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I'm converting a bedroom into a bathroom. It looks like the current subfloor is 1/2" MDF or Particle board (I'm not really sure). This sits on top of 2x8 tongue and groove.

I've read that MDF and Particle Board both are not ideal for bathrooms because of how they retain moisture. We plan on using ceramic tile throughout the whole bathroom so I was wondering if I should replace the subfloor with something else like plywood or if I can just use some sort of underlayment. If I use an underlayment, what should I use?

Here's a close-up of the subfloor: enter image description here

Here's a picture of the room we're converting: enter image description here

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    FYI, you're asking about underlayment. The solid wood boards you mention are the subfloor.
    – isherwood
    Aug 1, 2016 at 20:35
  • The white stuff is the dust that is leftover from taking down all the drywall. It's much cleaner now, but that's what it looked like before cleaning up. Or are you referring to the white stuff on the studs?
    – bigmike7801
    Aug 1, 2016 at 20:47

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Your underlayment is particle board and is not suitable for ceramic tile. Most tile mortars specifically disallow bonding to particle board.

Replace the underlayment with 1/2" cement board or 1/2" (or thicker) CDX plywood, or overlay 1/4" cement board if finished height isn't a concern. Either option should be screwed down thoroughly using corrosion-resistant screws in an appropriate grid pattern, such as 8" by 8".

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  • Would using a thinset on top of the particle board be an option?
    – bigmike7801
    Aug 1, 2016 at 20:48
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    No. "Thinset" is the mortar I mentioned. It's just a modern product that isn't applied in as thick a layer as with older mortar techniques.
    – isherwood
    Aug 1, 2016 at 20:56
  • you need to add a layer of wonderboard or hardy backer designed for mortar and tile
    – shirlock homes
    Aug 1, 2016 at 22:35
  • Do I add the wonderboard/hardy backer on top of the plywood or cement board or do I use it in place of the plywood/cement board?
    – bigmike7801
    Aug 2, 2016 at 14:06
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    My concern would be the ability to screw it down well, but your solid wood subfloor should take screws well (through the particle board). Then there's the possibility that the particle board gets wet and swells. That can ruin your day. You'd probably want to use a moisture barrier or seal the particle board.
    – isherwood
    Aug 2, 2016 at 19:17

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