cement board over tub?

how much space should i be giving over my tub? ill be infront of the bath tub lip because of some issues trying to make everything flush.ive heard to just butt it up against the tub, but then ive heard give a 1/8 of a space for grout. any1 know?


I'd leave a very slight gap to allow for any movement in the floor/walls. Tile will go over this anyway, and leave a slight gap between the tub and tile also. You are not going to grout between the tub and tile - use silicone caulk instead. If you grout it you will end up with a crack.
May 11, 2017
If there is a lip on the tub that is usually around an 1/8 thick I use thin strips of wood to fur out the stud creating a flat smooth surface from the ceiling to the top of the tub. Then the tile backer is screwed to the studs and leave approx 1/8 clearance at the bottom to avoid any water that may eventually creep under if the caulk fails from wicking up into your backer. You could use the end of a paint stirring stick on the tub to create the small space I am suggesting. I haven't used cementatious backer board ( WonderBoard or Durock) in the last 10 years. I have found the best product for ease of installation and durability is Hardiebacker. It is a little more than the cement boards but worth every penny. You can cut it using a circular saw, use drill bits, and hole saws without destroying your tools like cement board and get even cuts without the cement falling out like the other boards. Use their Backer On screws, NOT drywall or deck screws. Half inch Hardibacker also mates up with 1/2 drywall utilizing 1 5/8 screws. This system has worked well for me on hundreds of tile jobs over the last decade.
May 11, 2017
The cement board goes over the lip of the tub to help prevent water from running down behind the tub. a small gap between tub and board is fine. When the tile is grouted, the gaps between the tub and tile will be filled. (the final step is to caulk the corners and between the tub and tile). The gaps in the cement board are filled with thin set and mesh tape before tiling. I have never used mesh tape between the tub and cement board. I fill the space with thin set. If it's installed properly and screwed down sufficiently, cement board should not move. It really doesn't expand/contract with heat/cold much. It's very stable.
May 11, 2017

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