Scratch Coats
A scratch coat, sometimes also referred to as a base coat, is the first layer of mortar that is applied to a wall before the final coats of render are applied. It provides a strong bond and anchor between the material of the surface and the layers of the render, protecting the render and creating a durable surface.
Although a scratch coat is primarily used as a base coat, it is also slowly becoming more common to use a scratch coat as a top coat.
Continue reading to learn more about scratch coats.
What is a Scratch Coat?
A scratch coat is the first coat of mortar applied to a wall to prepare the surface for the following layers of render. It provides a solid surface for the render layers to adhere to and bond with, minimising the risk of moisture getting into the structure and cracks forming. A scratch coat also makes the surface stronger and more durable.
Scratch coats can be applied to concrete, cement, stone, brick, and metal lath (a mesh formed by a metal sheet) surfaces. They can also be used with most types of render (cement, silicone, acrylic, etc.).
Why Do You Need a Scratch Coat?
A scratch coat has a critical role to play in the rendering process; it forms a strong base that supports the render and allows it to bond more easily with the wall’s material to create a smooth, even surface that can withstand the natural movement of the building over time and exposure to the elements.
In simple terms, the purpose of a scratch coat is to provide support, protection and adhesion to the finished render.
The Benefits of a Scratch Coat
A scratch coat has many functional benefits, including:
- More effective bonding: The textured surface of the scratch coat creates an interlocking feature that the render grips onto, allowing the render layers to bond better with the surface.
- Smoother finish: The scratch coat helps prevent uneven surfaces by helping to level the render layers.
- Moisture regulation: The scratch coat can act as a type of buffer against moisture, distributing it evenly. It can also slow down the movement of moisture from the surface material to the outer render layers, minimising the risk of moisture damage or cracks forming.
- Temperature regulation: The scratch coat provides a consistent base that regulates temperature changes, which decreases the risk of the render cracking or becoming damaged due to temperature and humidity.
- Breathability: An effective scratch coat is breathable, which helps prevent any water damage.
- Load distribution: A scratch coat enables weight and pressure to be distributed equally along the wall.
- Insulation: The scratch coat acts as a barrier between the inside of the building and the exterior, providing insulation for the interior of the building.
- Soundproofing: The thick layer of the scratch coat reduces the amount of noise that travels through the walls.
- Aesthetic appeal: Although the scratch coat isn’t visible, it does impact how the render layers look. It helps provide a smooth, strong surface for the render to look its best.
- Adds texture: If you use a scratch coat as a top coat, it can add interesting texture to the building’s appearance.
How is a Scratch Coat Created?
Scratch Tool
A scratch tool (also known as a scarifier or a comb) is a specially designed tool that creates a textured effect on the scratch coat with its serrated edges. The textured surface provides a supportive grip for the layers of the render to bond to, which improves the overall strength of the render and prevents cracks from forming.
Regular rendering and plastering tools such as a notched trowel and floats can also be used to create a textured finish on the scratch coat.
The Scratch Coat Mixture
A scratch coat can contain different ingredients, but the most common mixture is sand, cement, water and lime. The ratio of these ingredients depends on the project and local building standards. The scratch coat mixture should be easily spreadable but not too runny.
Applying the Scratch Coat
- Prepare the wall: Before applying the scratch coat, remove debris, dust and dirt from the wall using a stiff brush and a hose.
- Mix the scratch coat mixture: Mix the sand, cement, and lime together, ensuring it has a spreadable consistency but is not too thin or runny.
- Application: Using a trowel, apply the scratch coat to the wall, starting from the bottom and working your way upwards.
Try your best to spread the scratch coat evenly, ensuring every spot of the wall is covered.
- Adding the texture: Once the scratch coat is thick enough and firm (but not completely dry), use a scarifier or notched trowel and run it horizontally along the wall to add notches and lines to the scratch coat.Try to be consistent with how deep the notches are and the distance between each one. Having neater patterns will ensure that the render bonds better with the wall.
- Drying and curing time: Allow the scratch coat to dry for about 48 hours (drying time will differ depending on the weather and the amount of scratch coat applied).Don’t let the scratch coat dry too quickly, and help the curing process by occasionally spraying the wall with a light mist of water.
Once the scratch coat has dried and cured completely, it is time to apply the render layers.
Scratch Coat Tips
- Adding lime to the scratch coat mixture makes it more crack-resistant.
- Before the scratch coat is applied, the wall must be free from all dirt, dust and debris.
- Apply the scratch coat during dry, mild weather – not too hot or too cold.
- The scratch coat should be around 9.5 mm thick to form proper support for the render layers.
If you want to know more about a scratch coat, don’t hesitate to give us a call! At Sydney Cement Rendering, we take pride in being Sydney’s trusted rendering experts. We provide our customers with the highest quality rendering services. Contact us at 0402 704 984 or visit our website: https://sydneycementrendering.com/ to find out more about what we offer!