What joint compound to use for cement board?

Traditional drywall joint compound is not used for cement board. Instead, thin-set mortar is the appropriate material for bedding cement board and treating its joints, typically in conjunction with alkali-resistant tape.

Understanding Materials for Cement Board

Cement board is a durable, water-resistant substrate commonly used in areas prone to moisture, such as bathrooms, kitchens, and laundry rooms, as a base for tile. Unlike drywall, it requires materials that can withstand moisture and provide a strong, rigid foundation for ceramic or porcelain tiles. Traditional drywall joint compound (often called "mud") is water-soluble and not designed for the high-moisture environments or structural demands of cement board applications, making it an unsuitable choice for seams or fasteners.

Thin-Set Mortar: The Correct Choice

For cement board applications, thin-set mortar is the primary material utilized.

• Composition: Thin-set mortar is a cement-based product, consisting of cement, fine sand, and a water-retaining agent.
• Primary Use: It is specifically designed to bond materials like ceramic or porcelain tile and cement backer board. When installing cement backer board, thin-set mortar is applied to the subfloor or studs before fastening the board.
• Working Properties: One of its advantages is its relatively slow drying time, which allows for crucial adjustments and repositioning of backer board sheets as needed during installation, ensuring a precise and level surface.

Treating Cement Board Joints

For seams (joints) between cement board panels and over fastener heads, a combination of materials is essential to create a strong, stable, and crack-resistant surface ready for tiling.

1. Alkali-Resistant Fiberglass Mesh Tape: This specialized tape is designed to be used with cement products. It is placed directly over the seams between cement board panels.
2. Thin-Set Mortar Application:
   • After placing the mesh tape over the joint, a layer of thin-set mortar is applied directly over the tape, thoroughly embedding it.
   • Additional thin-set is then used to skim coat the seam and cover all fastener heads, creating a smooth, monolithic surface that provides reinforcement and prevents cracks from transferring through to the tile.

This method ensures the joints are strong, resistant to moisture, and provide a stable base for tile, which is critical for the longevity of the tiled surface, especially in wet areas like showers.

Types of Thin-Set Mortar

Thin-set mortars come in various formulations, typically categorized as modified or unmodified:

• Modified Thin-Set: Contains polymers that enhance its bonding strength, flexibility, and water resistance. It is generally recommended for bonding cement board to subfloors and for setting most types of tiles, especially in high-traffic or wet areas.
• Unmodified Thin-Set: Pure cement, sand, and water-retaining agent. It is often used in conjunction with liquid waterproofing membranes, where the membrane itself provides the necessary flexibility and bonding properties. Always check the specific recommendations of the waterproofing membrane manufacturer.

Summary of Materials