Replacing tile in large area w/Large Format - subfloor worries [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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08-05-2019, 08:10 AM
Hi all,
This forum has been great in answering all my questions without having to ask one over the years, however, I'm in the need of some good advice.

I'm planning to remodel our kitchen which includes replacing the existing tile floor (porcelain 12x12's) that also flows into the foyer, mudroom and dining area. The existing tile was installed directly over a plywood subfloor with modified thinset; The subfloor is 3/4 T&G plywood, glued and nailed (ring-shank) to 2x12 I-Joists 16" OC.

Underlayment atop the 3/4" subfloor is 3/8" plywood that is adhered to subfloor with construction adhesive (PL400) troweled on wit 1/16" v-notch trowel and screwed down. In my opinion, this bond almost makes it like a solid 1-1/8" thick.

The current floor has been down for 25 years and show no sign of wear, cracks, loose tile or loose grout.

Although the replacement tile hasn't been selected yet, it will large format, likely in the 12x24 or 18x18 range.

The current subfloor has proven itself worthy based on the current install, however, I'm a bit worried and concerned with larger format tiles and don't know if I should be.

Idealistically, replacing over the current subfloor with same direct-to plywood means would eliminate floor height concerns with adjoining rooms, stairs, doors, etc. Removing the 3/8 underlayment will be nearly impossible without shredding the floor beneath it.

So my friends, I'm looking for some assurance that my subfloor is probably solid enough or cautions based on others experiences. I did notice a fiber-reinforced thinset that might be good for such an application as well. It's called ARDEX X 77.

Your comments and opinions much appreciated!

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08-05-2019, 09:00 AM
Welcome, Ray. :)

Your subfloor/substrate does not qualify for a direct to plywood ceramic tile installation as described, proving once again that we can no more guarantee tile installation failure than we can guarantee successful installations. We can, and will, however, tell you what the ceramic tile industry recommends and where the smart money is betting.

If you can remove your existing tile installation without damaging the top layer of plywood (doubtful) and you want to tile directly to what you find under there, you can certainly do that. Keep in mind that with your larger format tiles, the industry standard for substrate flatness now becomes no deviation from intended plane of more than 1/8th" in ten feet and no more than 1/16th" in two feet. A very strict requirement and one you're unlikely to have under your current installation.

While there is still a TCNA accepted method in the industry for tiling directly to plywood (F150), the requirements are fairly strict. One of the requirements is that the top layer of plywood, the tile underlayment, must be a minimum of nominal 1/2" exterior glue plywood, which you don't have. And while not specifically stated in that published method, the industry recommends not gluing the second layer to the first, but attaching it using mechanical fasteners only and fastening only to the bottom layer and not into the joist structure.

I think the only reasonable approach at this point for you is to remove the existing tile and see what you've got left, then decide how you want to proceed.

My opinion; worth price charged.

08-06-2019, 09:00 AM
Thanks for the comments CX. I had a "feeling" about this and what you shared.

I'm reasonably confident that the floor is very flat, given the I-joists straight as an arrow, but will verify once everything is torn up. There's no way the existing underlayment will come off, so if I were to improve the subfloor to an acceptable spec, can you suggest the minimum material thickness? 1/4 or 3/8 cement board, Ditra or something thicker? If it weren't for abutting hardwood floors in multiple areas it would be an easy choice.

Thanks again

08-06-2019, 09:46 AM
Nothing inherently wrong with your subflooring, Ray, it's just not acceptable to the ceramic tile industry for a direct-to-plywood tile installation. If you end up with a useful surface after tile removal, you could certainly use a CBU or tiling membrane over what you've got. Or you could, of course, just tile again over what you've got. Your floor. Your tile. Your choice.

But first you gotta know what you've got, eh?

Height transitions from tile to hardwood flooring are one of the easiest to make.

My opinion; worth price charged.