dadiOH wrote:
Dante M. Catoni wrote:
a) Is the ply firmly attached?

No, there was a drywall screw about every foot.

b) Ditto the "wavy" boards and what do you mean by wavy?

They are not perfectly flat. I don't think the very thin ply was
enough to compensate.

I think you're right. If you put down another layer of ply

That is, take off old, put down ne in mortar.

dadiOH