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Old 04-29-2020, 09:33 AM   #1
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Bathroom Subfloor


Currently in the midst of a bathroom reno. Old floor was mudded with tile surface, on top of plank subfloor, which looks to be in decent shape. Taking everything out above the subfloor. Plan is to put in 1/4" cement board (or equivalent) and tile over that. Is there anything wrong with that plan, or should I put down another layer of subfloor—OSB? Ply? Something else?—before the CBU goes down?
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Old 04-29-2020, 10:10 AM   #2
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Re: Bathroom Subfloor


It depends on the span (spacing of joists) and the thickness of the planks. Rigid tile (ceramic, porcelain, etc.) needs a fairly stiff backing.



A real pro I worked with once put thin-set mortar between every layer, to provide full support and provide composite action between the layers. Relative stiffness = relative thickness cubed (thickness to the 3rd power); thus a 1" thick subfloor (or 2 layers of material attached sufficiently to act as a single panel) has more than twice the stiffness of a 3/4" subfloor at the same span between joists. All that to say, spread a layer of mortar on your plank subfloor (with a notched trowel, of course) before putting down the cement board (or tile backerboard). If you can get the consistency so that it fills in the gaps between the planks, that's even better.
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Old 04-29-2020, 10:24 AM   #3
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Re: Bathroom Subfloor


Should I step up to a 1/2" CBU for the floor?
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Old 04-29-2020, 11:11 AM   #4
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Re: Bathroom Subfloor


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Should I step up to a 1/2" CBU for the floor?

If you don't put the mortar down under it, definitely. With a plank subfloor, there will be alot of gaps under CBU.


If you mortar as I suggested, it's probably unnecessary unless your joist spacing is very wide.
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Old 04-29-2020, 12:11 PM   #5
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Re: Bathroom Subfloor


Assuming that the subfloor boards are stiff enough to not give when you step on them, you can do as @HotRodx10 suggests. My only concern is what is below the bathroom. If you fill in between the boards with thinset, where will it drop onto?

If you don't want to add 3/8" or thicker plywood, you can add diamond mesh to the subfloor and then screed the thin set into it and then put down more mortar with a notched trowel for the CBU. Use backer board screws as recommend.

Be sure to moisten the CBU and floorboards so that they don't draw out too much moisture from the mortar before it has a chance to set up.

The diamond mesh provides another layer of stiffness without too much height.

If you put down the plywood, use both screws and construction adhesive to get a mechanical and chemical bond.

Use Ditra or similar on top of the CBU or plywood to create an isolation membrane to prevent cracking. You can use Red Guard or similar product to achieve the same effect.

In my bathroom redo, I was going to put down plywood over CVT, but, I pulled up all of the tile. And then I put down the 3/8" CDX plywood. And then Ditra.

Just sayin..
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Old 04-29-2020, 12:24 PM   #6
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Re: Bathroom Subfloor


If you took off the old mortar base, your floor is sure to be low now w.r.t. transitions to other floor surfaces. Also it is not going to be stiff enough.

I would add 3/4" plywood, then Ditra on top of that.
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Old 04-30-2020, 11:57 AM   #7
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Re: Bathroom Subfloor


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Originally Posted by jeffnc View Post
If you took off the old mortar base, your floor is sure to be low now w.r.t. transitions to other floor surfaces. Also it is not going to be stiff enough.

I would add 3/4" plywood, then Ditra on top of that.
I'll need to check (it's not where I'm living) but I'm pretty sure the adjacent 3/4" h/w floor is on the same subfloor, albeit with a thin black underlay. If I add 3/4" ply and then Ditra, I'm already above the adjacent floor before adding my tile.

Beneath the plank subfloor is the finished part of the basement, so any goop that falls through the cracks will just land on the top of the basement ceiling—no worries there.
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Old 04-30-2020, 12:32 PM   #8
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Re: Bathroom Subfloor


Well this is a little confusing, because the only way I can imagine they tiled over a slat subfloor is by adding an old school mortar base, which is typically about an inch thick, or at least 3/4". So normally adding 3/4" plywood fits this scheme perfectly. I don't understand what you took out of there exactly.
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Old 05-01-2020, 10:02 AM   #9
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Re: Bathroom Subfloor


No, it doesn't fit perfectly. Adding 3/4" ply immediately adjacent to 3/4" hardwood only puts the surface of the ply at the same height as the hardwood, when both are laid on top of the same plank subfloor. Then you add your backer on top of the ply, and then the tile and thinset, and you're at a much higher height than the hardwood.

This is what was there before:
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Old 05-01-2020, 10:42 AM   #10
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Re: Bathroom Subfloor


Oh, so it was already higher before. So can you use the same transition you used before? Point being, plywood is a structural element, cement board is not. i.e. you're going to want to stiffen the floor for tile, and cement board will not help much. So 3/4" plywood with 1/8" Ditra on top is a better solution than 1/4" plywood with 1/2" cement board on top. I would definitely use at least 1/2" plywood. You could even go with 5/8" plywood, and the 1/8" Ditra makes it an even 3/4".
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Old 05-02-2020, 04:56 AM   #11
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Re: Bathroom Subfloor


I suggest you head on to HotRodx10's advice. He knows what to do in this situation.
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Old 05-02-2020, 09:21 AM   #12
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Re: Bathroom Subfloor


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Originally Posted by jeffnc View Post
Oh, so it was already higher before. So can you use the same transition you used before? Point being, plywood is a structural element, cement board is not. i.e. you're going to want to stiffen the floor for tile, and cement board will not help much. So 3/4" plywood with 1/8" Ditra on top is a better solution than 1/4" plywood with 1/2" cement board on top. I would definitely use at least 1/2" plywood. You could even go with 5/8" plywood, and the 1/8" Ditra makes it an even 3/4".
Alas, the transition was destroyed in removal.

So you feel using ply and Ditra would be superior than mortar and CB?
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Old 05-02-2020, 10:42 AM   #13
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Re: Bathroom Subfloor


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Alas, the transition was destroyed in removal.

So you feel using ply and Ditra would be superior than mortar and CB?

The transition per se is not important, you can get them anywhere. You would probably want a new one anyway. Transition profiles from Schluter et al are less bulky than that big wooden one anyway.

I've never made a mortar base, because I think they're too much trouble and wood and Ditra are better anyway. Obviously they work because the old ones lasted a long time. But that is true of some of the oldest architecture in existence too, and the Romans used uncoupling. I'm more a fan of uncoupling than brute force mega structure, but the floor has to be structurally sound, and IMO if you have that then CBUs are basically a waste of space nowadays, since they are not structural. You're adding 1/2" of stuff for no other advantage than to have a dimensionally stable substrate that the tile will stick to. Not worth it.
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Old 05-03-2020, 09:59 AM   #14
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Re: Bathroom Subfloor


Thanks.

FWIW, just because someone mentioned joists, they are 16" o/c.
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Old 05-03-2020, 11:01 AM   #15
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Re: Bathroom Subfloor


Quote:
someone mentioned joists, they are 16" o/c.

That being the case, if you fill in on top of the planks, so that your underlayment is fully supported on the subfloor, using 1/4" or 5/16" tile backerboard, or the 1/4" CBU, should provide sufficient stiffness for rigid tile. Be sure to add blocking to any free edges of the subfloor (at heat vents, etc.) if you have them. I also add blocking around toilet closet flanges, whenever I redo a bathroom floor, due to the high local pressures, particularly at the back edge of the base.


I've never used Ditra or seen the results of using it. I suspect it would be more necessary for new construction where everything still has to 'settle in' than for remodeling, where the subfloor, joists, etc. are fairly stable and not likely to shrink, swell, warp, or otherwise move around after the floor is down. The exception in my mind would be if you add plywood underlayment, since it does typically shrink a little, so you'd probably want to isolate the tile from the underlayment with the Ditra or something equivalent.
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