What is a Scratch Coat?

If you’ve ever been involved in masonry work, plastering, or some types of tiling, you may have come across the term “scratch coat.” This essential step, though it might sound trivial, can determine the quality and longevity of the final application. Let’s delve deep into understanding what a scratch coat is and why it’s so crucial.

Defining the scratch coat

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 consistency that’s spreadable but not too liquid, ensuring it stays in place once applied.

The scratch coat is typically applied to a thickness of about 3/8 inch. This 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 isn’t smooth like a finishing coat. Instead, as its name implies, it has horizontal scratches or grooves. These grooves are crucial because they provide a mechanical bond, giving the next layer of material something to grip onto.

The scratch coat can adhere to a variety of substrates, including masonry blocks, brick, concrete, and even certain types of sheathing. Its adaptability is one of its strengths. When working with smoother surfaces, additional bonding agents might be required to ensure proper adhesion of the scratch coat.

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’s crucial to wait for at least 24-48 hours before applying the next layer. This ensures the scratch coat has been set fully and can provide a strong base for additional coats.

Purpose of the scratch coat

1. Enhanced Adhesion

• Mechanical Bonding: The textured nature of the scratch coat, characterised by its grooves or scratches, creates an interlocking surface. This mechanical bond ensures that subsequent layers, such as the brown coat in plaster or the mortar for tiles, interlock with these grooves, resulting in a stronger and more secure attachment.
• Chemical Bonding: The composition of the scratch coat can sometimes facilitate chemical bonding with the layers that come after. For instance, when certain additives or bonding agents are included, they can react with the subsequent layer, creating a stronger bond.

2. Surface Preparation and Levelling

• Uniformity: Uneven surfaces, which are common, especially in older structures, can lead to an inconsistent finish. The scratch coat helps in levelling out these irregularities, ensuring a consistent thickness and a smoother final appearance.
• Filling Voids: Cracks, holes, and other surface defects can compromise the integrity of the final layer. It acts as a filler, ensuring such defects don’t propagate through to the finish.

3. Shrinkage Control and Crack Prevention

• Uniform Moisture Distribution: The scratch coat serves as a buffer, absorbing and redistributing moisture. By doing so, it reduces the potential for the top layers to dry out unevenly, which can lead to cracks.
• Thermal Expansion: Different materials can expand and contract differently under temperature fluctuations. The scratch coat helps to mediate this by providing a consistent base that responds uniformly to thermal changes, minimising potential cracks or detachment of the top layers.

4. Moisture Regulation

• Barrier Formation: While not entirely impermeable, the scratch coat can act as a semi-barrier, slowing down the movement of moisture from the substrate to the outer layers. This helps in controlling the curing process of the subsequent layers and also protects the substrate from potential water damage.
• Vapour Permeability: In cases where the substrate needs to breathe, a properly formulated scratch coat can allow for a certain amount of vapour permeability, ensuring that trapped moisture can escape without compromising the finish.

5. Structural Strength

• Load Distribution: A scratch coat can help distribute weight and pressure more evenly across the substrate. This is especially useful in flooring applications where tiles or stones might be subjected to significant loads.

Application

Starting off, one must ensure that the mixture of sand, cement, and occasionally lime is blended according to precise ratios. This mixture should strike a balance in consistency: spreadable, yet not too fluid, ensuring it remains steadfast once applied. Using a trowel, this blend is then uniformly spread over the desired substrate, typically to a thickness of about 3/8 inch. This is thick enough to mask any substrate irregularities but sufficiently thin to allow for proper curing and to avert potential cracking.

As the coat reaches a semi-dry state, a scarifier or notched trowel becomes the tool of choice to etch horizontal grooves into the surface. These grooves are essential, not just in name, but for the mechanical bond they provide for subsequent layers. Once this is accomplished, patience is key. The scratch coat requires an unhurried drying period, commonly spanning 24-48 hours, dependent on environmental conditions. This drying time is pivotal, ensuring the coat sets fully, ready to provide a robust foundation for the subsequent layers.

When do you need a scratch coat?

Stucco application

For exterior finishes that utilise traditional stucco, a scratch coat is fundamental. This is because the texture of the scratch coat ensures that the stucco adheres seamlessly to the underlying structure, providing longevity to the facade. Even for Exterior Insulation and Finish Systems, which use synthetic stucco, the application of a scratch coat becomes crucial, especially when the system demands a multi-layered approach.

Plastering over certain substrates

Brick and stone, with their inherently rough and uneven surfaces, can lead to an inconsistent plaster finish. A scratch coat can bridge the inconsistencies, ensuring a uniform layer upon which the plaster can adhere. When rejuvenating or restoring old plaster walls, a scratch coat can help create a suitable bonding surface, ensuring that the new plaster adheres effectively.

Masonry veneer installations

Whether it’s a natural or manufactured stone veneer, a scratch coat provides the necessary grip, ensuring that the veneer adheres properly to the substrate. Much like stone veneer, brick veneer can benefit immensely from a scratch coat, especially when being applied over surfaces that aren’t naturally conducive to adhesion.

Tiling on uneven substrates

In places like bathrooms or outdoor patios, where tiles might be exposed to consistent moisture, a scratch coat can provide an added layer of protection, ensuring better adhesion of tiles and reducing the risk of water infiltration. Instead of removing old tiles, sometimes it’s feasible to tile over them. In such cases, it creates a surface that’s more receptive to the new tiles.

Restorative work

When dealing with historic or aged structures, there are often layers of various materials, some of which might not be compatible with modern finishes. Here, it serves as an intermediary. Therefore, it ensures that newer finishes bond well without damaging the integrity of the original structure.

Do you use a scratch coat in your rendering work? Let us know below!