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Unread 12-28-2020, 04:38 PM   #1
rmean
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1-1/2" Diagonal Plank Subfloor

I'm a proud owner of John's book and followed it to the tee to install travertine on the first floor that has outlasted Harvey and a 2" foundation repair without a single crack in a grout-line.

I'm now remodeling our third floor master bathroom with travertine.

Floor structure is pine open joists that are 18" high and 3-1/2" wide (2x4's on top and bottom of joists). Joists are 24" apart oc and they are 8' long (support to support).

The subfloor is 2x6 T&G planks (1-1/2" thick) and are laid diagonal to the joists (45deg). Should I replace this with osb/plywood or just lay a 1/2" osb on top of it?

Thank you for all the advice over the years.
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Last edited by rmean; 12-29-2020 at 12:06 PM.
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Unread 12-29-2020, 06:22 AM   #2
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Hi Armin,

Sounds like a interesting project. What’s the total unsupported lengths of the beam? Travertine is very soft and will require a very stiff floor.
If the top planking is in good condition, a top layer of plywood sounds like best option. Just need to make sure the beams aren’t over spanned (my first question) and the planks are secure and flat. I’d say 1/2 inch would be fine but the travertine worries me. What size of stone are you planning?
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Unread 12-29-2020, 09:11 AM   #3
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Welcome aboard, Armin.


If you were using ceramic tiles I would say go ahead with the plywood over the planks, but travertine is a very soft material that contains numerous "faults" or veins. I hope CX picks up on this. He's the guy with all the tech info. I've been "retarred" too man years now.
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Unread 12-29-2020, 09:40 AM   #4
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Welcome, Armin.

A geographic location in your User Profile is often helpful and in this case would also give meaning to your comment about "Harvey," eh?

PC, I think he's saying his unsupported span of his 18" open-web joists is only 8 feet unless I'm misinterpreting something there. That would leave no doubt in my little pea brain that the joist structure meets the required L/720 deflection criterion for his natural stone tile installation, but I been wrong before.

The stone industry doesn't acknowledge a joist spacing greater than 16" on center and you'll find no tile industry standard for natural stone over your current structure. That said, if I really wanted a natural stone tile on that floor I think I'd be content to add a layer of nominal half-inch exterior glue plywood with no face of grade lower than C to my existing subfloor and install my Travertine tiles. That would satisfy the stone industry requirement for a double layer subfloor and I think you would have the between-joist equivalent of the required /720 deflection there as well. See my warranty information below. I think you'll not find an industry published method for your combination of materials.

My opinion; worth price charged.
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Unread 12-29-2020, 11:44 AM   #5
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Quote:
Originally Posted by Armin
Joists are 24" apart oc and they are 8' long (support to support).
Quote:
Originally Posted by CX
PC, I think he's saying his unsupported span of his 18" open-web joists is only 8 feet unless I'm misinterpreting something there. That would leave no doubt in my little pea brain that the joist structure meets the required L/720 deflection criterion for his natural stone tile installation, but I been wrong before.
Agree - Sorry I missed the 8' length, I was on my phone and its easy to stuff on that that small screen.
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Unread 12-29-2020, 11:56 AM   #6
rmean
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Thank you for the responses. The unsupported length of the joists is 8' and I'm in Houston (Harvey was the supposedly once in a century flood that covered my first floor for a few days). Travertines are 18"x18"x1/2".

CX picked up on the reason why I posted. The dimensions of my structure is just outside published standards, so I wanted to see if anyone had experienced it before. I would cut out the planks except that the planks cover the entire span of the building and continues into the neighbors unit as we have a building of six three story condo units wall to wall. Our master bathrooms on the third floor is wall to wall with my neighbor's. So I'm not sure if it provides any other structural purpose.

I'm not holding anybody accountable, but what is your recommendation?
- rip out the planks, block the heck out of the joists and install two 3/4" osb's?
- or screw the heck out of the plank, sand it level and install a 1/2" OSB (Schluter calls it in their manuals over plywood)? This would also raise the bathroom floor at the entry by 1/4"?

I am installing kerdi frameless shower system with ditra.
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Unread 12-29-2020, 12:26 PM   #7
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Wait a few years and it'll be easy to miss the whole screen, PC.

Armin, I believe you'll find my recommendation in post #4.

I would not recommend OSB in your application if you plan to use Ditra or any other bonded to the floor membrane as your tile substrate. For a CBU it would be fine. Schluter thinks it's fine, but some manufacturers of thinset mortars don't recommend their products for bonding to OSB. I've never found OSB to be a good surface for any kind of bonding with any sort of adhesive, but that's a different discussion. Buy a good exterior glue plywood with no face of grade lower than C.
Quote:
Originally Posted by Armin
...or screw the heck out of the plank, sand it level...
Neither your tile nor your second layer of subflooring cares a whit about level, but they do care about flat.

My opinion; worth price charged.
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Unread 12-29-2020, 02:58 PM   #8
rmean
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Thank you. I will go with fastened and "flat" planks covered with plywood; except for the section where I have to install the recessed ledger for the sloped Kerdi shower pan. Will post the results.
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