[ChuckHammer]

Hello all, first timer here. I'll keep it to the point.

Old-ish house (early 1970s), discovered plywood on all three walls surrounding shower when removing old tile. Can I just screw CBU directly over this old plywood without using thinset in between?

Thank you for any responses!

 

[jproffer]

I wouldn't. The thinset is meant to fill any voids behind the CBU. If you leave it out, and if the plywood isn't perfectly flat (which it probably isn't), the wall could flex and crack the tile and/or grout.

 

[ChuckHammer]

jproffer. Ok. Then I need to remove all the old plywood and sister new studs to the old studs to get back the 1/2 in of plywood (actually more than 1/2 in). I was trying to avoid having to do this. Thanks.

 

[jproffer]

I dunno about that....

If the plywood is in good shape and tight, thinset and CBU over the top of it. On a wall, flex isn't such a problem, but it's still possible.

 

[ChuckHammer]

You're right, it's not high quality ply. I think I'm going to remove it. What I didn't mention was that on top of the plywood was 1/2" drywall, then on top of the drywall was some kind of board I had no idea what it was (1/2" as well). I took a piece of it to my local hardware store and they didn't know what it was either. It is brown in color and crumbled like a kind of clay. On top of this mystery material was the tile. So I need to add 1" of furring or add sister studs to get flush with the wall. Leaving the old ply in would make an easier job.