Attaching plywood to plywood for ceramic tile?

We were told this morning by the guy who is laying ceramic tile in our porch that the 1/2" plywood I put down is not suitable and will have to be replaced. Apparently, the problem is that the type of plywood is something used for cabinets, and he said the tiles won't adhere properly to the surface. At a glance, the texture of the upper surface resembles masonite
When I bought the plywood, I specified what it would be used for and this is what they sold me. I dutifully put down most of a 4X8 sheet with screws every 4" and construction adhesive!
I don't want to have to tear up the plywood again, and I'm hoping I can use very thin plywood glued or otherwise attached to the 1/2" plywood that is there now. I can get away with a little extra height for the tiles, but I want to use the thinnest plywood I can get.
Can anyone recommend a glue or other method to attach thin plywood (or even veneer) to whatever the hell surface is on the plywood I already put down?
_ snipped-for-privacy@REMOVESPAM.operail.com
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Gord wrote:

Step #1 is to find out exactly what it is (MDF?) that you put down.
Step #2 is to determine if it would be suitable should you put a 1/8"-1/4" layer of ply on top. I suspect not...not because of lack of adhesion (which is horse hocky) but general characteristics.
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It probably is MDF on one side. I didn't think about it when I put the stuff down.
Anyway, wouldn't attaching more plywood by gluing and screwing it down effectively give me a thicker plywood?
_ Gord
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What's underneath the plywood?
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"What's underneath the plywood? "
That's what I I was wondering too. That's a big part of the equation here. The floor has to be stiff enough for the tile. . Plus as already pointed out, finding out exactly what the material is that you have put down is essential.
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The subfloor is 3/4" tongue and groove. I also put extra blocking between the 10" joists underneath that, so there should be no problem with stability. What I hope to do is attach additional plywood that can get around the problem with adhesion for the tiles.
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I have an apartment that I rehabbed and I had the same softwood 3/4" t&g, and I put ordinary sheathing plywood over it (1/2"). This was before all the cement board products were out. I put down ceramic tile with a high grade mastic (from American Olean, I think). This was not cement based like thinset. Not a single tile or a single grout joint has budged in 23 years. I believe your contractor is just wanting to use cheap thinset and making you pay for underlayment that will tolerate the cheap adhesive. It might be a good idea to simply change contractors at this point, and get someone that knows adhesives.
bill

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Is there an adhesive that would stick tiles to an MDF surface on the plywood?

Someone in an old post recommended "tile backer board", but this seems to be available only in 1/4" thickness. I don't need extra thickness and I would like to get something 1/8" or less.
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I don't know, but that's a pretty specific question you could ask someone in the tile adhesive business.
bill

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I would like to thank everyone for their suggestions. It turns out the surface was MDF, judging by the amount of smelly, noxious dust produced when I tried sanding it off with an angle grinder.
I gave up on the goal of keeping the level the same, and opted to put down additional 1/4" plywood using screws and yellow glue. The building supplies store manager gave the materials for free when I told him how I was sold the wrong type of plywood, so I'll just chalk it up to yet another bitter experience.
Thanks again,
_ Gord
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