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Recently I had a water leak in the bathroom. I noticed water spilling out from the tiles grouting and I removed them to prevent any more water penetration.

I noticed that the following layers have been used:

  • Ceramic tiles

  • glue/cement

  • 3mm underlay

  • carpet glue (or some kind of glue)

  • plywood

The underlay did a remarkable job of absorbing many liters of water but I cannot find what kind of underlay it is.

I am about to redo the work and I am asking around for advice. Some professionals strongly recomment AVOIDING the use of the underlay.

So.. My question is.. Should I use underlay? or should I simply glue the tiles on top of the plywood?

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  • Might as well make it heated while you're down there :-)
    – MDMoore313
    Commented Apr 24, 2013 at 15:48

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If you have wood underlayment that has been water logged, you really should get rid of it. i would remove all layers of old flooring and underlayments down to the primary flooring. If the primary flooring is still wet, dry it and seal it with a primer. If there are any signs of rot, replace it.

Let's assume the primary floor is salvageable. Tile should have a full 1.5 inches of solid base so flexing is not an issue. Any flex or bounce in the floor will cause grout to crack and tiles to come loose.

Tile is best laid over a cement based underlayment. Concrete board or "Hardy Backer" is a good choice. This underlayment should be screwed down and screw indents filled with leveler or embossing compound. Plywood sub-floors have been used for years and tile installed on them, but with that said, masonry underlayment is still so much better, just as easy to install and not much more expensive.

The main issue here is not to install tile over a wet or damaged sub-floor. It would be a waste of time and money when your tile lifts or the grout cracks. Mold is always a possibility where moisture is trapped.

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  • Masonry/cement based is easiest from a DIY perspective and cheaper than buying HB. It does seem like a pain to throw an inch of concrete on a floor but after you factor in cutting your HB, installing it, and filling gaps... Laying down an inch of cement is easier, just need to make sure you get it level.
    – DMoore
    Commented Apr 24, 2013 at 15:05
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    pouring and leveling an inch of concrete on a floor is a real trick. Stick with the hardy backer. Poured concrete on plywood is not a great idea, more water. Also, it can't be removed easily if one wants to change the floor at a future date.
    – shirlock homes
    Commented Apr 24, 2013 at 21:40
  • I have done this in bathrooms since I was ummm 15. One of my first times doing this was my mom's guest bathroom. I retiled her floor a few years ago and kept the concrete. Not sure why it would be removed. I chiseled the crap out of the old tile to get it out and just a few chips in the concrete after...
    – Tom
    Commented Apr 25, 2013 at 5:30
  • Shirlock - Never had an issue with concrete on plywood. Can you expand on this? I honestly do it this way because it has worked and it is cheap/easy. Mix a batch of concrete and laying it is maybe a 2-3 hour project. Given I have not done this in a huge bathroom before. Just wondering the issues you think there is with this. Also wondering what instances you have found that people want to remove this subfloor - I would LOVE to find this under any tile removal. Maybe I am not thinking of all angles.
    – DMoore
    Commented Apr 25, 2013 at 17:02

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