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Unread 03-24-2018, 12:24 AM   #1
Gadus
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subfloor suitability

I have a 80 ft2 bathroom floor to do (my own) which currently has only a 2" X 6" T & G fir subfloor, over 4 X 12 beams spaced at 36". It is over-nailed but some cracks have appeared over the (13 yr) life of it but doesn't appear to be any flex or creaking; I have planed down the high spots. Can I get away with putting hardiebacker directly on this floor or must I absolutely have some 3/8" ply between the two? I have a 1.25" sill so my finished floor has to be 1" preferably, for a 1/4" reveal.

I am aware of the Shluter-Ditra membrane but would rather not go that route unless someone has an incredibly persuasive reason why I should go that way.

I am planning to use 12" X 12" travertine stone tiles.

Thanks.
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Unread 03-24-2018, 07:01 AM   #2
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Welcome back, Gadus.

Unfortunately, I don't think you'll be able to use that stone on your floor at all due to the joist system you have, plus the type of subfloor you have.

Natural stone requires two layers of plywood over regular joist spacing, and you'll have neither.

What you can do is add a layer of 1/2" plywood, properly secured, and then a tile underlayment and ceramic tile. If you is a tile membrane like Ditra, you'll save some of the height of the assembly, but only 1/8" vs. regular cement board.
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Unread 03-24-2018, 07:33 AM   #3
Gadus
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Thanks, I'm glad to be back where the experts are so helpful.

Am I ok with any other variety of tile? Would other tile allow me to make do with hardiebacker directly over my flooring or would I still need the plywood?

Thanks
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Unread 03-24-2018, 01:41 PM   #4
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There is no tile substrate that you can put directly over your subfloor. They all require plywood as a base.
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Unread 03-24-2018, 04:41 PM   #5
Gadus
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So, even two full inches of solid wood does not meet the standard for tiling...For some reason I thought 1 1/8" was the minumum req, guess I didn't realize it had to be engineered wood.

Can I use 3/8 ply, 1/4 hardie backer plus ceramic? Or must it be 1/2" ply?

Many people say plywood is perfectly suitable for tile but I don't want to open that can of worms, except to ask, would it make more sense to install 3/4" ply and tile directly to that? That would still give me the 1/4" reveal that is ideal.

Thanks again.
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Unread 03-24-2018, 06:03 PM   #6
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It's not the thickness that's the problem, but the fact that it's dimensional lumber, and not as stable as plywood. You can do some checking if you want, but you won't find any tile substrate that can be installed over what you have.
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Unread 05-26-2022, 08:23 AM   #7
Gadus
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prep question (hardieboard over 2" spruce.

I've got a second-floor subfloor, 2X6 spruce, aged 17 years. Used to have squeaks but since I've laid hardwood floors on top of it and nailed the shit out of it, it stopped squeaking. The only remaining room to finish is the bathroom and I want to put down same 12X12 travertine tile I installed recently on my downstairs bathroom.

So while it is amply nailed, there are still joints which have opened up slightly and knot holes you can nearly see through. I have planed the whole area flat. The question is when I mortar in the 1/4 hardieboard, should I preliminarily fill those deeper holes or just do it as I lay the board? I think the mortar is fine enough to go into and fill entirely such gaps as exist on the first pass, so I guess the question (or two) would really be, should I use some other material first or possibly do the board install in two separate passes?

Thank you.

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Unread 05-26-2022, 10:18 AM   #8
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Welcome back, Gadus.

This is the same car plank subflooring over joists on 36" centers we've visited a couple times before?

If so, the only technically correct method of installing ceramic tile over what you've got is still to add a minimum of nominal 1/2" exterior glue plywood over your existing planks before adding the tiling substrate of your choice.

Still no way around that. And whether that technically qualifies for a natural stone installation is somewhat questionable.

My opinion; worth price charged.
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Unread 05-26-2022, 12:38 PM   #9
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SOlid wood planks just move too much in between seasons and just the cbu on top of it won't isolate it enough. THen, industry standards call for two layers of subflooring before prepping for stone tile. To help limit the height buildup, you could put something like Ditra down versus cbu which would save a bit over 1/8" when applied over the plywood versus 1/4" cbu.

The spacing between joists, if 36", exceeds any tile industry standards, and isn't likely a suitable structure without augmentation, especially for natural stone tile which has lots more internal variations than a manufactured ceramic tile.
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Unread 06-21-2022, 06:25 AM   #10
Gadus
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Apologies for my tardiness in reply. Damned life sure gets in the way.

Thank you both for your replies. Yes, I had a senior moment, as mine was a repost but I guess it doesn't hurt to ask the same question twice.

To qualify a tad, the joists under the 2" subflooring, are 4X12 spruce. I haven't heard any squeaking since nailing everything down - except the occasional squeak of my planer going over exposed nail heads.

I have approx. 1 3/8" to play with so it looks like I could do 1/2" ply, then 1/4" hardieboard and install 3/8 stone travertine and still not go above my installed door sill. I do realize this doesn't give me the two thicknesses of plywood but I am willing to take that chance.

Thanks again.
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Unread 06-21-2022, 07:17 AM   #11
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I've combined one of your previous threads on this floor here, Gadus. We still don't know nearly enough about your structure to say whether it qualifies for a natural stone installation.

I'm guessing that the subfloor structure, the car plank over 36-inch centers, will be sufficiently rigid and with the addition of the half-inch plywood will likely be suitable, even though it doesn't technically meet the stone industry requirements, but see my warranty information below.

The joist structure, on the other hand, is a mystery. You need to know at least the unsupported span of the joists in question.

My opinion; worth price charged.
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Unread 06-21-2022, 04:53 PM   #12
jadnashua
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The deflection requirement has two components, along the joist and in between them. The 2" planks are likely strong enough between them, but it is not certain along them is. Plus, as has been mentioned, dimensional lumber just moves too much, so some plywood on top is required.

If you read the installation instructions on any CBU or Ditra, they'll say that. Any other use is suspect.
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