NASCLA Accredited Commercial General ContractorFinishesMedium
A contractor is installing gypsum plaster on a ceiling. To ensure proper bond and prevent cracking due to differential movement, what is the purpose of applying a bond coat (e.g., a dash coat or scratch coat) directly to the substrate before the brown coat?
- ATo provide a smooth, level surface for the finish coat.
- BTo act as a vapor barrier against moisture penetration.
- CTo create a mechanical key and absorb suction from the substrate.
- DTo increase the fire resistance rating of the assembly.
Show answer & explanationAnswer & explanation
Correct answer: C. To create a mechanical key and absorb suction from the substrate.
The bond coat, often a scratch coat, is designed to penetrate the substrate and provide a rough, textured surface (a 'mechanical key') for the subsequent plaster layers to adhere to. It also helps equalize the suction of the substrate, ensuring uniform drying and preventing cracking.
Why the other options are wrong
- A. The brown coat, not the bond coat, is primarily for leveling and plumbing surfaces.
- B. A plaster system is not designed to be a primary vapor barrier.
- D. While plaster systems contribute to fire resistance, the primary purpose of the bond coat is adhesion, not solely fire rating.
Plaster Bond Coat (Scratch Coat)
The first layer of a multi-coat plaster system, applied directly to the lath or substrate, designed to provide strong adhesion for subsequent coats.
- Creates a 'mechanical key' (rough surface) for the brown coat to grip.
- Helps equalize and control the suction (absorption) of the substrate.
- Essential for preventing delamination and cracking of the plaster system.
Memory trick: The BOND coat is the KEY that GRIPS the wall, preventing cracks.