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I am laying floating porcelain tile that is a half inch thick. I also have to lay down an underlayment under the tile. After accounting for both of these items I come up about a half inch too short under the door jambs, trim, etc.

I am wondering what is the proper way to add roughly half inch of height to the subfloor. Can I just use any type of OSB or plywood? Should I use cement board even though the tile I am laying doesn't require any type of mortar?

I should note that this is in an unconditioned entryway to a 100 year old apartment building in the Midwest. So there is not heat or AC in this entryway and is subject to moisture from peoples shoes coming in an out.

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    You haven't said what you're installing over.
    – isherwood
    Apr 7 at 12:57

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There is no need to use any kind of a cement board under your floating tile flooring. It appears to be no different that a floating wood floor except for the pattern in the top-most layer. If you were installing actual tile, you'd probably want a cement backer board to resist the moisture in the mortar that holds the tiles. This stuff, though, just sits on the floor.

Plywood would be slightly more water resistant than OSB, but not by much. You could use exterior grade and/or pressure treated plywood, but that's probably overkill.

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