Installing Cement Underlayment

If you're planning on installing ceramic tile flooring in an area where moisture will be a concern, such as a bathroom or a kitchen, you'll need to lay down cement underlayment. Cement underlayment or cement board is rigid and totally stable-even when wet. That's the good news. The not-so-good news is that cement underlayment is expensive, heavy, and difficult to work with. But it is by far the absolute best choice as an underlayment for ceramic tile.

Cement board typically comes in 32" by 60" sheets and is 1/2" thick. Although you can cut it with a utility knife, a power saw fitted with a masonry or carbide blade is your best bet.

Note: If you're laying ceramic tile in areas where moisture won't be a factor, consider using fiber/cement board (a thin, high-density underlayment) or even plywood. Both of these are less expensive and are easier to work with than cement board.

1. Apply mortar Mix your thin-set mortar according to the manufacturer's instructions. You'll find that a mixing attachment for a portable drill will speed up this task Starting along the longest wall, begin applying the mortar with a notched trowel In most cases, you'll want to use a Y4" trowel and spread the mortar in a figure-eight pattern. Spread only enough mortar for one sheet at a time. Just as with any other underlayment, make sure that the seams of the cement board don't match up with the seams in the subfloor.
2. Fasten sheets with screws Place a sheet of cement board on the mortar with the smooth side facing up. Then secure it to the subfloor with 1/2" galvanized deck screws every 6" or so along the edges and about every 8" throughout the rest of the panel. Drive the screws in so they're slightly countersunk beneath the surface of the cement board. Continue like this, working along the wall; then start the next row, making sure the end seams are offset.
3. Straight cuts You can make straight cuts on cement board similarly to cutting drywall. Start by marking the cut with a straightedge and marker or chalk line. Then score a line with a sharp utility knife (it will dull quickly while cutting this abrasive stuff). Next, slide the cement board on your work surface so that the scored line is at the edge, and press down to snap the board (or slip a 2x4 or dowel under the score line and snap it). Finish the cut with the utility knife. Alternatively, use a masonry blade in a power saw.
4. Obstruction cuts Quite often you'll need to cut cement board to fit around an odd-shaped obstacle. This is a situation where a saber saw fitted with a masonry or carbide blade is the best tool for the job. If you don't have a masonry blade, you can make short cuts in this stuff with a standard blade, but the cement board is so abrasive, it'll quickly dull the teeth. A tile saw or rod saw can also be used in a pinch to cut this.
5. Tape and level seams When all the cement board is down, your final task is to cover the seams and screw heads. To do this, apply thin-set mortar to the screw heads and then place fiberglass mesh tape over the seams. Spread a layer of thin-set mortar over the tape with a wide-blade putty knife or drywall knife. Feather the edges away from the seams to create a smooth surface. Allow the mortar to cure for a minimum of two days before beginning to lay tile.