Leveling 3/4" OSB for Ditra - SLC or plywood? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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07-11-2019, 03:24 PM
I've been working on preparing a 3/4" OSB subfloor for Ditra. So far I've been using a belt sander to bring down ridges at the joints, but am finding that I've still got dips of more than an 1/8", even after removing a fair amount of wood from the high spots (all at joints). I called Schluter, and they recommended using an SLC, but I'm wondering if I'd be better off adding a layer of 1/4'' or 3/8" plywood, especially since I'm planning on installing 12" X 24" ceramic tile over the Ditra. I'm also wondering if the wood I've sanded off will weaken the OSB subfloor too much. The OSB is nailed 16"oc, and it seems solid, with no creaking or noticible deflection under foot. Any advice would be much appreciated.

Thanks in advance.


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07-11-2019, 03:43 PM
What is under the OSB? 3/4" may not be eough. Use the Defleco in the blue bar at the top to see if what you have is adequate,,,and I doubt it is. Adding quarter inch or even three eights may or may not strengthen it enough. If it does, use the Ditra.

07-11-2019, 04:05 PM
Welcome, John. :)

You say your OSB is nailed on 16" centers. Is that along the joists, or is that perhaps your joist spacing?

Have you evaluated your joist structure to determine if it qualifies for a ceramic tile installation?

You do understand that your tiles don't care a whit if the floor is level, they care only if it's flat. The larger the tiles, the more they care.

OSB, in my opinion, is not a good substrate for a self-leveling compound, but I believe some manufacturers will approve the application with the proper primer and lath. You'll need to check that.

If your existing subfloor is not in pristine condition, 'specially with all your sanding, I'd recommend you install a second layer of nominal half-inch exterior glue plywood with no face of grade lower than C. Then proceed with any necessary flattening.

The ceramic tile industry requirement for substrate flatness for tiles the size you have is no deviation from intended plane of more than 1/8th" in ten feet nor 1/16th" in two feet. That's a very stringent requirement and you'll be glad to have it come time to set those tiles.

My opinion; worth price charged.

07-12-2019, 04:39 PM
Thanks for the advice - this is a really great forum!

The deflectolator gave me a score of 567, so I think I'm OK on that count.

I don't want to raise the floor more than necessary, since I'll be installing the tile in a hallway that leads to a stair case, and I don't want to throw off the height of the first stair. Hence my thinking that I should go with either SLC or 1/4" plywood.

Do you think adding 1/4" plywood do much to improve stability? I'm also wondering if maybe leveling with troweled product like Henry 547 would be the way to go (I think it's approved for OSB), rather than pouring an SLC.

07-12-2019, 05:13 PM
1/4" plywood...commonly referred to as Luan is a No No. Use at least 3/8" ply for that.......:gerg:

07-12-2019, 07:51 PM
1/4" ply that is okay for use under tile is kind of hard to come by, plus, it doesn't have a whole bunch of additional strength for the actual effort to install it. 3/8" is generally the thinnest, and finding some that is actually able to lay flat is the bigger issue with most of it.

Not all OSB is created equal. Even then, it still needs to be installed properly. Generally it wants a gap at the edges. Having the edges swollen usually indicates that gap wasn't maintained when installing, and then, the floor saw moisture (often, before it was closed in). Some OSB has some pretty robust glues used and, lots of crossing layers of the wafers/strands of material, similar to plywood, to actually become stiffer than the equivalently thick ply. Some don't. Some are pretty stable if they get wetted, some aren't. Some have a waxy substance applied that helps it remain intact during construction when wetted, some don't. That can be a bond breaker for thinset. I've found that unless you try to tile over it immediately after installation, walking around it during the subsequent building process tends to roughen it and wear off some of that, giving the thinset a decent bite, if the panel isn't too dirty and contaminated first.

Tiger Mountain Tile Inc
07-13-2019, 11:19 AM
If you are just trying to fill in some dips then using a product like Planipatch or Feather Finish would work just fine.

If there's more flattening then that then a self leveler might be the way to go. Custom Level Tech and Ardex Liquid Backer Board both don't require lath. Not sure if Level Tech is approved for OSB, or not.

I'm not sure that adding additional plywood would take the dips out of the floor. If you're just fastening it to the OSB underneath then I would think it would dip wherever the OSB does. Might not dip as bad, though.

However, additional plywood is a good way to add rigidity to the floor if that's something that you are looking to accomplish.