How many coats of gypsum plaster are required over concrete for proper application?

For the proper application of gypsum plaster over concrete, a system generally requires two coats. The first coat, known as the scratch coat, creates a bonding surface and provides the necessary thickness. It is applied directly to the concrete substrate, which may be smooth and non-porous. To enhance adhesion and ensure that the plaster adheres properly to the substrate, the scratch coat is scored to create grooves that help the subsequent layer to bond effectively.

The second coat, which is the finish coat, is then applied over the scratch coat. This coat not only provides a smooth, aesthetic finish but also adds the required durability and performance characteristics to the plaster application. Using two coats allows the plaster to achieve the necessary thickness and finish quality, which is critical for long-lasting results and to prevent issues such as cracking or peeling.

In contrast, applying just one coat may not provide sufficient thickness or proper bonding, potentially leading to a less durable and aesthetically pleasing finish. More than two coats, while possible in specific applications or finishes, is not standard practice and can be unjustified for typical residential plaster applications, making two coats the accepted requirement in construction standards for this type of work.