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I want to re-tile and extend my old hearth floor which means the new (large) tiles would be laid partially over the old concrete substrate and partially over new plywood. Will Ditra membrane covering these two different substrates suffice to prevent cracks in the new tiles?

  • The membrane will help with cracks caused by movement in the horizontal plane, such as might be caused by different expansion/contraction of the dissimilar materials. Nothing will prevent cracks if there is too much movement, or if there is vertical movement. – Jimmy Fix-it Mar 26 at 4:58
  • How is the plywood section related to the concrete section? Are they all on the same slab? Would the plywood ever move independently of the concrete? Also, for large tiles there is Ditra XL. – JPhi1618 Mar 26 at 15:10
  • Thank you, great question - This is on the first floor, not a slab. The old tile was a hearth over concrete next to hardwood floor (hardwood over 2X subfloor). I replaced the endcap of the hardwood with 3/4" plywood thinking to use self-levelling mortar over the concrete and then cap both with Ditra. But now I'm thinking I should remove the plywood and use Durock over all of it. I can attach the Durock to the concrete with thin-set, but should I let the part over the wood subfloor float or screw it to the subfloor? – JAY Mar 27 at 15:57

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