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Harker
07-02-2019, 05:58 PM
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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cx
07-02-2019, 06:04 PM
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.

Harker
07-02-2019, 08:48 PM
Glue yes, T&G was not available anywhere locally in 5/8.

cx
07-02-2019, 09:56 PM
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.

Harker
07-02-2019, 11:25 PM
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.
Larry

Kman
07-03-2019, 01:56 AM
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.

Harker
07-03-2019, 06:20 AM
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?

Kman
07-03-2019, 07:43 AM
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?

Harker
07-03-2019, 08:17 AM
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?

Kman
07-03-2019, 08:55 AM
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. :)

zeaflal
07-04-2019, 03:09 PM
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.

cx
07-04-2019, 04:21 PM
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. :)

Kman
07-04-2019, 07:49 PM
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. :dunce:

zeaflal
07-04-2019, 08:35 PM
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.

Harker
07-05-2019, 02:30 PM
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.

PC7060
07-05-2019, 03:40 PM
Plywood is fine; glued and screw as recommended by CX. Use 1 5/8” grip rite screws with locktite PL adhesive.

cx
07-05-2019, 04:52 PM
What PC said.

And pay attention to what Kevin said in post #6: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 don't want your added block touching the joist on either side on accounta it can cause squeaks.

My opinion; worth price charged.

Kman
07-05-2019, 08:33 PM
Plywood strips would work fine. Remember, you're just joining the two edges of the plywood together so they don't move independently of each other. I would use screws that are 1 1/2 -2" long, no longer than that.

They don't have to be attached to the joists, although it doesn't hurt if they are. Of course, you can't do that with plywood rips, just 2x's. And if you decide to use dimensional lumber, 2x4's will work just fine.

Harker
07-06-2019, 03:31 PM
Which you be better, blocking connected to the joists or just going across the joists bay? Going across seems easier, no risk of squeaking, but less secure? In one location I have to connect to joists, should I use glue in that location to avoid squeaks?

Harker
07-06-2019, 05:35 PM
This is what I’ve started doing. Just wanted to confirm this is what you all had envisioned. You can see the butt joint at the top. There are 4 screws sunk from above 1/2 inch from the plywood joint. Where I can only screw from below see second image. If this isn’t right please let me know.

Harker
07-06-2019, 05:38 PM
2nd photo

cx
07-06-2019, 05:44 PM
I recommend using glue on all joints in a subfloor construction. Did you put glue under that piece we see in the photos?

Not sure how you could have a situation where you couldn't screw into those blocks from the top, but what you've got there is probably adequate if there's glue involved.

My opinion; worth price charged.

Harker
07-06-2019, 06:24 PM
Shoot. I did not glue these. The gaps between the plywood sheets were glued. Will have to pull these and redo. As for the areas that can’t be accessed from above I put leveling concrete down in a couple of dips. I very much appreciate your input though you are not my back’s friend.

cx
07-06-2019, 06:38 PM
You might survive with what you've got, Larry, but I'd take those blocks off and do some gluing, 'specially since you've probably used screws that are technically too short if you didn't screw all the way up through the top layer.

Harker
07-06-2019, 06:56 PM
Will do. On say and 8x8 block how much should be covered in glue?

cx
07-06-2019, 10:14 PM
I dunno. Partly that's because I would never use an 8x8 block nor would I use that sort of 1x material it looks like you have there. And partly because I dunno what kinda glue you're using. And partly because I don't give it a whole lotta thought when I do it, I just put enough glue on there to make me comfortable that my two edges will be solidly supported.

On a 5-inch wide rip of 3/4" plywood I'd likely run two beads of PL Premium construction adhesive the length of the piece on each side of center such that the edge of each of my subfloor panels would be over the two beads. You could use other products, including wood glue if your pieces fit properly.

When doing existing subflooring, I'd generally apply the glue, tack or screw the piece in place from the bottom and then screw each subfloor edge through the piece from the top. End up with a whole lot more support than even a glued T&G edge, but it's a place where I like a little overkill.

When installing square-edge panels from the top, I glue and screw the rip of plywood under the edge of the first panel with half the rip extending out such that I can then glue and screw to it when I lay the second panel. Much easier to do than try to properly fit T&G edges in tight quarters.

Just make sure your panel edges are fully supported very permanently so there is no possibility of any differential movement between your subfloor panels.

My opinion; worth price charged.