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Old 07-22-2019, 07:35 AM   #1
atct86
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Underlayment for Mosaic Marble

I am reading many different opinions on the best underlayment for a mosaic marble floor. Looking like 1x2 basket weave.

I am working on renovating the powder room of my 1920 house. I currently have the room down to the the 3/4" T&G diagonal subfloor. My thoughts were to glue and screw another layer of 23/32 plywood. The Joists are true 2x10's, with one end of the room resting on a steel beam, and stretching 17 feet to a stone foundation wall.

I know I cannot use Ditra with my tile size, so was planning on putting CBU. If I do this, do I need to pour self leveling concrete on top?

Also, Is any exterior grade plywood better than the other? I have heard that PLYTANIUM is somehow stronger.
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Old 07-22-2019, 08:42 AM   #2
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Welcome back, Andrew.

Not the same bathroom from ten years ago, we hope?

Over your sawn board subfloor the minimum you can use is nominal half-inch exterior glue plywood with no face of grade lower than C. With the diagonal orientation, a bit more would likely be better.

Why do you want the self leveling material? Your tiles don't care about level, they care only about flat.

There is no "exterior grade" plywood. The plywood grade reflects the surface ply grades, usually one higher than the other, such as AC, BC, etc. You do want an exterior glue plywood, though, as indicated on the grad stamp by EXT or Exposure 1. You can usually find a decent BC grade of exterior glue plywood at your local home center. The Plytanium will also serve, but you'll be paying a premium that's just not justified by your application.

My opinion; worth price charged.
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Old 07-22-2019, 12:53 PM   #3
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New to me house, the bathrooms from 10 years ago are still in perfect shape.

Would it be better to cut out the diagonal sawn board subfloor? Would 2 new layers of plywood reduce deflection, as I seem to be very close on the calculators to not being able to use marble.

Good thought on the leveling, I assumed it might help with strength, but I would be glad to skip the step and tile directly to the CBU.
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Old 07-22-2019, 01:38 PM   #4
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Andrew, the plywood over the sawn boards doesn't technically meet the requirements for the first layer of a subfloor for a natural stone tile installation, but is usually treated as though it did. The primary purpose of the required double layer of subflooring, aside from rigidity, is being able to have no subfloor joints extend from joist top through the subflooring. You can generally achieve that with the board floor as your first layer, which the Natural Stone Institute requires to be a minimum of nominal 5/8ths" plywood (they don't even recognize OSB). The ceramic tile industry presumes a board subfloor to be T&G and be installed perpendicular to the joists, which you do not have. But I think if you were to use a good nominal 5/8ths" plywood as your second layer, Properly installed, you'd be fine. See my warranty information below.

By "close on the calculations" I presume you're talking about your joist structure and that needs to be addressed separately. Very unusual for a joist structure not designed for a natural stone tile installation to meet the required L/720 deflection requirements, but I can't see your floor from over here.

My opinion; worth price charged.
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Old 07-22-2019, 01:42 PM   #5
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Just do not confuse pressure treated plywood with what CX is recommending. Do not use pressure treated plywood.

Look at this link: https://www.homedepot.com/p/3-4-in-x...1060/202088096

This is listed in the details as rated for "Exposure 1". That link is CD grade, which is a bit worse than what CX recommended. You would want like ABX or BCX. The link below has some more info. HD kind of sucks sometimes to find good stuff. Don't be surprised if the floor help tell you "it's fine", when it is not.

https://www.andersonplywood.com/exterior-plywood/
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Old 07-22-2019, 06:14 PM   #6
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Understood on the subfloor process. I will end up cutting out the diagonal planks, and glueing and screwing 23/32" (I cant seem to find 3/4" larger than 4x4 anywhere) BC plywood perpendicular to the joists. I will then install another layer of 23/32" or 15/32", perpendicular to the other layer, with screws into the plywood, not the joists.

Right now I am getting a deflection of L/616. Would sistering every other joist over the whole span be enough? Of note, the bathroom previously had marble 12x12 tile applied with thinset to 3/4" pine t&G flooring for 26 years, and not a single tile was cracked.

Also, am I correct that thin-set for the CBU and the tile will end up being less than the thickness off the trowel once the tile is pressed into it? I am trying to see if there is a chance that I can end up with tile flush to the wood floor transition.

Thanks!
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Old 07-22-2019, 07:02 PM   #7
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Quote:
am I correct that thin-set for the CBU and the tile will end up being less than the thickness off the trowel once the tile is pressed into it?
Yes. If the trowel was held 90 degrees to the floor to get the full height of the notches, the compressed thickness should be half the notch height. Since the trowel is usually held at an angle when combing out thinset, the finished thickness is normally less than half the notch height.
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Old 07-22-2019, 08:40 PM   #8
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Andrew,

I ran the numbers you provided for the joist structure and came up with L/375. This will be fine for porcelain but will not meet the L/720 requirement for natural stone.

FWIW: You can either add another support beam at 4’ from either of the existing beams or double up the existing joists.
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Old 07-22-2019, 08:49 PM   #9
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Andrew, if that finished dimension is important to your installation, the only real way to determine what it will be is to make a test board using your materials, your tools, and your technique and measuring the result. The only standard involved is that the minimum thickness of the setting material must be 3/32nds of an inch installed and the maximum must not exceed the manufacturer's recommendation.

Quote:
Originally Posted by Andrew
Understood on the subfloor process. I will end up cutting out the diagonal planks, and glueing and screwing 23/32" (I cant seem to find 3/4" larger than 4x4 anywhere) BC plywood perpendicular to the joists. I will then install another layer of 23/32" or 15/32", perpendicular to the other layer, with screws into the plywood, not the joists.
Let's be very careful there. You should have no trouble finding the appropriate nominal 3/4" plywood in 4'x8'sheets wherever you are, and for that first layer they must have T&G edges on the long side or you will need to install blocking for the between-joist seams.

Your second layer, and all other layers of structural subflooring must be installed with the strength axis perpendicular to the joists. You do not want the second layer oriented opposite the first layer. Nominal 1/2" plywood is sufficient for that second layer.

You can, of course, tile over anything you want on your project. We can tell you only what the industry requirements are and where the smart money is betting.

My opinion; worth price charged.
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Old 07-22-2019, 08:57 PM   #10
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Some mortar manufacturers will OK their products to be used over subfloors under L/720. I'm pretty sure Laticrete's requirement is L/480 and I think Ardex is somewhere in that ballpark also. There might be others too.
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