In construction, a “scratch coat” refers to the first coat of plaster or stucco applied to a masonry surface before the brown coat or finish coat. The scratch coat provides a rough surface for the subsequent coats to adhere to and ensures a strong bond between the plaster or stucco and the masonry substrate. Proper application of the scratch coat is essential for ensuring the durability, weather resistance, and aesthetic appeal of the finished surface.
Function
The primary function of a scratch coat in construction is to provide a rough surface for the subsequent coats of plaster or stucco to adhere to. Key aspects of the function of a scratch coat include:
- Adhesion: The scratch coat creates a strong bond between the plaster or stucco and the masonry substrate, preventing delamination, cracking, and separation.
- Keying: The scratch coat forms a keying or gripping surface for the subsequent coats of plaster or stucco, ensuring they adhere properly and uniformly.
- Moisture Management: The scratch coat helps manage moisture and prevent water penetration, ensuring the durability and weather resistance of the finished surface.
Types of Scratch Coats
There are several types of scratch coats commonly used in construction:
- Plaster Scratch Coat: A plaster scratch coat is made of a mixture of sand, cement, and lime, applied to masonry surfaces before the brown coat or finish coat.
- Stucco Scratch Coat: A stucco scratch coat is made of a mixture of sand, cement, and water, applied to masonry surfaces before the brown coat or finish coat.
- Exterior Scratch Coat: An exterior scratch coat is specially formulated for outdoor applications, providing enhanced durability, weather resistance, and adhesion.
Applications
Scratch coats are used in a wide range of construction applications, including:
- Residential Buildings: Scratch coats are used in residential buildings to provide a rough surface for subsequent coats of plaster or stucco, ensuring a strong bond and durable finish.
- Commercial Buildings: Scratch coats are used in commercial buildings to provide a rough surface for subsequent coats of plaster or stucco, ensuring a strong bond and durable finish.
- Industrial Buildings: Scratch coats are used in industrial buildings to provide a rough surface for subsequent coats of plaster or stucco, ensuring a strong bond and durable finish.
Installation
Proper application of the scratch coat is essential for ensuring the durability, weather resistance, and aesthetic appeal of the finished surface. Key aspects of the installation process include:
- Prepare the Surface: Prepare the masonry surface by cleaning, dampening, and applying a bonding agent, ensuring proper adhesion of the scratch coat.
- Mix the Materials: Mix the plaster or stucco with water according to the manufacturer’s instructions, ensuring a consistent and workable mixture.
- Apply the Scratch Coat: Apply the scratch coat to the masonry surface using a trowel, combing it with a scratcher or rake to create a rough texture for the subsequent coats to adhere to.
Conclusion
In construction, a scratch coat refers to the first coat of plaster or stucco applied to a masonry surface before the brown coat or finish coat. By providing a rough surface for subsequent coats to adhere to, proper application of the scratch coat ensures the durability, weather resistance, and aesthetic appeal of the finished surface in residential, commercial, and industrial buildings.
