Plastering is an essential finishing process in masonry construction that involves applying a thin layer of mortar over the surface of brick or concrete walls. This layer not only acts as a damp-proof coating but also provides a smooth and firm surface that enhances the overall appearance of the building. Beyond aesthetics, plastering plays a crucial role in protecting masonry surfaces from weathering, covering defects, and preparing the wall for painting. This article explores the standards, procedures, and care involved in plastering work.
Understanding the Purpose of Plastering
Plaster serves multiple important functions in masonry construction. Primarily, it acts as a protective barrier against moisture and atmospheric elements, thereby increasing the durability of the structure. It conceals any uneven or defective workmanship, fills porous surfaces, and creates a uniform base that is ideal for paint adhesion. The finished surface improves both the functionality and visual appeal of the building.
Standard Specifications for Plastering
Plastering work is governed by various standards worldwide to ensure quality and consistency:
- American Standards: ASTM C926 specifies requirements for the application of Portland cement-based plaster.
- New Zealand Standards: NZS 4251-1:2007 covers solid plastering, including cement plasters for walls, ceilings, and soffits.
- European Standards: BS EN 998-1 defines specifications for rendering and plastering mortar.
- Indian Standards: Several BIS codes apply to plastering materials, such as IS 383 for aggregates, IS 1542 for sand, IS 2645 for waterproofing compounds, and standards for different grades of Ordinary Portland Cement (IS 8112, IS 269, IS 1489).
Procedure of Plastering Work
1. Preparation of Surface
A proper surface preparation is crucial for a durable plaster finish. The masonry surface should be roughened to promote good bonding. All mortar joints must be left coarse, and the wall cleaned thoroughly to remove dust, grease, or any loose material. Any cavities or holes should be pre-filled. In the case of old or smooth surfaces, the mortar joints are raked out to a minimum depth of 12 mm to improve adhesion. The surface is then wetted for at least six hours before plaster application to prevent premature drying.
2. Groundwork for Plaster
To ensure an even plaster thickness, small patches called “dots” are first applied at regular intervals (about 2 meters apart) both horizontally and vertically. These dots serve as guides and their vertical alignment is checked using a plumb bob. Between these dots, vertical strips of plaster called “screeds” are created, which act as gauges to maintain consistent thickness throughout the plastering process.
3. Applying the Under Coat (Base Coat)
The base coat generally has a thickness of about 12 mm for brick masonry, and between 9 to 15 mm for concrete masonry. The mix ratio of cement to sand varies from 1:3 to 1:6. This coat is applied within the screeded areas using a trowel, then leveled with wooden floats and straight edges. Once leveled, the surface is left to set partially—not completely dry—and then roughened with a scratching tool to provide a good mechanical key for the finishing coat.
4. Applying the Finishing Coat
The finishing coat is thinner, typically 2 to 3 mm thick, with a cement-to-sand ratio ranging from 1:4 to 1:6. Before applying this coat, the base coat is evenly dampened. The finishing plaster is then applied smoothly using wooden floats and steel trowels, aiming for a uniform, flawless surface. It is advisable to apply the finishing coat from top to bottom in one continuous operation to avoid visible joints.
5. Curing of Plaster
Proper curing is vital to ensure the strength and durability of the plaster. After application, the plastered surface should be kept moist by sprinkling water regularly for at least seven days. In external applications, materials like gunny bags are often used to retain moisture. Insufficient curing can lead to cracks and unsightly efflorescence, compromising both strength and appearance.
Post-Plastering Care
Once plastering is completed, thorough cleaning of door frames, window frames, and floor areas is necessary to remove any plaster splashes or debris. Curing should start as soon as the plaster has hardened enough—usually 24 hours after application—and continue for at least seven days to allow full hydration and strength development.
Conclusion
Plastering is a critical finishing operation that not only beautifies a masonry structure but also protects it from environmental damage. Adhering to proper standards, following the correct procedures, and ensuring adequate curing can significantly extend the life and appearance of plastered surfaces. Whether in brick or concrete masonry, careful attention to plastering details ensures a durable, attractive finish that enhances the overall quality of construction.