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I have to rip out a rotted section of sub-floor in a handicapped bathroom. It's 3/4 in. osb with 1/4 in. plywood on top. The contractors that had built this apartment just 4 years ago put no cement board down and needless to say it's in bad shape. Now comes the interesting part, everything I've been researching says says I need at least 5/8 in sub, then some kind of backer board. Now that's all fine and dandy, but the problem is the company wants it tiled. I only have an inch to play with when it comes to the lip of the shower stall, and tile is 3/8 in. thick ... how am I supposed do this?

Pᴀᴜʟsᴛᴇʀ2
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joe in nd.
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  • What does the lip of the shower stall have to do with the tile install? Maybe a picture would help... – Jack Mar 05 '15 at 19:19
  • How did the shower lip work before with 1" substrate and tile? Why can't you replace the subfloor with 3/4" T&G plywood plus 1/4" cementboard and arrive at the same thickness? – isherwood Dec 02 '15 at 01:50

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You could frame below the level of the rough floor and have the subfloor flush with the joists. That would give you room for 7/16 hardie board, glue, and tiles. (But what a huge pain!)

Alternately, get vinyl and your thickness problem goes away.

I'm concerned that if something as mundane as a floor failed, then what of the shower stall? Isn't it going to have the same (if not worse) problems? If that's a goner, then you don't have a thickness problem any more...

Aloysius Defenestrate
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Some ideas you can explore:

Can you get a single piece of subfloor to cover the entire tiled area? If you can avoid joints in the subfloor altogether, you might be able to use some shower membrane (like Redgard) instead of backerboard. If you do have joints, can you tie them together really well? Use a CDX plywood, not OSB. you'll probably have to use mastic instead of thinset to bond the membrane to the subfloor.

What is your joist spacing? Maybe you can get away with thinner subfoor if you use CDX plywood instead of OSB? I thought I read somewhere that 7/16 was OK for CDX on 16-in centers.

Along the lines of Aloysius's answer, use a thin subfloor, but glue and screw additional almost-joist-spacing-wide strips of plywood to underside the subfloor. This will reduce the flex between joists, but I doubt this will meet load requirements.

Paul
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