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Unread 10-23-2020, 08:17 PM   #12
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 91,866
That second layer of plywood is structural subflooring, S, not underlayment. The CBU will be your tile underlayment. Yeah, yeah, I know what that article says, and what's in some of the TCNA Methods, but the second layer of subflooring would be an underlayment only if you intended to install tile directly onto it.

Another point: There is no CDX grade of plywood. Nor ACX or BCX. There are AC and BC and CD grades. The X we all see now in the big box stores is, I suppose, to represent the exposure rating, but that rating should be found in the grade stamp and expressed as EXT or Exterior or Exposure 1, all indicating an exterior glue was used in the manufacture.

End of rant.

And I'll point out again that the Method 250 for Stone in the TCNA Handbook us for 16" joist spacing. You don't have that. Nor the requisite L/720 joist deflection. Also, I think you'll not find a method in that handbook for using CBU as a tile substrate with any joist spacing exceeding 16" on center. Individual manufacturers' of those products may have differing recommendations.

All that said, you install your CBU per the CBU manufacturer's instructions. If they don't indicate doing anything with the subfloor panel joints, don't do anything. Just pretend they're not there. No, it's not intuitive to intentionally create those joints and then fill them with mortar while installing the CBU, but it's done all the time. I'm told there are some membrane manufacturers out there that now recommend bond-breaker tape over the joints, but I've not see any of them in writing. I know at least one has always said to pay no attention to the joints; don't fill them with mortar, don't try not to fill them with mortar.

If you think it better to tape over them or fill them with a flexible sealant, I suppose you can do that and I don't know that it will make any difference at all. I do know that if you have a tile installation failure, the CBU manufacturer is gonna say, "We didn't tell you to put that caulking in there!" That'll be right after they say they never said you could install stone tile over 19.2" joist centers with their product. Or somethin' else.

But if you think it's all gonna work out well, I'll just hope you're right.

My opinion; worth price charged.

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