My wife and I recently purchased a house (our first!) in which the previous homeowner added a shower stall that sits on top of a stand constructed from plywood to accommodate the shower drain.

After a couple months of use, I noticed several of the tiles were not sitting flush. At this point, I realized the contractors had used caulk to join the tiles together instead of grout. I pulled up the loose tiles and noticed some black mold spots underneath. Water was leaking through the gaps in the caulk. Being cautious, I pulled up the rest of the tile and scrubbed/bleached the surface.

I could use some advice with the path forward. I know how to do tiling, but I'm not sure if there's a correct underlayer to put between the wood and the tile. I don't want to remove the shower if possible, but I'd like to minimize water infiltration and the recurrence of mold.

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  • Can you get under the shower stall and see if the drain is leaking? If so everything must come out. If the drain is intact and not leaking, cover the OSB with cement board or tile backer and lay your tile on that with the proper thinset mortar and grout. Even with that the base may not be sturdy enough to prevent tile from cracking or coming loose. Sorry, but I just see this as a bad idea.
    – RMDman
    Commented May 20 at 21:09
  • It's worse than that, it's Oriented Strand Board (OSB) which turns into Wheetabix® when it gets throughly wet. You'll be ripping the whole thing out to redo it correctly. If it was exterior or marine grade plywood, you might stand a chance. Even interior plywood takes water better than OSB.
    – Ecnerwal
    Commented May 20 at 23:02
  • Yep, that OSB is what make me see this as a bad idea.
    – RMDman
    Commented May 21 at 0:20

2 Answers 2


This is a prefabricated shower cubicle, so assuming it's installed correctly, it's almost certainly not leaking.

Tiles (in particular the grout between them) are not waterproof, so to prevent the OSB from getting damp you'll need back them with a waterproof layer. the location of the first wet footprint seems to be where there is most mould.

  • Your second sentence needs revision. It's a bit of a mess. Is it supposed to be two?
    – isherwood
    Commented May 21 at 14:11

It's not the tiles being caulked that was causing the leakage, it's that the substrate was incorrect—it'd still have leaked if the tiles had been grouted.

The good news is that since it's a prefab shower unit, you still have the ability to fix it—remove the shower and the OSB, then install CBU or some "system" (Schluter, etc.) material instead and re-tile. Make sure the drain is done correctly too!

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