I am installing a 12" granite floor in a bathroom- 40 Sq. ft. Currently, there is a 3/4" OSB subfloor. My plan is to add a 1/2" Exterior Grade Ply to this- screwed down. I've read on this forum that the second layer should not be screwed to the joists, but only to the OSB. Is this correct? Which direction should I orient the ply? The OSB is 90 degrees to the joists.

Subfloors always go across the joists. The addition of the second sheet of plywood is mandatory as the subfloor for stone tiles. What are you installing over that? A cement backer, membrane such as Ditra?

Jaz

I will be installing a 1/4" cement backer over the ply with thinset. I prefer Fiberock, but I'm not sure I can get it around here anymore. I might have to use hardi.

Just to clarify- do I want to screw the ply to the joists or to the OSB, as I've read in other posts?

I will be installing a 1/4" cement backer over the ply with thinset. I prefer Fiberock, but I'm not sure I can get it around here anymore. I might have to use hardi.

Just to clarify- do I want to screw the ply to the joists or to the OSB, as I've read in other posts?

Also, what is the difference between exterior grade ply and interior? What is the advantage for this application?

Use screws that equal the thickness of the total subfloor/underlayment and do not screw the underlayment layer to the joists.

You should use an exterior grade plywood for your tile floor underlayment. It is constructed with waterproof resins.

Interior grade plywood is generally made from a lower quality grade of wood.

I am screwing 1/2" ply over an OSB subfloor. Should I screw the ply to the joists?

Also, I planned on butt jointing the ply together, if I remember correctly from my last tile experience. Is this correct? Thanks!

RawDeal,

Randall just told you what to do about the screws.

Plywood sheets are always gapped about 1/8". What does it say in the instructions of your thinset mortar? What brand and type will you be using.

We didn't talk about your joists system yet. This is very important especially with natural stone tiles. What kind and size are they, also grade, species, spacing and unsupported span?

Jaz

I will be using Flexbond- Marble and Granite Thinset- it's $26 for 50 lbs. at Home Depot. Should I also use this for the thinset under the hardi or am I okay with a cheaper mortar, like Versabond, as long as it is modified? Also, should I use water or the flexbond additive for better flexibility?

RC states that I should not screw the underlayment into the joists. I am familiar with only referring to the backerboard as the underlayment. Does this mean the plywood as well should not be screwed to the joists? I guess I'm still not clear on this because of the equivocal language.

I'll get back about the Joists with specifics. I know they are 2x4 construction like a truss. Thanks again for the assistance.

Any additional plywood layer thats installed over the existing subfloor should not have any fasteners that attach to the floor joists.

Verabond is fine for the thinset under the Hardie.
There's no need to add a latex additive to a modified thinset.

Thanks Randall, I appreciate the assistance!