Backer board, also called backing board, is a type of submaterial used under a variety of building projects, most often, tile projects. The most popular type of backing board is made of compressed cement pressed between two sheets of fiberglass. Another type of backing board is one-half inch (1.3 cm) water resistant greenboard. The determining factor on what type of backer board to choose is the area where it will be installed.

Backer board does not add any structural strength to the floor or walls. If strength is a concern, reinforce the area first, before adding the additional weight of the board. Cement board, in particular, is heavy, so it is important to place it on strong subflooring.

The use of backing board makes tiling an easy do-it-yourself project. Prior to the development of backing board, laying tile required the person handling the job to lay a thick mud bed with mortar. This created a waterproof barrier and provided a level area to install tile. Installation is simple with backer board, which it is screwed directly into the sub floor. A thin application of mortar under the backer board fills any voids between the board and the sub floor.

Greenboard is water resistant, but not waterproof. Greenboard is not for use in areas where it will be exposed to a great deal of moisture. While it will do well in areas with some moisture, such as on a bathroom wall or under a backsplash, when exposed to a great deal of moisture, the greenboard can break down and disintegrate.

Cement backer board is a much more durable material. Typically available in one-half inch (1.3 cm) thickness, cement backing board is impervious to moisture. Cement backer board is the ideal choice for showers, bathroom floors, and other wet weather applications. Cement backing board is long lasting; many types advertise a projected life of 50 years. It is also resistant to mold, rotting, and warping.

The major disadvantage of cement backer board is that it is heavy and can be difficult to handle during installation. Available in 3 by 5 foot (.9m by 1.5m) and 4 by 6 foot (1.2m by 1.8m) sizes, depending on the manufacturer, bringing the backing board into a small bathroom or laundry room for installation can be challenging. It is often necessary to bring the full sheets of backing board into the area where it will be installed to make accurate cuts along pipes and other fittings.