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Unread 10-26-2017, 08:48 PM   #1
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Ditra and SLC

I am installing Ditra over plywood soon. Tiles will be 12x24. The existing floor is out of level approximately 3/8" over 8'. Is it feasible to use SLC prior to installing Ditra, poor SLC over Ditra, or simply make up the difference with thinset?

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Unread 10-26-2017, 09:16 PM   #2
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Thinset is NOT designed to be used to level things, and many of them cannot be installed very thick or they shrink and potentially crack if you do. It's MUCH harder to get the tile in plane if you do try, as well.

SLC can be used, but not all of them can be installed to a feather edge over plywood (but can over a slab or cbu). Ardex's liquid backerboard can, though.

On most SLC installations, it is MUCH easier to try to pour it thicker (many of them want at least 1/2" above the high point), otherwise, it's tough to get it to flow properly and level. The stuff is not like water and it levels much easier if it is applied thicker.
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Unread 10-26-2017, 10:03 PM   #3
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Flat and level are two totally different things.The floor does not have to be level for tile but it has to be VERY flat for those big tiles .If you put a 10 foot straight edge on the floor you should not have more than a 1/8 inch variance.If you do than you need to flatten it with a patching compound designed for wood before you install your ditra.
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Unread 10-27-2017, 06:12 AM   #4
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Depending on where you're getting your materials, Lowes carries our Self Leveler Plus which can go over plywood and under uncoupling membrane. Typically that is the order we recommend as the uncoupling membrane can act as a cleavage membrane if a high-shrinkage leveler is used on top of it. That can lead to hollow sounding spots under the tile.

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Unread 10-27-2017, 09:18 AM   #5
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Hi Paul,

I'll touch on what Mark said. If the floor is just out of level but on plane (flat) I don't know that I would worry about 3/8 in.
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Unread 10-27-2017, 12:25 PM   #6
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We do “ flat “ more often than “level” in older homes . ... in fact
“ level “ can be a mistake in many homes as it simply may not be relative to the rest of the room.
Minor deviations can be addressed with thinset under the ditra prior to installing it. Anything more than 3/16”- 1/4” and thinset starts to get problematic at best.
If the room and walls are plumb and more or less level but the floor is off ... then we often turn to SLC under Ditra.
In the Grand scheme of things ...3/8” in 8’ probably isn’t that bad...provided it’s gradual and even .. if it drops 3/8” in the last foot ...then that’s another matter .
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