I have a joint between two sheets of cement board that forms a small ridge elevated maybe 1/8" relative to the centers of either board. I was wondering if extra thinset on both sides can be used to even it out or is it better to use self-leveling compound beforehand. I am using expensive marble tiles and don't want them to break to an unevenness.

1 Answer 1


You may break a few pieces(of marble) just picking them up.

A skim coat on either side of the joint will help.

Block feather with a concrete 'rubbing' block.

My marble floor always have Schluter Ditra under them. I also use it with any questionable floor.

Ceramic tile can tolerate L/360 deflection, stone is twice as sensitive at L/720.

If you have any doubts about floor springiness, use an isolation membrane or stiffen joists or add a beam. Cement board does little for stiffness.

  • a skim coat of what?
    – amphibient
    Apr 8, 2013 at 20:59
  • 2
    Thinset mortar.
    – HerrBag
    Apr 8, 2013 at 21:32
  • what if i just dump some self-leveling on either side and wait for it to solidify before setting thinset?
    – amphibient
    Apr 8, 2013 at 21:35
  • as far as Schluter Ditra, do i set that using thinset and then again thinset on top between it and the tile?
    – amphibient
    Apr 8, 2013 at 21:44
  • You can use leveling mortar first. Ditra is set based on the substrate, usually polymer modified thinset over ply or cement board. For most marble, a white thinset is best.
    – HerrBag
    Apr 8, 2013 at 22:59

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