
A scratch coat is the first layer of plaster applied to a wall. It is typically made from a mix of sand, cement, and sometimes lime, with a consistency that is spreadable but not too liquid. The precise ratios can vary based on the specific application and local building standards. The scratch coat is applied to a thickness of about 3/8 inch or 5mm, which is thick enough to even out irregularities but thin enough to prevent cracking. While the coat is still wet, it is scratched using a trowel, serrated blade, or special comb to create a web of surface scratches that help the second coat of plaster to adhere properly.
Characteristics | Values |
---|---|
Ingredients | Sand, cement, sometimes lime, and other additives |
Sand-to-cement ratio | 3:1 or 4:1 |
Thickness | 3/8" or 5mm |
Surface | Rough with horizontal lines/scratches/grooves |
Application | Trowel, serrated blade, or sprayer |
Setting time | 2-5 days |
Bonding | Mechanical and chemical |
Purpose | Enhanced adhesion, surface preparation, shrinkage control, moisture regulation, structural strength |
What You'll Learn
- A scratch coat is the first layer of plaster on a wall
- It is made of sand, cement, and sometimes lime
- The coat is scratched with a trowel or serrated blade to create a web of scratches
- The rough surface with indentations helps the second coat of plaster to adhere
- The scratch coat is typically applied to a thickness of about 3/8 inch
A scratch coat is the first layer of plaster on a wall
The scratch coat can be applied to a variety of substrates, including masonry blocks, brick, concrete, and certain types of sheathing. Its adaptability is one of its strengths. When working with smoother surfaces, additional bonding agents may be required to ensure proper adhesion. The setting time for a scratch coat can vary based on environmental conditions and the specific mix used. Typically, the coat will start setting within a few hours, but it is crucial to wait for at least 24-48 hours before applying the next layer. This ensures the scratch coat is fully cured and can provide a strong base for additional coats.
The scratch coat is usually made from a mix of sand, cement, and sometimes lime. The precise ratios can vary based on the specific application and local building standards. The mixture should be of a spreadable consistency that is not too liquid, ensuring it stays in place once applied.
The purpose of the scratch coat is to enhance adhesion, prepare and level the surface, control shrinkage and crack prevention, regulate moisture, and provide structural strength. The grooves or scratches on the surface create an interlocking surface that ensures subsequent layers interlock, resulting in a stronger and more secure attachment. The scratch coat also helps to fill in any cracks, holes, or other surface defects, ensuring a consistent thickness and a smoother final appearance.
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It is made of sand, cement, and sometimes lime
A scratch coat is the first layer of plaster applied to a wall. It is made of sand, cement, and sometimes lime. The precise ratios of these components can vary based on the specific application and local building standards, but a common recipe is 3 parts sand to 1 part cement. Lime and other additives can also be included but are not always necessary. The mixture should be spreadable yet not too fluid, ensuring it remains in place once applied.
The scratch coat is typically applied to a thickness of about 3/8 inch. This thickness is crucial as it is thick enough to even out irregularities in the substrate and provide a sturdy base for subsequent coats, but thin enough to allow for proper curing and prevent cracking. The surface of the scratch coat is not smooth like a finishing coat. Instead, as its name implies, it has horizontal scratches or grooves that are made using a trowel or a serrated blade. These grooves are essential as they provide a mechanical bond, creating an interlocking surface for the next layer of material to grip onto.
The scratch coat can be applied to a variety of substrates, including masonry blocks, brick, concrete, and certain types of sheathing. Its adaptability is one of its strengths. However, when working with smoother surfaces, additional bonding agents may be required to ensure proper adhesion.
The setting time for a scratch coat can vary depending on environmental conditions and the specific mix used. Typically, the coat will start setting within a few hours, but it is crucial to wait for at least 24-48 hours before applying the next layer. This ensures that the scratch coat is fully cured and can provide a strong base for additional coats.
The scratch coat serves several purposes in construction. Firstly, it enhances adhesion by creating a mechanical bond between layers. Secondly, it helps in surface preparation and levelling by filling in cracks, holes, and other surface defects. Thirdly, it contributes to shrinkage control and crack prevention by reducing the potential for top layers to dry out unevenly. Additionally, it plays a role in moisture regulation, acting as a semi-barrier to slow down the movement of moisture and protect the substrate.
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The coat is scratched with a trowel or serrated blade to create a web of scratches
The scratch coat is the first layer of plaster applied to a wall. It is essential in masonry work, plastering, and tiling, as it determines the quality and longevity of the final application. The coat is made from a mixture of sand, cement, and sometimes lime, with a consistency that is spreadable but not too liquid. This mixture is then applied to the wall using a trowel, to a thickness of about 3/8 inches.
Once the scratch coat is applied, it is scratched with a trowel or serrated blade to create a web of surface scratches. This process is crucial and gives the coat its name. The scratches should be distributed all along the surface of the wall, and they can be made in a variety of ways. While there are comb-like tools specifically designed to create these scratches, it is more common to use a trowel or even a serrated knife. The trowel or blade is used to etch horizontal grooves into the semi-dry surface of the scratch coat.
These grooves have several important functions. Firstly, they provide a mechanical bond for the next layer of plaster, as the overcoat will enter these grooves and create a strong bond between the two layers. This interlocking surface results in a stronger and more secure attachment. Secondly, the grooves increase the surface area of the scratch coat, which further enhances the bond strength between the coats. Finally, the grooves help manage the drying process by allowing the scratch coat to dry and shrink evenly, reducing the likelihood of cracking.
After the grooves are created, the scratch coat needs to be left to dry for 24-48 hours before the next layer can be applied. This drying period is crucial, as it ensures that the coat is fully set and can provide a robust foundation for the subsequent layers.
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The rough surface with indentations helps the second coat of plaster to adhere
A scratch coat is the first layer of plaster applied to a wall. It is typically made from a mixture of sand, cement, and sometimes lime, and is applied to a thickness of about 3/8 inch. This layer is crucial in ensuring the quality and longevity of the final application.
As the name suggests, a scratch coat has horizontal scratches or grooves on its surface. These are created using a trowel, scarifier, or some type of serrated blade while the plaster is still wet. This process leaves behind a rough surface with tiny indentations.
The rough surface with indentations is essential for the adhesion of the second coat of plaster. When the second layer of plaster is applied, it fills in the scratches and indentations on the scratch coat. This creates a strong mechanical bond between the two layers, as the second coat interlocks with the grooves of the first. This bond provides greater stability to the finished plaster.
The scratch coat acts as a sturdy base for the subsequent coats. It helps to level out any irregularities in the wall, ensuring a consistent thickness and a smoother final appearance. The rough surface created by the grooves also increases the surface area of the scratch coat, further enhancing the bond strength between the two layers of plaster.
In addition to mechanical bonding, the scratch coat can also facilitate chemical bonding with the second coat. This occurs when certain additives or bonding agents are included in the mixture, creating a stronger bond between the layers.
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The scratch coat is typically applied to a thickness of about 3/8 inch
The scratch coat is the first layer of plaster applied to a wall. Typically, it is applied to a thickness of about 3/8 of an inch. This thickness is important for several reasons. Firstly, it needs to be thick enough to create a sturdy base for the subsequent coats. Secondly, it should be thin enough to allow for proper curing, which prevents cracking. This thickness is usually achieved by applying the scratch coat with a trowel, although it can also be sprayed on.
The scratch coat is applied to a variety of substrates, including brick, stone, concrete, and certain types of sheathing. It is adaptable and can be used on both interior and exterior walls. However, it is not recommended for brick walls as the small amounts of salt can collect in the niches of the brick surface over time, delaying the drying process of the plaster.
The scratch coat is typically made from a mix of sand and cement, with a ratio of 3 parts sand to 1 part cement. Sometimes, lime or other additives may be added to the mixture, depending on the specific application and local building standards. The mixture should be spreadable but not too liquid, ensuring it stays in place once applied.
The scratch coat is left to cure for at least 24 to 48 hours before the next layer is applied. This curing process is crucial, as it ensures that the scratch coat is fully set and provides a strong base for the subsequent coats. The curing time can vary depending on environmental conditions and the specific mix used.
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Frequently asked questions
A scratch coat is the first coat of plaster applied to a wall.
A scratch coat is typically made from a mix of sand, cement, and sometimes lime.
The purpose of a scratch coat is to create a strong base for subsequent coats of plaster and to improve adhesion between the coats.
A scratch coat is usually applied using a trowel or sprayer to a thickness of about 3/8 inch or 5mm. While still wet, lines are scratched onto its surface using a trowel, serrated blade, or scarifier.