Lay a Ceramic Tile Floor

Cutting and mortaring the underlayment

Cutting the Underlayment

Cement board, sold as WonderBoard and Ducock, is an excellent base for tile: rigid, waterproof, cheap, and flat. The only disadvantage: It instantly dulls knife blades. We use a jumbo drywall square, a utility knife, and plenty of blades to cut cement board. Cut one side (the label tells which), fold and break the board, and cut the other side. Stagger the joints so four corners don't meet, and place joints on top of the joists where possible.

Mortaring the Underlayment

The whole point of underlayment is to make a flat, stable base for the tiles. Underlayment gets laid in the same mortar bed you'll use for the tiles.

Once the boards are cut, mix thinset tile mortar and start laying underlayment:

1. Trowel mortar onto the plywood, following the same procedure used to mortar tiles.

2. Working quickly, lay the boards down, taking care to press the edges down. Don't wait too long, or the boards might rest not depress the mortar uniformly.

3. Screw the boards with 1¹⁄₄" cement-board screws, placed about 8" apart. If the screws are hard to drive, drill quick pilot holes for them. We screw underlayment with a screw gun, which shuts off when the screw reaches the right depth, but an electric drill would also work. Don't drive the screws too deep; that damages the cement board.

4. Use a straightedge to check flatness as you work.

5. Fill joints between boards with mortar.

Step 2: Where to Start Tiling?

Finding the starting line can be the toughest part of tiling. It's helpful to start tiling in a corner. A triangle 3' × 4' × 5' is a right triangle. Measure a “3-4-5” triangle in your starting corner to check if it's square. You can get into big trouble by assuming your starting corner is square.

There are two methods for finding a starting line, depending on how the tile fits the room. In the next section we'll take a look at both.