Plaster is a thin stratum of mortar that acts as damp-proof coat over brick masonry. It is also used to protect the external surface from atmospheric influences. Plastering also provides a suitable surface for painting and smooth surface which enhance the appearance of the building.
Countries and their standard specification used for plastering work
1. American Standards
- ASTM C926 Standard Specification for Application of Portland Cement-Based Plaster
2. New Zealand Standards
- NZS 4251-1 (2007): Solid plastering – Cement plasters for walls, ceilings and soffits
3. European Standards
- BS EN 998-1 Specification for mortar for masonry – Part 1: Rendering and plastering mortar.
4. Indian Standard (BIS codes)
- IS 383 Specification for coarse and fine aggregates for natural sources for concrete.
- IS 1542 Specifications for sand for plaster
- IS 2645 Specifications for integral cement waterproofing compound
- IS 8112 Specification for 43 grade OPC
- IS 269 Specification for 33 grade OPC
- IS 1489 Specification for Portland Pozzolana Cement
Procedure of Plastering Work
1. Preparing the surface for plastering
- The mortar joints of walls should be kept rough, in order to give good bond.
- Roughen the entire wall to be plastered.
- The entire joints and surfaces of walls should be clean with wire brush, and there should be no grease, oil or any lubricant substance on the surface.
- Make sure any cavity or holes does not exist on the surface before plastering. But if there is, fill it in advance with the right material.
- If the surface is smooth or the wall to be plastered is old one, then rake out the mortar joint to a depth of at least 12 mm to give a better bonding to the plaster.
- The wall to be plastered should be kept wet for at least 6 hours before plaster is applied.
- If the projection on the wall surface is more than 12 mm, then knock it off, so as to obtain a uniform surface of wall. This will reduce the consumption of plaster.
2. Groundwork for Plaster
- In order to get uniform thickness of plastering throughout the wall surface, first fix dots on the wall. A dot means patch of plaster of size 15 mm * 15 mm and having thickness of about 10 mm.
- Dots are fixed on the wall first horizontally and then vertically at a distance of about 2 meters covering the entire wall surface.
- Check the verticality of dots, one over the other, by means of plumb-bob.
- After fixing dots, the vertical strips of plaster, known as screeds , are formed in between the dots. These screeds serve as the gauges for maintaining even thickness of plastering being applied.
3. Applying Under Coat or Base Coat
- In case of brick masonry the thickness of first coat plaster is in general 12 mm and in case of concrete masonry this thickness varies from 9 to 15 mm.
- The ratio of cement and sand for first coat plaster varies from 1:3 to 1:6.
- Apply the first coat of plaster between the spaces formed by the screeds on the wall surface. This is done by means of trowel.
- Level the surface by means of flat wooden floats and wooden straight edges.
- After leveling, left the first coat to set but not to dry and then roughen it with a scratching tool to form a key to the second coat of plaster.
4. Applying Finishing Coat
- The thickness of second coat or finishing coat may vary between 2 to 3 mm.
- The ratio of cement and sand for second coat plaster varies from 1:4 to 1:6.
- Before applying the second coat, damp the first coat evenly.
- Apply the finishing coat with wooden floats to a true even surface and using a steel trowel, give it a finishing touch.
- As far as possible, the finishing coat should be applied starting from top towards bottom and completed in one operation to eliminate joining marks.
5. Curing of Plastering works
- After completion of the plastering work, it is kept wet by sprinkling water for at least 7 days in order to develop strength and hardness.
- Use of gunny bags or other materials is used to keep the plastering works wet in external works.
- Improper curing may lead to cracks formation or efflorescence in plaster work.
- Care be taken after Completion of Plaster Work
- Cleaning of doors or frame and floor area is necessary at the completion of work.
- Curing should be started as soon as the plaster has hardened sufficiently and must be cured for at least 7 days.
- Curing shall commence, 24 hours after the plaster is laid.