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I am redoing my bathroom and figuring out how to do the floor.

Previously it had lino on top of 26mm of tongue and groove foam-backed chipboard, which also locked into the hall floor. This chipboard was rotten and had to be removed. It sat directly on solid concrete, which is the structural floor of my flat.

I now want to put down ceramic tiles, but am unsure what to put between the tiles and the concrete (tiling directly onto the concrete is not an option)

Many places I have looked suggest plywood but they assume you already have wood underneath to affix it to, and I think that without that it may expand too much in the humidity.

Another option in concrete backer boards, but would I still need wood underneath them to hold them together?

I would also like to include a new foam layer for insulation/sound proofing.

Can anyone recommend a solution that will ensure my tiles will stay fixed down and not crack?

Thanks for reading

  • 2
    Out of curiosity, why can't you tile directly to the concrete floor? – JPhi1618 Feb 3 '16 at 19:17
  • It's partly because I need some extra height to match levels, but mostly that the flat is lease hold so I don't think I can really do anything that would permanently alter/attach to the sub-floor without being removable. – user1111284 Feb 4 '16 at 11:29
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No no no. You do not put a subfloor on good concrete. You are then allowing for a moisture sandwich (which is bad). Buy a modified thinset, put tiles on thinset, grout, then you are done.

  • Would moisture from the slab be a problem? Should they apply a decoupling/waterproof membrane such as RedGard first? – Jason Hutchinson Feb 3 '16 at 21:22
  • @JasonHutchinson - If there is moisture then your optimal install is probably a layer of kerdi membrane. However in my opinion that is only needed if there is movement too. Tile has been installed on concrete via thinset or similar mortar for 100s of years and last just fine. Thinset does a really good job absorbing moisture, letting moisture seep through and usually doesn't expand or crack. If I had a basement that I knew would have moisture problems my install would be tile right on the concrete or some sort of pure vinyl click lock. – DMoore Feb 3 '16 at 21:30
  • As the flat is leasehold I don't think I can permanently bond to the subfloor like that. I think I'm obligated to keep some level of sound proofing, especially when putting a hard floor down. – user1111284 Feb 4 '16 at 11:31
  • @user1111284 - Huhhhhh?? Concrete board under is way more permanent and a much harder demo. I am giving you the right way. Talk to your landlord about options. – DMoore Feb 4 '16 at 15:46
  • On the post below you advise against putting concrete backer boards between the tiles and the floor. What would you advise I can do to raise the level? Putting tiles straight onto the concrete will leave quite a step down. I know there are levelling compounds, but all the walls are plaster partitions and it will be tricky not to have the stuff run away. – user1111284 Feb 5 '16 at 9:55
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My guess is that you're trying to raise the tile to match the existing hall floor level. For my fraction-oriented friends, the material that was removed is about 1 inch thick.

Foam is generally not suitable as a tile substrate. However, a direct mortar/cement bond with the concrete slab will do very well to dampen sound.

There's no reason that you can't install 1/2" cementboard in a bed of thinset mortar, then install your tile on top of that. A 1/4" by 1/4" notched trowel should do well. I suggest fiberglass mesh tape embedded in thinset for the seams.

This will get you nearly as high as the hall floor. A tapered transition strip would finish the job (if necessary).


As there seems to be enough controversy here to raise doubt, I'll mention another solution. Self-leveling underlayments, such as USG's Levelrock, is also a good approach. It's probably more expensive, somewhat difficult to work with, and can be messy, but you'll likely end up with a very nice subfloor.

  • I don't know of any cement board manufacturer that allows installation directly on concrete. Just thinking about it, I can think of a slew of issues with water getting trapped, securing the board to the floor, and air gaps between the board and concrete. I would not suggest that anyone do this. Also this costs a lot of time and money to do something that will cause a lot of problems. Concrete is a great surface, don't ruin that substrate. – DMoore Feb 3 '16 at 21:37
  • This installation will have no more water issues than concrete itself, which is already very porous. No, manufacturers will probably not explicitly permit such installations as they're a very rare use case. It's not worth the ink. – isherwood Feb 3 '16 at 21:44
  • Also, I've done this myself. 15 years later, no issues. Granted, that basement isn't regularly wet, but concrete in my area is always at least a little moist. – isherwood Feb 3 '16 at 21:47
  • I don't know. Durarock and HB love selling concrete board and they have a lot of uses for them. I am sure they would tell you you could install on concrete if they though it was a good idea. When a manufacturer tells me not to do something I listen if it is a big job, and layer board over concrete isn't fun. I have demo'ed concrete board over concrete because of issues in houses I bought but I don't know if I would blame the install method or the installers... – DMoore Feb 3 '16 at 21:56

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