Prepping A Plywood Subfloor For Tile After Demolition

bluethumbnail

Hi guys.
I am demoing up a very sound tile floor in a large kit/din rm for a second (referral) job. The last floor was qtr.inch, porcelain-over-plywood-over-plank subfloor; (had to be diligent with the demo hammer to strip the tile). The plywood subfloor had such a tough residue, I just went over it with cement board screwed to the plywwod rather than take it up. Then, marble over that. The application raised the level of the floor a half inch(3/8 backer and 3/8 marble, less the demo). Turned out very nice anyway!
Question: Should I have used an adhesive remover to clean the plywood and apply the marble directly to the plywood after painting on a sealer coat? If I can improve on this job, I'll be a hero! (again).

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Mike Finley

You should have applied a coat of thinset between your backer and your plywood which would have done much more for you then the process you are asking about.

Most tile jobs will look pretty nice when completed, the proof in the pudding is 2 years from now.

troubleseeker

You are much better off IMO with the backer board than sticking the tile to the plywood, whose movement will cause the tiles bond to fail.

It is recommended that you bond the backer to the sub-floor with thinset as Mike Finley has stated, but I have not seen any floors fail because of lack of this, as long as the backer was nailed down sufficiently.

Mike Costello

Im confused. How can you improve on the job if its already in?

Just for the record and in the future ALWAYS thinset your backerboard down .If you dont Id bet a Dodge Challenger those tile will crack or come loose

bluethumbnail

I fully screwed the backer to the plywood in an 8" grid with 1-1/4" screws. I would think an 8" grid would be sufficient to stabilize the backer board. Ater all, the previous 1/4" tiles were set (thinset) directly to the plywood. This is not to say that I disagree with you to set the backer with thinset. Makes good sense.
However, being the kind of person that I am, I must consider the poor slob who has to take up the floor some day! I just hope it's not any time soon!
Thanks guys.

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Move your damn THUMB!

Mike Finley

I've gone into the subject of thinset under backer before and how I am of the firm belief that you CAN get by without it in certain circumstances. BUT When you say this:

Because of all that this has little bearing on success of this floor over time:

You've got STONE over backer, with backer with No Thinset over plywood with lots of areas of left over THINSET on it, absolutely ensuring voids between the backer and the plywood which will lead to cracked grout and worse within probably 8-18 months without a doubt. I'm assuming to add insult to injury based on your descriptions that you probably have 3/8 or 1/2 plywood over the dimensional lumber floor.

Unfortunately, you've broken just about every rule there is to a properly installed stone floor. Sorry for the bad news, but either tear up the floor and install it properly or memorize this customers phone number so you recognize it when they do eventually start hounding you for the problems with this floor.

Brock

oh my! this job will need to be redone. it wont last 2 weeks.

Floordude

It's already cracking now!!!

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Bill_Vincent

If it's not installed properly, and there's a whole lot more to take into consideration than tiling over CBU. Leave out or shortcut just one spec, and the floor's coming back up.

You don't "bond the CBU with the thinset. It's there more as a cushion, to take out the vibration between the CBU and subfloor. That's why you screw it down, as well. AS someone once put it, the screws hold it down, while the thinset holds it up.

One way or the other, that thinset DOES need to be there, especially being you have old thinset on the floor CREATING voids.

One last thing-- NEVER use any kind of chemicals to remove adhesive or thinset residue on a floor surface which will directly receive tile afterward. Any residue left behind (and no matter how meticulous, there WILL be some) can and will inhibit the thinset bond, later. if I've got to remove old thinset from plywood, this is what I use, hooked up to a shop vac:

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