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Unread 02-27-2020, 07:09 PM   #76
vbtalent
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Dan, what's the finished floor height with 1/2" ply? 3/4 + 1/2 + 3/16 + 1/8 (Ditra) + 1/2 = 2"?


CX, Dan recommended DITRA.. you'd go with 3/4 + 1/4 + 1/2 (CBU) why not the Ditra??
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Unread 02-27-2020, 07:14 PM   #77
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The difference is that you'd be bonding the Ditra to the existing 1/4" plywood, which may or may not be flat against the subfloor in all places and result in some minor, but vertical, movement.

That same condition of the 1/4" plywood exists under the CBU, of course, but the CBU would be fastened with mechanical fasteners every 8 inches or so and might mitigate some of that problem. Still not a good idea, but a better idea.

You would not, by the way, use a half-inch CBU unless you just want the additional height. All manufacturers recommend their thinner product for floor applications.

My opinion; worth price charged.
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Unread 02-27-2020, 09:24 PM   #78
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So, a 1/4" CBU (https://www.lowes.com/pd/DUROCK-Bran...ard/1000383885) on top of the 1/4" plywood fastened directly to the floor joists, got it.

1/4" CBU would meet my floor height requirement and although I realize not a perfect install - one that should be acceptable assuming not a lot of vertical movement, if I understand correctly.

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Unread 02-27-2020, 10:20 PM   #79
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Nope.

You do not want to fasten the CBU to the joists. You want to fasten the CBU only to the subfloor with screws that fully penetrate the subflooring per the schedule specified by the CBU manufacturer. And you must read and follow the CBU manufacturer's instructions carefully, including the bed of thinset mortar under the CBU panels.
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Unread 02-28-2020, 05:37 AM   #80
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When reattaching some of the 1/4" plywood to the sub floor is it preferred to use construction adhesive or simply nail and screw it to the floor joists?

Can I also use the same thin set I was going to use for setting the Ditra for setting the CBU?
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Unread 02-28-2020, 07:37 AM   #81
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I suggested removing the 1/4" due to the reasons cx outlined; you just don't know what's under it, how well it's attached everywhere, and to provide an opportunity to screw down the 3/4", thus being certain IT is firmly attached. I suggested the 1/2" on top to further decrease the between joist deflection of the 3/4", making for a very robust substrate, thereby bettering the odds of a long lasting installation.

If you leave the 1/4" ply, add 1/4" backer, you don't gain any significant subfloor stiffness, you only gain height.

But that's how I would do it (and it should be acknowledged that I'm a OCD afflicted DIY'er), if doing so did not create too much of a height difference between the bath and adjoining room.

IF you simply don't have the height, I'd still strip the 1/4" (because not knowing how the 3/4" is attached would make me crazy), then just add whatever combination of backer and membrane thickness is necessary to reach the height you need.

The 1/4" ply is just a spacer, and as installed creates variables that I just wouldn't be comfy with. Just my opinion though, jah?
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Unread 02-28-2020, 08:55 AM   #82
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ss3964spd, I hear all of the items you're saying. The subfloor is attached via nails and it looks like the 1/4" is stapled, it's very flat and very stable.

My plan is to screw the existing 1/4" ply to the floor for the area that is already there to firmly attach it to the floor joists. Then to attach new 1/4" ply where needed using construction adhesive and also to screw it to the floor joists.

Then to lay a CBD on top of that staggering the joints and not screwing to the floor joists..??

Thoughts on the construction adhesive??

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Unread 02-29-2020, 05:18 AM   #83
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No thoughts on construction adhesive between the top 1/4" layer and the sub floor?
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Unread 02-29-2020, 06:47 AM   #84
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No construction adhesive, VB, the gunk is thick and will leave voids between the beads - even after they're smooshed down. If you're compelled to glue it use a waterproof wood glue and spread it evenly and completely over the area. Work fast, don't want it to skin over.
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Unread 02-29-2020, 11:42 AM   #85
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While a good wood glue will make a stronger lamination, if you don't do it well, you'll end up with a less desirable solution. Should you go that way, use a small notched trowel to evenly spread the glue. Note, all of the TCNA guidelines assumed you've screwed it down per the recommendations. Construction adhesive is just too thick over a large area to get that needed full coverage. And, it costs lots more than a good wood glue. WOrks great on the top of a joist, since there's relatively small area when attaching the ply to it for your subflooring.
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Unread 03-01-2020, 05:09 AM   #86
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Thanks... the project rages on, lol!
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Unread 03-02-2020, 08:06 PM   #87
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Can I use the same thin set I was going to use for setting the Ditra for setting the CBU?
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Unread 03-02-2020, 08:29 PM   #88
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Probably, but I dunno what you were fixin' to use for the Ditra.

Bear in mind that the thinset mortar under the CBU is for the purpose of filling any voids and providing a 100 percent footprint for the CBU panels, not for bonding them. The least expensive thinset mortar available to you is probably adequate to the task.

And even if you were bonding, you'd be bonding to the stapled down 1/4" plywood rather than an appropriate subfloor.

My opinion; worth price charged.
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Unread 03-04-2020, 05:17 AM   #89
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Gotch on the flattening of the surface as opposed to bonding for the CBU...

I bought Versabond for the Ditra and I assume that will suffice for the leveling?

VB,
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Unread 03-04-2020, 07:12 AM   #90
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"Leveling", VB? Not sure what you're attempting to level.

The VB (no relation) will be fine for setting the CBU's and for adhering the Ditra.

EDIT: The "regular" VB is what you want for the two applications above, not the "LFT" variant.
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