What type of underlayment is typically used for tile installations over concrete?

Cement backer board is the correct type of underlayment for tile installations over concrete due to its durability and moisture resistance. It provides a stable and level surface that is essential for tile application, particularly in areas subject to high humidity or moisture, such as bathrooms and kitchens.

This type of underlayment also ensures that the tiles adhere well, minimizing the risk of cracking due to movement or settling. Cement backer board is specifically designed for use in tile installations and helps to create a solid foundation that can handle the weight of the tiles without compromising the overall integrity of the installation.

Plywood lacks the moisture resistance required for tile applications and is not suitable for wet areas. Foam board, while it can provide insulation, does not offer the necessary structural support for tile installations. Gypsum board is also unsuitable as it is not water-resistant and can deteriorate in moist environments, making it a poor choice for underneath tile.