Another plywood job

[f24c8c]

Hello All,
Read as many of the items here and elsewhere on laying tile on plywood so here is my case and i'm hoping for some advise. I'm laying over a subfloor of 5/8" OSB sitting on 16" centers on 2x8 joists. Span for 2/3 of the job is 10ft and the last third about the joist span is about 11ft.

The plan was to use 1/2" fir on top of the OSB. Saw some debate over to screw vs nail, to glue vs nail, but plan was to glue the 1/2" (predrilled for screws), screw it down (grain oriented appropriately, screws not penetrating joists). Then apply my modified thinset (FlexBond) to that. I would have entertained 1/2" CBU but i feel that the 5/8" OSB and no contribution of stiffness from the CBU that the deflection might be excessive.

1) Besides thoughts or recommendations on this approach is what i considered the simplest aspect (the plywood), but after reading the CDX stuff, etc I'm wondering what is it that I need and ask for at my friendly Home Despot/Loews? Is it BCX and will most of these guys have a clue what that is. An I assume this is non-sanded exterior type stuff?

2) Now assuming i'm still screwing and gluing ply to my OSB by the end of the forum dialog ;-} , I was planning on using flooring screws of appropriate length to secure the ply to the OSB. Since the thinset will be on this ply layer is there any corrosion problem? since i notice the CBU screw seem to have a more substantial coating, or is a treated screw similar to a decking screw a better idea?

Thats about it for now. Looking forward to walking on anything other than OSB, thanks!

-roman

[f24c8c]

Thanks for the speedy response!
The existing OSB is "OK" but overall I think it needs some bolstering.
I'm assuming these exterior grades will typically be unsanded (Just browsing the APA plywood guide)?

Is the main reason for not gluing to preserve the subfloor for the next flooring iteration (hopefully not in my lifetime)?

Now, only screwing down into the OSB 1/4" vs the full 1/2" or 5/8" is that to essentially secure it, but essentially decouple it from the sub floor? However, If there was going to be CBU or Ditra or something on top of this 2nd layer, then a person would typically secure the ply to the OSB with longer screws?

Given a reasonably budget conscious scenario, assuming my joist geometry, and the fact i'm more of less stuck with the 5/8" OSB sub floor (that i'll classify as "OK" in terms of stiffness for lack of a better term), and I'm going to be butting up to 3/4" hardwood on same sub floor, what would be the recommended approach keeping a reasonable height?

Thanks!

[Jayme568]

Given that you were going to use half inch plywood to begin with, why don't you use exterior glue 3/8 plywood over the 5x8 then use Ditra? Heights would be approximately the same, but a much more stable substrate to tile over.

[cx]

Welcome, Roman.

The primary reason for not gluing the subflooring layers, especially if tiling directly to the top one, is to gain the uncoupling effect. Testing shows it can make a difference. Another reason is that OSB just doesn't glue well.

Originally Posted by Roman

Now, only screwing down into the OSB 1/4" vs the full 1/2" or 5/8" is that to essentially secure it, but essentially decouple it from the sub floor?

Not what Injineer Bob meant. By "penetrate the OSB by 1/4" he means you go alla way through 'till there's a quarter-inch stickin' out the bottom.

My recommendation for your installation would be to add a minimum of nominal half-inch exterior glue plywood and then your CBU or other tiling membrane. Get you a useful subfloor for your tile installation and then deal with any necessary transitions to adjacent floor covering. Transitions to hardwood flooring are among the easiest to do.

My opinion; worth price charged.

[trevor_ash]

Roman,

I just recently went through the whole "what plywood do I need, I'm confused by all the choices" game and will try to share what I wish I knew before I started.

1. Google "APA stamp" and learn how to read them.
2. Besides dimensions, you're interested in the Exposure Rating and Grade.
3. Simplified, exposure rating is how well it can handle the elements (like water). Exposure 1 means it can handle limited exposure.
4. Simplified, grade is the quality of the face veneers.
5. Grades range from A (best) to D (worst).
6. Grades are listed in pairs since there are two faces to a piece of plywood (usually one face is higher quality than the other)
7. One special grade is called "Sheathing". It's my understanding that it's equivalent to C-D.

The biggest confusion I had was item #7. I kept looking at the plywood and couldn't find anything on the stamp that had AA, AB, CD or whatever on it (not realizing that sheathing WAS the grade). Finally, I found the "nice" plywood in a separate part of the store and low and behold I finally saw some AB type grades. It was in the "Finish" lumber section as opposed to the "Construction" section.

While you're learning, you may as well learn what treated lumber is and it's purpose.

If you know all of this, I apologize. But I finally pieced it all together when I inspected every piece of plywood in the entire store

The experts will correct my errors (which I'm sure I have made) but hopefully I'm not leading anyway too astray.

[Shpedly]

Originally Posted by Trevor

The biggest confusion I had was item #7. I kept looking at the plywood and couldn't find anything on the stamp that had AA, AB, CD or whatever on it (not realizing that sheathing WAS the grade). Finally, I found the "nice" plywood in a separate part of the store and low and behold I finally saw some AB type grades. It was in the "Finish" lumber section as opposed to the "Construction" section.

There has to be a grade somewhere on the sheet or the shelf label. If your shopping at the big box stores, your most likely to find exterior grade BC, which is fine. You dont want or need to pay for cabinet grade AB plywood for under your tile. You probably know that, but just making sure.

[trevor_ash]

Thanks. I wasn't suggesting buying the fancy lumber. I mentioned it because it was the first time I was able to confirm that I wasn't reading the labels wrong, there just wasn't an A thru D number on the ply. Maybe it's the store or the ply they were selling (Georgia Pacific stuff I think).

[cx]

You've got it down pretty well there, Trevor.

Originally Posted by Ed

You dont want or need to pay for cabinet grade AB plywood for under your tile.

Actually, you wouldn't want any kind of cabinet plywood at all anywhere in a subflooring application, Ed, even if it were less expensive, eh?

And the grading is different when a fella gets into the "hardwood" plywoods.

[Shpedly]

Cx, As you said, Trevor's "got it down pretty well there". I think he unnerstand the "dont want" part of my sentence.

[PetrH]

if it is awailable in your area, look into 3/8" halex. It's a 7 ply birch plywood specifically designed for tile underlay and will give you the strength of a much thicker conventional plywood. It will be a bit more expensive, but well worth it.