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Old 07-06-2016, 12:45 PM   #1
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Subfloor prep for Tile


Hi all,

I am about to tile my second floor bathroom (212 joists, 24in appart, 5/8in plywood subfloor).

Since the 5/8in plywood is not stiff enough for large tiles, I want to beef it up by adding a 3/8in or 4/8in layer of plywood then thinset then 1/4in hardie backer cement board.

Which minimum plywood thickness do you recommand that I add (as the rest of the floor is 3/4in hardwood Tongue & groove and I want to minimize the height difference)?

How should I attach the second plywood layer to the first one, there is a lot of contradicting information about that on internet:
  • Screw or not screw through the joist?
  • What distance between the screws knowing that when the backerboard will be screwed on top of that, the backerboard screws will probably reinforce the link between the two plywood layers
  • Glue or not glue the two plywood layers with wood glue or construction adhesive?

Thank you a lot for your help!
John
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Old 07-06-2016, 02:16 PM   #2
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Re: Subfloor prep for Tile


Screw the second layer to the first but miss the joists - sounds weird but you want those two layers to have a little ability to move independently of each other.

Hang tight on the thickness of plywood but in the meantime could you tell us the unsupported span of those joists?
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Old 07-06-2016, 02:32 PM   #3
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Re: Subfloor prep for Tile


Quote:
Originally Posted by stick\shift View Post
Screw the second layer to the first but miss the joists - sounds weird but you want those two layers to have a little ability to move independently of each other.

Hang tight on the thickness of plywood but in the meantime could you tell us the unsupported span of those joists?
Hi stick\shift,
If I screw and glue the two layers together (even missing the joists), How can they still move independently of each other?

Concerning the unsupported span of the joists, I'd say around 16ft.
John
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Old 07-06-2016, 02:45 PM   #4
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Re: Subfloor prep for Tile


I didn't say glue, just screw.

Hang tight for one of the pros, I'm beyond my range here for the rest of your questions.
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Old 07-06-2016, 08:00 PM   #5
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Re: Subfloor prep for Tile


Hi guys,

Do not glue the new underlayment to the subfloor....unless you can use thin wood glue and spread it with a trowel to give you 100% coverage, and fasten before it dries. (hard to do) This is called 'laminating', but creates lots of problems if not done right and if it needs to be removed some day. Fasten only and use fasteners that are just a bit longer than the two layers. This way you don't have to actively try to miss the joists. Re-fasten the subfloor to the joists before installing the underlayment.

As for what thickness the new underlayment should be. In this case I wouldn't consider anything thinner than a true ", but I recommend thicker. Why not go with ⅝" or " and use Ditra instead of " Hardie? Now that would be a good floor. Ditra is ⅛" installed where " Hardie will be close to ⅜".

Quote:
Originally Posted by John
Concerning the unsupported span of the joists, I'd say around 16ft.
That answer will not do. We need something more accurate like this; 13' 8".

Quote:
Originally Posted by John
If I screw and glue the two layers together (even missing the joists), How can they still move independently of each other?
Yes, the two layer will still move a bit, plus you need to install expansion joints at min. around the perimeter for a small room.

Jaz
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Old 07-06-2016, 09:32 PM   #6
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Re: Subfloor prep for Tile


You'll need less than 14.5' of unsupported joist span to ensure you meet the deflection criteria for ceramic tile at minimum.
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Old 07-06-2016, 09:41 PM   #7
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Re: Subfloor prep for Tile


Hold on there HDS, we have no idea what species or grade the joists are yet. Some species would need to be more like 12.5 ft.

Jaz
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Old 07-07-2016, 11:09 AM   #8
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Re: Subfloor prep for Tile


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Hold on there HDS, we have no idea what species or grade the joists are yet. Some species would need to be more like 12.5 ft.

Jaz
Hi JazMan,

Sorry I meant the 2x12 are 16 ft long, but they are supported by both lateral walls of the bathroom located exactly below, so the the unsupported span is 5ft. I also remeasured the span between the floor joist and it is 16in not 24in. Bathroom size is only 5ftx9ft.
So the framing deflection should really not be an issue, The only issue is that the subfloor plywood is not thick enough.

Finish floor height today is 3/4in on the rest of the floor (3/4in hardwood on top of the 5/8 plywood)

Estimated floor height of the finished bathroom with 1/4in cement boards not counting the additional plywood is 1/8 thinset + 1/4 cement board + 1/8 thinset + 1/4 tile = 3/4in. Since the rest of the floor is exactly 3/4, the bathroom finished floor would match exactly with no additional plywood. That means the additional plywood thickness will exactly be the final floor height difference. 3/4in of additional plywood would be a huge difference for a transition. This is why I was hoping for no more than 1/2in.

Concerning the screwing, if I understand you correctly I should just screw the new plywood (no glue at all) with 1in deck screws and. I will also avoid aligning the seams of both layers and will stagger the boards of the new layer. I guess the cement board screws should also not penetrate the joists, can you confirm?

Thank you,
John
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Old 07-07-2016, 08:37 PM   #9
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Re: Subfloor prep for Tile


Your corrected numbers makes a huge difference. 5 ft. span and 16" oc vs. 16 ft, span and 24" oc. You're fine now regardless of the species and grade of the joists.

I didn't suggest new " ply and Hardie, I suggested using Ditra which would be a good 3/16" thinner than " Hardie. But, that was with 24" oc.

Actually...... the industry says ⅝" plywood subfloor (t&g of course) is thick enough as the base for either Ditra or any CBU. So there's several ways for you to proceed. Another method is to use Ditra XL (5/16") which is especially made to transition to " hardwood just like your situation. I personally don't like going over ⅝" subfloor, but it's been tested and proven good for ceramic. If an uneven threshold doesn't bother you, go with ⅜" or thicker underlayment grade ply.

Decide on the ply and whether to use CBU, before you worry about the screws too much.

CBU screws are 1 1/4", they're not gonna penetrate the joists to any degree if at all.

Jaz
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Old 07-08-2016, 11:01 AM   #10
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Re: Subfloor prep for Tile


Hi JazMan,

I'll either go with 3/8in plywood as you suggest or the size just above as I prefer to make the floor stronger. I also have my toilet drain opening too large for screwing the toilet flange. The additional plywood layer will help with that.



I'll probably also put a plywood patch the exact shape of the opening glued below the new plywood at the exact location to offer more screwing thickness for the toilet flange.

John
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