Cement board, commonly sold under trade names such as Durock, Hardiebacker, and WonderBoard, is now the standard underlayment used for ceramic, porcelain, or stone tile laid with thinset mortar adhesive. Cement board is used for most floor tile and has all but replaced plywood and drywall backer materials for wall-tile applications in wet areas like showers and tub surrounds.
Cement board forms a durable, stable base for tile, and it has no organic materials (unlike drywall, greenboard, or plywood) so it is not prone to mold, rot, shrinkage, or decomposition due to moisture. Ceramic tile laid over cement board is one of the most durable floor or wall surfaces you can install.
The Surface Below the Cement Board
Because cement board can be used either for floors or walls, the surface below the cement board will vary:
- Flooring: For ceramic tile floors, the cement board usually rests on a plywood subfloor (minimum 5/8-inch-thick exterior-grade plywood or OSB). If new ceramic tile is being laid on an existing floor, the cement board can usually be laid right over the existing floor finish, provided the floor is in good shape and the extra thickness is not a problem. There is no need to install backer board on a concrete substrate—ceramic tile can be applied right over concrete, provided the surface is flat, smooth, and free of significant cracks.
- Walls: For highly wet areas, such as showers and bathtub surrounds, cement board typically is attached directly over the wall studs, although the studs may be covered with an intervening moisture membrane. (Some cement board products may not require the underlying membrane if the seams are properly waterproofed.) In such wet areas, never install cement board directly over drywall or green board. Instead, first, remove the existing surface material, then install the cement board over the studs and membrane, as applicable.
Cement board is most commonly sold in 3 x 5-foot sheets and in thicknesses ranging from 1/4 inch to 5/8 inch. A 3 x 5 panel of 1/4-inch cement board weighs about 30 pounds; a 1/2-inch sheet, 36 to 44 pounds. Larger, 4 x 8-foot panels are also available, but their weight makes them hard to manage for DIYers.
Choose a sheet thicknesses based on the application:
- Floors: A 1/4-inch minimum thickness is needed, but thicker panels are also fine.
- Walls: Use 1/2-inch or 5/8-inch-thick cement board, not 1/4-inch. The extra thickness is needed when spanning the studs and it provides a solid base for the tile.
- Countertops: 1/4-inch, 1/2-inch, or 5/8-inch-thick panels are all suitable for countertops.
Before You Begin
You will need to cut your cement board to various sizes to suit your project, and knowing how before you start your project can keep you from making costly errors.
You can cut the cement board with a utility knife and a T-square. Score the cement board deeply with two or three passes with the knife, using the square to ensure a straight line, then snap the waste area of the sheet back to break the board along the cutting line. Cut the fiberglass mesh along the backside of the board, as applicable, to complete the cut.
To make round or curved cuts, use a jigsaw with a carbide blade or a drill and a hole saw bit.
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