Plastering is a vital process in construction, transforming rough, coarse wall and ceiling surfaces into smooth, appealing finishes. It involves spreading wet materials like cement, lime, or gypsum onto walls or ceilings and then leveling them for a seamless, hard finish. This smooth surface can then be painted or left exposed for aesthetic appeal. Let’s dive deeper into the materials, techniques, and types of plastering finishes commonly used in modern construction.
I. Understanding the Plastering Process
Plastering is not just about making surfaces look smooth; it’s about strengthening and protecting walls and ceilings. Typically, wet materials like lime, cement, or gypsum are spread over surfaces, and tools are used to achieve a flat, smooth finish. While the plaster layer should ideally be thin, sometimes thicker layers are needed to cover irregular surfaces. For larger irregularities, a two-layer process is often used, where a coarse undercoat is applied first, followed by a finer finish coat for a smooth appearance.
II. Benefits of a Two-Layer Plastering System
Applying two layers of plaster is beneficial for walls with substantial irregularities. Using a coarse undercoat helps level the surface, while a second fine coat gives a refined, smooth appearance. This method is especially useful for surfaces with variations over 3 cm. Thinner layers are less likely to sag under their weight, making a two-coat approach more stable and effective.
III. Types of Plaster Based on Materials
Different types of plaster are chosen based on the desired properties, cost, and application purpose. Here’s a look at some of the most commonly used plaster materials:
1. Lime Plaster
Lime plaster, a mixture of lime and sand, is often used in restoration work for its historic authenticity and flexibility. To minimize cracking, builders sometimes add animal hair to the mix. Lime plaster provides a durable, yet flexible finish, which is less prone to cracking than some other materials.
2. Cement Plaster
Cement plaster is made by mixing Portland cement with sand and sometimes adding a plasticizer for ease of application. This mixture is ideal for strong backgrounds, such as brick walls. It creates a hard, durable surface, though it does require skilled labor for proper application. Adding plasticizers or lime enhances its workability and adhesion to surfaces.
3. Gypsum Plaster
Gypsum plaster, made from natural or synthetic gypsum, is widely used for its versatility and low shrinkage. It comes in various types, such as anhydrous and hemihydrates, with each type heated to different temperatures to achieve unique properties. Gypsum plaster can be used as both an undercoat and a finish coat and is particularly valued for its resistance to cracking.
IV. Preparing Background Surfaces for Plastering
The effectiveness of plaster depends heavily on the preparation of the surface it’s applied to. For optimal adhesion, the surface should be rough, solid, and capable of absorbing moisture. Machine-pressed bricks and dense concrete blocks provide good “suction,” which helps the plaster adhere. Lightweight blocks, however, can absorb too much water, reducing plaster adhesion. Solutions like PVA bonding agents and polymer bonding agents help improve adhesion on surfaces with low suction.
Reinforcement Techniques: Galvanized steel beads and stops are often used to reinforce plaster at angles and junctions, such as wall-to-ceiling intersections. These materials add stability to the plaster, ensuring it remains smooth and intact.
V. Traditional and Modern Plastering for Timber Structures
Traditional plaster on timber structures involves applying a lath-and-plaster method, where plaster is spread over wooden slats (laths). However, this has largely been replaced by gypsum plasterboard, which is easier to install and provides better fire resistance. Gypsum boards are available in various thicknesses, allowing builders to choose based on the specific needs of a ceiling or wall.
VI. Decorative Elements: Skirting and Architraves
Skirting boards, placed around the wall base where it meets the floor, add a decorative touch and protect the walls from impact. They come in various materials, including timber, metal, and tile. Architraves, another architectural detail, are moldings placed around doors and windows, adding a classic decorative element that enhances the aesthetic value of a room.
VII. Types of Plaster Finishes for Enhanced Aesthetics
Plaster finishes come in various textures, allowing builders and designers to create unique appearances for walls and ceilings. Here are a few popular finishes:
- Smooth Cast Finish: This basic finish uses fine sand and cement in a 1:3 ratio, applied with a skimming float to create a smooth, level surface.
- Rough Cast Finish: Also called “spatter dash,” this finish involves dashing a mixture of cement, sand, and coarse aggregate onto the surface. It is commonly used for exterior walls.
- Sand Faced Finish: Achieved by applying a two-layer plaster coating, with a fine sand finish on top for texture and uniformity.
- Pebble Dash Finish: A mortar layer is applied, and small pebbles are pressed into the surface for a unique, textured effect, ideal for exterior walls.
- Scrapped Finish: A final layer of plaster is allowed to dry partially, then scraped to reveal a rougher surface that is resistant to cracking.
- Depeter Finish: Similar to pebble dash but uses gravel or flint for a textured effect.
- Textured Finish: Using stucco techniques, various textures and shapes are created on the final plaster layer for artistic effect.
VIII. External Rendering for Weather Resistance and Aesthetics
Rendering the external walls of buildings not only improves their appearance but also enhances durability. External render is typically a mix of cement or lime and natural aggregates, applied in two or three coats for a strong bond. This finish adds resistance to rainfall and other weather elements, helping protect the building’s structure.
Conclusion
Plastering is an essential element of construction, providing both protective and aesthetic benefits to buildings. By choosing the right plaster material and finish, builders can enhance the durability, functionality, and visual appeal of a structure. Whether restoring an old building or finishing a new one, understanding the materials, techniques, and finishes of plastering is key to achieving the desired results.