Laying a Ceramic Tile Floor
Tiling Over a Subfloor
I am putting a ceramic tile floor down in our bathroom. I will be installing cement tileboard before I put down the tile. Some say thinset should go under this board and others say it is not necessary. The subfloor is a solid, level plywood floor that was under wall-to-wall carpet.
JD from Centerville, OH
I can't imagine why you would put thinset under the tile board. You can, if you wish, use construction adhesive (caulking-tube type) to give it a more secure hold onto the plywood. Though thinset is an adhesive, its adhesion to plywood is questionable, witnessed by me literally hundreds of times in loose tiles and sometimes entirely loose floors! Plywood's absorbency can cause thinset to dry too quickly.
In all honesty, if your floor is strong and solid, you can do away with the tileboard (which is difficult to work with) and use 1/2" plywood instead, screwed and glued to the old floor. The rule of thumb is 1 1/4" of solid floor under tile. Thus a 3/4" subfloor with a 1/2" AC plywood (not lauan because mastic doesn't always take to it) over it is plenty strong. Of course, the seams between the new floor must be staggered over the old seams.
Instead of using thinset, I recommend using either a mastic or Laticrete "Quick Trowel," which is a pre-mixed, sand-fortified mastic that is so much like thinset it fools people!
As you can gather, I am not a fan of thinset for do-it-yourselfers. It's too easy to mix it incorrectly or let it get too old in the bucket. Even the pros mess up with thinset, which can literally turn back to sand if not used properly! It is almost impossible to mess up a tile job with premixed adhesive.
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