Cement Board Installation

DEAR MIKE: I want to install ceramic tile in an upstairs bedroom that has carpet. I have laid tile on a ground floor that is concrete, but never upstairs. I know I have to prepare the floor so that the tiles will not crack or come loose. What needs to be done? -- Gabriel R.

DEAR GABRIEL: You've got to make the floor harder than Chinese arithmetic.

You can do this with cement board, which comes in sheets that measure 3X5 and cost about $12 apiece. The sheets come in thicknesses of either one-quarter or one-half inch..

I remember seeing an upstairs bathroom floor where the homeowner just laid the ceramic tile right over the floor without cement board. Tiles were loose, and the grout was cracked and chipped out.

The cause was deflection. The floor joists had a long span, which gave it more deflection -- the nemesis of ceramic tile. Any give in the subfloor will cause movement which will crack tile and grout. By stiffening the floor, you can reduce movement.

Before you lay down the cement board, get rid of any little squeaks that have been bugging you over the years. Get rid of the carpet and tack strip and screw the wood subfloor down to the floor joists. Coarse drywall screws are great for this.

Next, make sure there are no ridges in the floor. Use a straightedge and hold it over the seams where two pieces of plywood meet. Hold it perpendicular to the seam to check for any "teeter- tottering" along the seam.

If there is, use a belt sander to knock down the high spots. Also, sink any screw or nail heads into the flooring.

Before you install the cement board, you will apply thin set in between the cement board and wood subfloor. The reason here is not so much to hold it down, but rather to eliminate any gaps between the cement board and the wood subfloor. This will make the floor rock solid.

Precut the cement board. Start laying the pieces out and when you need to cut a piece, use a utility knife and score the fiberglass mesh on one side. Snap the board back and then cut the mesh on the other side. Once all the pieces are cut, you can number them and remove them.

Mix up a batch of thin set and, using a trowel with one-quarter inch teeth, spread a layer of the stuff as large as the cement board.

One by one, place the cement boards on top of a layer of the thin set and smoosh it into the mixture. Use 1 1/4-inch cement board screws and screw the cement board to the subfloor every 4 to 6 inches.

When that chore is done, lay some fiberglass tape on top of each seam. One side of the tape is tacky, and sticks on top of the cement board.

After the taping, mix up some more thin set. Use a flat trowel or the back of your notched trowel and apply a thin layer on top of the tape. Let it all sit at least overnight.

Once you add more thin set and finally the tile, you will notice the floor will have raised. For this reason, consider removing the baseboard and re-installing it after the tile is down. You also may have to trim the bottom of the door. 

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