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Unread 02-18-2020, 04:43 PM   #1
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Sub floor

Confused should be title..
Ok I have checked deflection calculator which is l558 2x10 fir joists 16” Center
With 5/8 std Tg plywood with a 1/2 “ subfloor. In kitchen. I am going to lay ceramic 12x24 tile using mapei light tile and stone mortar. 1/16 Th inch spacers.
Can I apply directly over the plywood?
If not and need a membrane what mapei product do I use under it?
Or should I rip up the 1/2” subfloor and use backer board
Thanks dave
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Unread 02-18-2020, 05:33 PM   #2
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Hi Dave, and welcome,

If you are satisfied that your joist structure meets the L360 requirement for ceramic tile I'll not run through that drill.

While technically you can set the tile directly to plywood there are reasons you don't wanna. One being the joints between plywood sheets can result in cracks in the tile(s) above those joints.

5/8" T&G is the absolute minimum recommended subfloor thickness for setting tile, and no backer board fixes that; it isn't structural. My house was done with 5/8" and there is just too much defection between the joists. Your 1/2" over the 5/8" is far better, so long as it was installed correctly, with the long edges of the 1/2" oriented perpendicular to the floor joists, and with none of the joints aligning with the 5/8" joints underneath. There is a tutorial in our "Liberry" that details the best way to install that 2nd layer of subfloor.

So, a layer of membrane, such as Schluter's Ditra over your existing 1/2" ply (and that 1/2" ply should be the exterior glue type (AKA Exposure 1) with no face grade less than "C" (B/C being pretty common) will do the trick. There are other membranes, I'm sure Mapei has one, but I don't recall their names.

Or you can use 1/4" backer, correctly installed, over the existing 1/2" ply. A bit less expensive but more time consuming to install. The membrane will result in less added height.
If I recall correctly my memory is excellent, but my ability to access it is intermittent.
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Unread 02-18-2020, 06:33 PM   #3
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Mapeis Uncoupling membrane is Mapeguard UM. As far as Uncoupling membranes go its a good one...
You could however us a crack isolation membrane as opposed to uncoupling like Nobleseal CIS for a lower profile.
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Unread 02-18-2020, 10:55 PM   #4
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Welcome, Dave.
Originally Posted by Dave
If not and need a membrane what mapei product do I use under it?
MAPEI's crack isolation product (or one of them) is Mapeguard 2, also a thin sheet like Eric's Nobleseal. I've never used the MAPEI product.

My opinion; worth price charged.

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