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Unread 03-02-2021, 08:25 AM   #1
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Subfloor Question

Hello everyone! I have a question and after searching, I've been unable to find the answer outright...

Finally am getting started on my powder room and after gutting it, I'm in the same spot I was with my kitchen, that being a 1/2" plywood subfloor. The tile that was chosen for the powder room was a Dolomite Premium Arabesque Polished Marble Tile (which is a mosaic tile in a 12"x12" sheet. Each piece is 3"x3").

Based on what I've been reading, it appears I need to add another layer of plywood for an underlayment since I'm dealing with natural stone (does this apply to the type of tile I'm laying or is it only for bigger singular pieces?). The plan is to use more of the Ditra I have but it appears that even their guide states another layer of plywood is necessary. My concern is building up the height too much with two layers of plywood and making the transition between the hallway (which is hardwood) and the bathroom obnoxious. For reference the hardwood is 3/4".

In the kitchen I had added 3/4 plywood since I was using large format tiles. Am I able to use 1/2" in this room? Room is 4.5'x4.8'.

Joists are 9.5" high, 1.5" wide and 10' long. I'm unsure of the species of wood. House was built in 1966.

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Unread 03-02-2021, 02:33 PM   #2
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Welcome, Bob.

First problem you have is the joist structure. If you knew the grade and species of your joists it's possible they meet the deflection requirement for a natural stone tile installation, but it's not something you want to guess at. And you do understand that you need to know the unsupported span of the joists, which is not necessarily related to the size of the room to be tiled, yes?

Second is the subfloor. The stone industry requires a double layer of plywood subflooring regardless the joist spacing. And that applies regardless the size of your tiles. I personally believe all mosaic tiles should be treated as natural stone installations on floors, but that's another discussion.

Your problem here is that you don't have a first layer of subflooring. That nominal 1/2" plywood does not have T&G edges and is too thin to even meet building code if it had blocking under the edges, which it does not. Seriously, Bob, it just doesn't even count. The absolute minimum you can use for the first layer is nominal 5/8ths" plywood with a second layer of a minimum of nominal 1/2" plywood properly, and differently, installed over that. Then the tiling substrate of your choice.

You need to make your floor suitable for your tile installation, then make whatever transitions necessary to accommodate adjacent flooring.

My opinion; worth price charged.

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Unread 03-02-2021, 05:40 PM   #3
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Thanks CX. I appreciate the insight. That's helpful information.
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