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I'm planning on tiling the entryway around the cutout door to my
basement (about 3' x 6') and a hearth in front of a fireplace. (about
2' x 4'). Both are going to be ceramic tile, and are going to be
surrounded by wall to wall carpet on 3 sides. Right now, the floor is
an unfinished cement floor, and the carpeters are coming in a week or
two, so I need to get moving on the tile.

After talking to a guy at the store, we agree that I probably need a
backerboard under the tiles, so that the height is proportionate to the
surrounding carpet. (Moderately thick pile on top of a 3/8" moisture
resistant pad). So I bought some 1/4" Durock cement backer boards, and
was getting ready to start slicing them up and putting them down, when
I thought of some more questions.

1) Do I need to (i) tape the seams and/or (ii) put down some moisture
membrane on the area by the door, or can I just seal my grout and
figure that it's cement all the way down?

2) How should I attach the backer board to a concrete subfloor? Will
thinset be enough to hold it in place, or should I mess with masonry
bits and screws, and if I need to screw it down, what kind of bit and
screw should I use?

3) Is there a good way to figure out if I've got the tile at the right
height? Would I be happier with 0" of backerboard (i.e. none), or
1/2"?

Thanks, TM


Strictly speaking, you don't need backerboard, the tiles can be set directly
on the concrete but unless you want a very thick bed of mortar, the cement
board is the best material to use for a build up. It depends a lot on the
thickness of the carpet and pad you plan to use

There are also metal beads for outside corners, edges and a ramp shape for
floor transitions if you want to go that way.

As long as the concrete floor is in good condition, probably any thinset
will hold it in place without nailing, it is just a small area (30SF) with
little traffic afterall . Premium thinset will bond better but a bit
overkill IMO. Concrete bonding adshesive painted on the concrete before
mortaring would probably help as well especially if the concrete is
presently painted. If it is open clean concrete, you could use masons
mortar (under the board, not for the tiles) and be OK. This is not as
critical as setting stone on a cieling, gravity and similar materials will
all work in your favor here. With no risk of floor deflection, you don't
need a flexible mortar either.

1. No, a membrane would prevent the thinset from sticking to the concrete.
Tapiing the saems is unnessary sionce the underfloor cannot deflect. Just
smooth it over with some thinset until you are ready to set the tile.
Fiberglass tape won't hurt if you have some. Tile and thinset are a
moisture barrior.

2, No, just stick it to the floor

3. Build a mockup using a sample of tile, plywood and a sample of carpet and
pad. Stack it up and see how it looks. Given the thickness you described,
it will probably be fine with 1/4" board. Add the tile thickness to ~1/4"
of thinset plus the board thickness and ~1/8" thinset under the board for an
estimate.