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Unread 05-28-2020, 09:28 AM   #1
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Can I lay tile on this subfloor?

Hi all! I was looking for some expertise on if I can safety lay either ceramic or porcelain tile on the floor in my kitchen. When we bought the house it was tiled in a quick "homeowner special" by the previous guy that failed and lifted up after only 6-9 months. We replaced it with a luxury vinyl product that I am just not happy with. Original tile was laid on a 1/2 plywood subfloor but this is what the subfloor is now after that little project:

Joists are 2x10 placed 16"OC and spanning 11.5 feet. At the center of the span the joists are supported with steel X bridging. Subfloor is the original 1/2 plywood that does not appear to be tongue in groove with another 1/2 OSB that I added running the opposite direction and screwed to the original sheets but not into the joists. Since I have a fixed dishwasher opening under a granite counter I have to stay under a certain floor thickness and that is a concern. I am looking to use 1/8 Ditra to save room and only have about 7/8'' of total end floor thickness above my current subfloor to play with. Is this possible? A "professional" I asked said it wouldn't be an issue installing just on the 1/2 original subfloor but I am pretty sure that's wrong and is what led me here. Thanks!
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Unread 05-28-2020, 11:38 AM   #2
Tool Guy - Kg
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Welcome to the forum, Wes!

Argh! Another builder who started with 1/2” subfloor!

The quick answer is no. The absolute minimum subfloor thickness for man-made tile installation is 5/8” with either: tongue and groove...or support (like blocking) under all the seams. This assumes the long axis of the ply is perpendicular to the joists.

Unfortunately, your 1/2” subfloor is too bendy that it doesn’t do enough to help support another layer of something less than 5/8” on top of it. Industry standards and all the manufacturer’s minimal standards would say you should act like there’s no support from your original 1/2” ply and dictate a minimal 5/8” layer be installed. And it would also require that the ply be installed with the long axis perpendicular to the joists. All layers should be in this same direction.

Please take a look at this article in post #11 about proper plywood installation.

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Unread 05-28-2020, 12:36 PM   #3
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Darn, that’s what I was afraid of. I knew I came to the right place. I want to do it right or not at all so I appreciate the response. Is there any way to sure up my current arrangement to make it acceptable for tile? Brace it more from the basement maybe? I thought the 1 inch total thickness might be acceptable with Ditra. I could always take that 1/2 layer that I added and replace it with 5/8 I guess? Or completely replace the entire subfloor and remove the original but that seems excessive. How I understand it, replacing the top sheet with 5/8 might be the best solution but then I lose another 1/8 of clearance for the dishwasher opening and I think I am already cutting it close. If I lay it right and can’t get that sucker back in there I will be less than excited about the prospect of finding a slightly shorter dishwasher but I guess I could do that.
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Unread 05-28-2020, 04:28 PM   #4
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Your best solution to save height would be to tear things out and go with a quality 3/4" t&g ply that you would glue and screw the joists. The x bracing does nothing to improve deflection ratings. 5/8" T&G ply on top would work as well.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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