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I patched a ~4x4’ section of subfloor in my bathroom where there was water damage. Glued and screwed the new plywood down to the joists (16” on center).

When I step on the patched subfloor, I can hear it cracking ever so slightly and am starting to see some hairline cracks running the length of the board.

My existing plywood is 5/8” (1980’s home). I used this plywood (5/8") for the subfloor patch. Did I use the wrong type of plywood? I was planning to add another 1/2” of plywood on top of the entire bathroom subfloor. Then 1/2" Durock. Then 12x12 or 12x24 tile. Will the extra 1/2" plywood resolve the issue?

I’d hate to have to rip the patched plywood out since about a foot of it sits under my tub that I am almost done tiling.

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    What is the floor covering going on top of the next 1/2"?
    – popham
    Jan 21 at 21:43
  • The next 1/2” will cover the entire bathroom subfloor. I’m adding 1/2” to raise the subfloor to the minimum of 1 1/8” to use durock cement board for tile floor.
    – Niko
    Jan 21 at 21:45
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    This is the second time I've heard that 1-1/8" around here from somebody doing floor tile. Where does that come from? The Durock people say 5/8": usg.com/content/dam/USG_Marketing_Communications/united_states/….
    – popham
    Jan 21 at 21:51
  • I picked the 1 1/8” minimum requirement up from a few sites while doing a google search. John Bridge forums in particular is where I have seen it called out.
    – Niko
    Jan 21 at 21:57
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    Looking at the TCNA Handbook, I see a 5/8" subfloor plus 1/2" plywood underlayment detail for installations without cement board. I suspect that's where the 1-1/8" comes from. There's a 5/8" subfloor plus 1/4" cement backer board detail for residential traffic, where alternatively 1/2" cement backer board provides a light commercial grade installation. It's detail F144 if you can look it up for specifics on the installation.
    – popham
    Jan 21 at 22:29

1 Answer 1

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Plywood has a "strength axis." The fibers of the outer plies must span across the supporting joists. They can't run parallel to them. If you see hairline cracks, then that indicates that you ran them parallel and the flexing is opening and closing spaces between the fibers.

Your floor has about 20% of the stiffness of correctly oriented plywood. It has about 33% of the strength of correctly oriented plywood. See the 40/20 row from Table M9.2-1 on page 52 of the AWC's Manual for Engineered Wood Construction. "Strength axis" is defined in footnote 1, although the plywood suppliers have webpages that should talk about it while describing how to read APA panel marks.

The 5/8" minimum subfloor thickness specified for Durock is about stiffness, not strength, so the question is whether your floor will have an equivalent stiffness. Taking 5/8" OSB as the spec stiffness, the handbook table from above provides a stiffness of 225000 #-in2/ft. Your incorrectly oriented layer of 5/8" has a stiffness of 56000 #-in2/ft (same table), and the correctly oriented 1/2" will have a stiffness of 125000 #-in2/ft. Without gluing them together, the sum provides a net stiffness of 181000 #-in2/ft which is only about 80% of the spec's. If there's something like a bathtub blocking the 1/2" upper layer end from bearing on a joist, then that would be a soft spot. Wood glue between the 1/2" plywood and the subfloor would stiffen it greatly, though, where I suspect that even 1/4" plywood would be sufficient with glue.

You could add another 5/8" of plywood with 3/8" Durock if you were planning on 1/2" Durock to arrive at the same elevation. Smaller tiles tend to be more sensitive to subfloor stiffness problems than large tiles, so 12x12 or 12x24 tiles have a better chance of tolerating a below spec stiffness than mosaic tiles.

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  • I wish I had done my research…Do you think adding another layer of subfloor on top of the patch in the opposite orientation would work?
    – Niko
    Jan 21 at 21:44
  • @Niko, what's your floor covering? If tile, what size of tile?
    – popham
    Jan 21 at 21:46
  • I’m going to use 1/4” durock and then a 13x13” tile, though the wife was thinking to change it to a 12x24” tile
    – Niko
    Jan 21 at 21:54
  • I appreciate the concise response! One last question - would 3/4” plywood + 1/4” durock be acceptable as well? 3/8” durock isn’t readily available and it was my understanding that floors should use 1/4 durock boards. Also thinking the 3/4” board will add more stiffness.
    – Niko
    Jan 21 at 22:57
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    @Niko, the Service Rating section of detail F144 from the TCNA Handbook: "Light commercial with minimum 23⁄32"-thick subfloor and minimum 1⁄4"-thick cement backer board." Your proposal is satisfies it.
    – popham
    Jan 21 at 23:28

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