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Unread 05-31-2007, 12:53 PM   #1
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1/2" Plywood Subfloor - Deflection?

I am getting ready to tile a bathroom in our 1961 house. I want to be certain that it is done right and I am concerned about deflection. I will do my best to describe my situation. I hope you guys can help me out.

The room currently has 2 layers of linoleum, 1/2" particle-board, roofing-felt, and finally 1/2" plywood sublfoor. There are 8" Floor joists on 16" center. The room is roughly "L" shaped (10' on the long side, 6' on the short side, and about 3' wide either way). The joists run parallel to the long-side of the "L" and in the basement there is a wall (and doorway) at about 8'.

So, effectively I have an 8' span with 8" joists (16" o.c.) and 1/2" plywood sub-floor.

My local tile shop suggested that 3/4" plywood is preferred but, that I could address it by using 1/2" Durock (+ mud and screws) over the plywood.

Is that sufficient? Would it be stronger to do 1/4" or 3/8" plywood, then 1/4" Durock/Hardibacker (+ mud/glue and screws)?

It will be MUCH more work to rip-out and replace the entire sub-floor because I would have to cut at the "toe" of the tub and vanity (or remove them entirely). Both are in nice shape, the vanity custom-sized to fit the walls, and the 1/2" particle-board does NOT go under either one (but of course the plywood does). I really prefer not to mess with the 1/2" plywood.

Please tell me what you can. I sincerely appreciate any advice!


In case it matters: Tile will be 12" and 6" porcelain laid in a hopscotch pattern with 1/4" grout lines.
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Unread 05-31-2007, 12:59 PM   #2
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Hold the phone!

I just clicked on "Deflecto" (above). It tells me that I am ready to roll even with my 1/2 plywood.

Can it be so? Am I dreaming?

This would let me use 1/4" Hardibacker and end-up flush with the hardwood floor in my hallway. I would really like that.

What is the catch?

Please still send your input! TIA!

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Unread 05-31-2007, 01:21 PM   #3
Brian in San Diego
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Don't want to be the bearer of bad news, but the deflecto is used to calculate the joist structure. It has no input for the plywood over the joist structure. The other type of deflection (here comes the bad news) is between the joists (i.e. the plywood). You are on the shakiest of ground here. I don't think there's a profeesional here that would encourage tiling over 1/2" plywood, but I could be wrong. I think most will tell you 5/8" minimum and 3/4" is better. That is for ceramic. Natural stone is an entirely different beast altogether. What the folks at the tile store forgot to tell you (or perhaps they don't know) is that CBU offers no structural support to your subfloor.

Hold on for some additional input. I think if it were mine I'd add 3/8" (the minimum that's structurally stable) to your existing and then use ditra over that. You'd be adding at least 1/2" plus tile to your existing subfloor.

If that doesn't work, I'll always think it should have.
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