The following topics are covered in this post.

  1. What is plastering?
  2. What are the requirements of a good plaster?
  3. Different types of mortar for plastering
  4. number of plaster coats
  5. How to prepare the background before plastering?
  6. What are the recommended mortars for different situations?
  7. What are the disadvantages of plastering?

Plastering is the process of covering the rough surfaces of walls, columns, roofs and other building components with a thin coat of mortar to create a smooth durable surface. The coating of mortar is called plaster.

Plastering is done to obtain the following items:

  • To protect external surfaces from the ingress of rainwater and other atmospheric agencies.
  • To provide a smooth surface in which dust and dirt cannot accumulate.
  • To give a decorative effect.
  • To protect surfaces from vermin.
  • To hide substandard material or defective workmanship.

Plaster material must meet the following requirements:

  • It should follow the background, and follow it during all changes in seasons and other atmospheric conditions.
  • It should be hard and durable.
  • It should have good working capacity.
  • It should be possible to apply it during all weather conditions.
  • It should be economical.
  • It should effectively check the penetration of moisture.

The selection of the type of plaster depends on the following factors:

  • Availability of binding material.
  • durability requirements.
  • finishing requirements.
  • Atmospheric conditions and weather changes.
  • The location of the surface (ie the exposed surface or inner surface).

cement mortar

Cement mortar is the best mortar for exterior plastering work as it is practically non-absorbent.

Lime plaster is also preferred in both rooms etc. and in damp climates. Cement mortar is stronger than lime mortar. Mixing Ratio (ie Cement.) , sand) may differ from 1:4 to 1:6, The sand used for plastering should be clean, rough and angular.

Cement plaster is applied either in two coats or in three coats, the first being more common.

Sometimes single coat plaster is applied for substandard work.

The background over which plastering is to be done depends on the type of wall construction, e.g. Random rubble (RR) masonry, Coarse rubble masonry, Brick masonry

background number of coats
stone work 3 or 2
brick work or hollow block 2 or 1
Concrete Cast In Situ 2 or 1

pay attention, If done in plastering single quote only, its thickness 12 mm. should not exceed Nor is it 6 mm. should be less than,

two coat plaster

The following procedure is followed:

  • The background is prepared by raking the joint to a depth of 20 mm, cleaning the surface and watering it thoroughly.
  • If the surface to be plastered is very uneven, before the first coat, a preliminary coat is applied to fill the hollow.
  • A first coat or rendering coat of plaster is applied, the thickness of which is 2 to 3 mm less than the specified thickness of plaster. 15cm x 15cm size to maintain uniform thickness of plaster. Two points are formed in a vertical line at a distance of about 2 m, and are dropped through a plumb. Several such vertical screeds are made at suitable intervals. Cement mortar is applied to the surface between successive screeds and the surface is properly finished.
  • Suitably worked to provide a mechanical key for the final or finishing coat, prior to hardening. The rendering coat is then sanded into the joints and onto the surface with a hard force mortar. The rendering coat is kept wet for at least 2 days, and then allowed to dry completely.
  • The thickness of the final or finishing coat can vary between 2 and 3 mm. Before applying the final coat, the rendering coat is moistened evenly. The final coat is applied to an even surface with wooden floats and finished with a steel trowel. as far as possible, Finishing coat should be applied starting from top to bottom And completed in one operation to eliminate the joining marks.

three coat plaster

The process for applying three-coat plaster is similar to two-coat plaster, except that an intermediate coat, known as swim coat is applied. The purpose of this coat of plaster is to bring the plaster to an even surface. The thickness of rendering coat, floating coat and finishing is kept 9 to 10 mm, 6 to 9 mm and 2 to 3 mm respectively. The rendering coat has been made rough.

The floating coat is applied after about 4 to 7 days Applying the first coat. The finishing coat can be applied approximately 6 hours after the floating coat is applied.

single coat plaster

It is used only in work of substandard quality. It is applied in the same way as two-coat plaster, except that the rendering coat, as applied for two-coat plaster, is finished soon after it has hardened sufficiently.

For plastering on new surfaces, all masonry joints should be raked to a depth of 10 mm in brick masonry and 15 mm in stone masonry to provide the key to plaster. All mortar droppings and dust, and lattices (in the case of freshly laid concrete) should be removed with the help of a stiff wire brush. Any inequalities are flattened before rendering is applied. To be applied in three coats, the local projection should be no more than 10 mm proud of the normal surface and the local depression should not exceed 20 mm. For two-coat plaster, these limits are 5 mm and 10 mm, respectively. The surface should be rinsed with clean water and kept evenly moist to generate optimum suction. In no case should the surface be kept so wet that it coats the mortar green, or so dry that it causes strong suction that draws moisture out of the mortar and makes it weak, porous and friable . If plaster is to be applied on the old surface, then all the dirt, oil, paint etc. should be cleaned. The loose and crumbling layer of plaster should be removed to its full thickness and the background surface should be exposed and the joints should be properly raked. The surface should be washed and kept moist to achieve optimum suction.

serial number Situation composition of mortar IS grading of lime
1 External plaster in areas where rainfall is less than 500 mm per year and where sub-soil water is not 2.5 m below the ground surface:
(a) below DPC 1 cement 6 sand ,
1 cement 2 lime 9 sand b or c
1 lime 2 sand a
1 lime 1 sand 1 drywall b or c
1 lemon 2 succulents b or c
(b) above DPC 1 cement 2 lime 9 sand b or c
1 lime 2 sand a
1 lime 1 dry sand b or c
1 lemon 2 succulents b or c
2 Exterior plaster in areas where rainfall exceeds 1300 mm per year and where groundwater is not 2.5 m below the ground surface:
(a) below DPC 1 cement 4 sand ,
1 cement 1 lime 6 sand b or c
1 lemon 2 succulents b or c
(b) above DPC 1 cement 2 lime 9 sand b or c
1 lime 2 sand a
1 lime 1 sand 1 drywall b or c
1 lemon 2 succulents b or c
3 External plaster in areas where underground water is within 2.5 meters of the ground
below dpc 1 cement 3 sand
4 Interior plaster in all areas 1 lime 2 sand a
1 lime 1 drywall 1 sand b or c
1 lemon 2 succulents b or c
1 cement 2 lime 9 sand b or c

pay attention, The proportion of lime varies with the % purity of lime and these ratios can be suitably adjusted depending on local practice.

The following defects may arise in plaster work:

  1. plaster surface blister

It is the formation of small patches of plaster swelling on the outside of the plaster surface, resulting from the late incorporation of line particles into the plaster.

  1. hoof

Cracking involves the formation of cracks or crevices in plaster work due to the following reasons:

  • Incomplete preparation of the background.
  • Structural defects in the building.
  • surface continuity.
  • Background movement due to its thermal expansion or rapid drying.
  • Movement in the plaster surface itself, due to either expansion (in the case of gypsum plaster) or shrinkage (in the case of lime sand plaster).
  • Excessive shrinkage due to thick coating.
  • faulty workmanship
  1. Madness

It is the formation of a series of hair cracks on the plastered surface, for the same reasons that cause the crack.

  1. flourish

It is white crystalline substance that appears on the surface due to the presence of salts in plaster making materials as well as in building materials like bricks, sand, cement etc. and even water. It gives a very bad look. This affects the adhesion of the paint with the surface of the wall.

Dry brushing and frequent washing of the surface can remove the inflorescences to some extent.

  1. flaking

This is the build-up of a very loose mass of the plastered surface, due to poor bonding between successive coats.

  1. peeling

This is the complete displacement of some part of the plastered surface, resulting in the formation of a patch. This is also the result of incomplete binding.

  1. popping

It is the formation of conical holes in the surface of the plaster due to the presence of certain particles, which expand upon setting.

  1. rust spots

These are sometimes formed when plaster is applied to metal laths.

  1. uneven surface

This is achieved purely because of poor workmanship.

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.