I am preparing to put down cement board for a bathroom tile install. I understand, I need to tape at the seams. Should I tape the edges where the cement board meets the drywall or the tub as well?

I was told to provide a 1/4 inch gap between each seam and the walls, so that makes me "feel" that I should do something, but what?

Also prior to putting down the cement board do I need to apply any caulk or anything to where the plywood meets the tub? Or will that be "sealed" with the mortar when applying beneath the cement board and then again when laying the tile.

Edit: Thanks for all the responses, I am worried I may have chosen the wrong product to put BENEATH the cement board. I am using James Hardie Backer Board and the two guys at the big box who both said they do tiling on the side use below.

Steps were advised as follow:

  1. Put down tile adhesive, put down the backer board let dry.
  2. Create thinset use between backer board with tape.
  3. After dry use thin set again to lay down tiles.

Mastic advised to use

  • 1
    Why is there plywood involved with anything in a bath room?
    – Mazura
    Sep 30, 2014 at 2:42
  • The layers are subfloor(plywood), then underlayment (another plywood) then mastic for the cement board, cement board and then mortar and then tile.
    – treeNinja
    Sep 30, 2014 at 13:05
  • Note: do not use mastic under the cement board, it does not dry rigid. Use thinset under the cement board and in the seams with tape.
    – user36043
    Apr 27, 2015 at 15:17
  • @treeNinja I'm pretty sure you can do without that second layer of plywood. I've never used it in any bath/kitchen remodel I've done and have never had a call back in years.
    – Dano0430
    Apr 27, 2015 at 17:32

3 Answers 3


You don't tape the edges. Not on walls or tub. I'm not sure about a 1/4 inch gap by tub (probably would do 1/8 there) be everywhere else is fine. Nothing goes in the gap. Gap will be covered by either tile on the wall or baseboard.

Also make sure you are using the right mesh "tape". Your cement board gets same thinset as your tile .

  • So you are you saying the tile should be butted up against the wall when I place it down? I read and told to make sure the space between wall and floor tile was also a 1/4 inch gap.
    – treeNinja
    Sep 30, 2014 at 13:07
  • No. Very close to tub so it can be caulked easily. The wall - it really doesn't matter since the gap will be covered by something. 1/8-1/4" is fine.
    – DMoore
    Sep 30, 2014 at 14:59

The cement board must be properly backed at tub edge to keep it stiff. The gap between it and the tub is grout filled. I don't tape corners. I do tape bevels simply to make a flush surface for the tiles. I generally silicon tiled corners. I use 1/8 inch gap and that give me more room for silicon on the corners. That's enough room. The idea is to avoid buckling from settlement and let you get more silicon in there.

You will notice the bevel edge on 2 sides of the cement board. When butt end joints are required it's a good idea to rasp a bevel into the ends using a wood rasp. The idea of the bevel is to allow you to get more joint compound in there making a stronger joint. The minimum for tub joints is Fast Set. It doesn't rot.

I silicon tub ledgers to avoid future squeaks. Just because you screw or clip the tub to the wall doesn't mean it won't squeak later if settlement occurs. Yes, fill the tub full when setting it.


Tape the seams if your going to use pro-red or its equivalent (highly suggested). If the manufacture says to leave an expansion gap, that's what you do; leave a gap. I must admit, that where it meets the walls, I wouldn't. Any expansion would buckle the tape and bubble the paint. I'd rather the whole wall shift a little. Also, the only place for caulk in a bathroom is on the outside of a shower door frame (and underneath it) and around fixtures (sparingly, leaving a drain hole at the bottom). Not the tub nor the tile.


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