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Unread 07-02-2019, 05:58 PM   #1
Harker
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Challenging tile installation

Hi,
I’ve tried to find a similar situation in the forums but I haven’t come across it. My house was built in the 1920s. When I removed the pre-existing tile it was set in 3 1/2 inches of concrete over 2 x 10 floor joists that were spaced 12 inches on center. The tops of the joists had been tipped (made into a triangle). I then sistered New two by eights to the pre-existing floor joists to provide a semi level surface. Due to space limitations I then placed a 5/8 inch plywood over that with a plan to then put Ditra and install tile over that. After reading here that 5/8 inch plywood is the minimum acceptable subfloor for tile installation I am a bit concerned. That being said we do have 12 inch on center floor joists. If anyone has any thoughts on this I’d really appreciate their input. The subfloor is very stable with almost no bounce.
Thank you.
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Unread 07-02-2019, 06:04 PM   #2
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Welcome, Larry.

I would personally never start a wood framed subfloor with anything less than nominal 3/4" plywood or OSB. Your 12" joist spacing certainly helps, and Schluter considers nominal 5/8ths" plywood sufficient over 16" joist centers, so you're at least above their minimum requirements. I still wouldn't wanna tile over it, but you certainly can.

You did, at least, use T&G plywood? You glue it to the joist tops?

My opinion; worth price charged.
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Unread 07-02-2019, 08:48 PM   #3
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Glue yes, T&G was not available anywhere locally in 5/8.
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Unread 07-02-2019, 09:56 PM   #4
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You've got a problem there, Larry.

Even with your 12" joist spacing you absolutely must have either center matched (T&G) edges on those panels or blocking under the between-joist seams.
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Unread 07-02-2019, 11:25 PM   #5
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If I have to I can put additional blocking in from below. It’ll be a lot of extra work as it’s not all open but it’s doable. The planned tile is 12x12. Please walk me through what you think will happen if I leave as is. Does putting Ditra on top of the plywood mitigate any of your concerns? Thank you.
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Unread 07-03-2019, 01:56 AM   #6
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There's a couple of ways to put blocking in at the plywood seams. But the best way is from above, not below.

You can use 2x blocks, or rips of 3/4" plywood, but whatever you use has to be the width of the joist bay, minus a 1/2" or so.

You'll want to screw them into place, with half the blocking on either side of the seam. The tricky part, in your case, is going to be getting the screws to go through the plywood subfloor, but not penetrate the surface. The appropriate length screws will work for that, as long as you don't over-drive them.

One other way, which involves a little more work, is to use a 2x material and screw it in place to the joists, tight to the subfloor. Then you can screw the plywood to the blocking from above without worrying about the length of the screws. 2" screws would be plenty long. Or, if you have a helper, they (or you) could hold the blocking in place underneath while the other one of you screws through the plywood from above.
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Unread 07-03-2019, 06:20 AM   #7
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Unfortunately will need to go from below. The subfloor is glued and nailed already. The ceiling below is plaster. I’m planning on using an angle grinder with diamond blade to get to what I need to. Do I need to put construction adhesive on the tops of the pieces or not needed? Any better way to get through the plaster?
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Unread 07-03-2019, 07:43 AM   #8
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You know what I would do in your situation, Larry? I'd forget the blocking and add another layer of plywood. That would strengthen the whole subfloor as well.

You'd just have to make sure to offset the long seams in the new plywood from the corresponding ones in your subfloor by at least six inches.

Is that an option?
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Unread 07-03-2019, 08:17 AM   #9
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Unfortunately no, there just isn’t enough space. I’ve only got about 3/4” in between the subfloor and the lip of the adjacent hardwood floor. If I add more plywood I’ll wind up way above the adjacent flooring. I should’ve cut the tipped floor joists to give myself more room but I was told by a local contractor that 5/8 plywood was sufficient. If I understand properly the potential movement at the sheet overlap could cause the tile above to fail?
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Unread 07-03-2019, 08:55 AM   #10
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Quote:
Originally Posted by Larry
If I understand properly the potential movement at the sheet overlap could cause the tile above to fail?
It absolutely would, no question about it.

If it's your only option, then start working from below and get the seams blocked.
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Unread 07-04-2019, 03:09 PM   #11
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Temporarily ignoring for the moment the practical implementation details, would it work to place 2X blocking under the joint and then use screws from the top to pull both sides securely to the plywood? You would need screws with a smooth shank at least through the plywood.
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Unread 07-04-2019, 04:21 PM   #12
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That would work just fine, Larry, and I've frequently recommended it here, except that I recommend using a 5 or 6 inch rip of 3/4" plywood instead of 2x material. I also recommend some construction adhesive.

Whole lot easier to do when the panels are being installed, though. But you knew that.
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Unread 07-04-2019, 07:49 PM   #13
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Quote:
Originally Posted by zeaflal View Post
Temporarily ignoring for the moment the practical implementation details, would it work to place 2X blocking under the joint and then use screws from the top to pull both sides securely to the plywood? You would need screws with a smooth shank at least through the plywood.
Apparently my explanation of that wasn't very clear in post #6 above.
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Unread 07-04-2019, 08:35 PM   #14
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Clear enough now Kevin. Some days I am just blind. I saw the part about screws not penetrating and led myself down the wrong path.
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Unread 07-05-2019, 02:30 PM   #15
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I’m not following some of the suggestions. Are you saying I could put plywood from below spanning the butt joint? My original thought is to put 2x8 blocking from below. There is one place where this will be very difficult as there is x-bracing right where 2 pieces of plywood come together. Putting plywood in the location would be much easier than cutting blocking to fit.
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