for a rough coat finish is prepared with coarse-grained sand. It is dashed against the surface to be plastered. The surface is roughly finished by light movements of a wooden float.
Smooth Coat Finish: Smooth Coat Finish is just similar to a rough coat finish. The only difference is fine-grained sand is used instead of coarse-grained sand.
Plastering of Decorating Piece
It should be finished before the finishing coat is applied.
Rounding of Corners
Rounding of corners and edges should be carried out along with the finishing coat in order to prevent any joint marks from appearing after some time.
Plastering Defects and Precautions
| Criteria | Defects | Precautions / Remedies |
|---|---|---|
| Crack Formation | Hairline crack formation on the plastered surface | Superior quality bricks should be used. Proper curing of the plastered surfaces should be done. |
| Peeling/ Falling of Plaster | Patch formation on the surface of plaster and falling of plaster | Proper bonding between coats of plaster should be provided. |
| Plastered Surface | Uneven surface due to poor workmanship | Workmanship should be good in brickwork and plastering. |
| Blistering | Small patches swell out beyond the plane of the plastered surface. | Proper trowelling should be done. |
| Effloresce | salts are present in bricks, cement, sand, etc. These salts appear as a white crystalline substance on the wall surface. It spoils the adhesion of paint or distemper on the wall surface. | During the manufacture of cement, bricks, etc., care should be taken about the quality of raw materials. Efflorescence is removed by rubbing brushes on the wall surface. |
| Excessive Dampness | Excessive dampness on the surface makes it soft | D.P.C. should be provided on the floor, roof, etc |