what to do with 1/2" sub-floor?

[lab17]

Thanks for the advice as I prepare for my tile job,been great to find this website and forum.
My questions is after removing my linoleum which was attached to 5/8" plywood there is only 1/2" plywood as the subfloor sitting on the joist (2x12, 16" oc). The deflection calculator says I'm good for anything including natural stone though I'm installing porcelain.
Can I install 3/4" over the 1/2" if I properly screw and am actually thinking of gluing the 3/4" to the 1/2" as well?
I know normally I would what the two plys switched with 1/2" on top but the 1/2" was screwed and an adhesive was used to additionally secure it to the joist- probably to reduce any possible squeaking.

[cx]

Welcome, Peter.

Sorry about your situation. Person or persons who installed that nominal half-inch plywood subfloor should be behind bars, but that's another issue.

The only way to make that subfloor usable for a ceramic tile installation is if the surface is in near pristine condition, you glue-laminate another layer of nominal half-inch plywood over what you've got using a full spread of wood glue and lots of screws.

If, instead, you elect to install your 3/4" plywood over what you've got, I recommend you do so as though the original half-inch material were not there. Fasten the 3/4" to the joists and consider that you have a single-layer subfloor that's thicker than the original.

My opinion; worth price charged.

[Kman]

And don't bother using glue between the 3/4" and 1/2" layers.

[jadnashua]

You sure it isn't actually a nominal 5/8" (today's ply is thinner than the 'nominal' measurement)?

One hassle with stuff that thin is that it won't hold a screw very well...it's really easy to strip the hole, and just have the screw spin and not hold. THus, the recommendation to treat it as it wasn't there and put screws into the joists, rather than avoid them, which is the normal procedure.

[lab17]

Its 1/2" as it comes out slightly less than 1/2".
Treating it as if the 1/2" is not there is a good idea, but I then would only have essentially 3/4" of substrate under the 1/4" CBU or uncoupling membrane. I'm trying not to go to much higher than the adjoining wood floor and that 1/2" ply is messing with my height now as it appears to be useless in its current position.

[rmckee84]

Removing the 1/2" is always an option. I install over 3/4" tounge and groove, with joists on 16" centers on a regular basis using uncoupling membranes without issue.