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Old 12-29-2009, 09:26 AM   #1
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Problems with Porcelain over Radiant Concrete

It's the middle of winter here in Philadelphia area, and I'm just wrapping up an extended kitchen renovation. The last piece in the puzzle is the flooring.

Concrete slab on 1st floor (no basement) with old-style radiant heat in copper pipe. There is a layer of what looks like black adhesive (petroleum based?) on the kitchen floor which was probably used for a prior vinyl tile installation or something.

I bought 18" porcelain tiling, VersaBond fortified thinset, grout.

When laying the tile, while being helped by my father, we spread the thinset in large areas, sometimes before cutting the tiles. I'm not even sure we mixed the thinset correctly. Because he did a single tiling job before, I was deferring to his judgment on how to do all this.

So, about 5 days later my wife and I are inspecting the job while preparing to add the grout. We realize several of the tiles are loud and hollow underneath. Now I'm not sure if this was due to thinset which dried before tiles were placed, or poorly mixed thinset.

Anyway, long story shorter, we started pulling some tiles. Underneath them were large, very damp spots of thinset, that came off in large chips. There were green spots which appeared like mold. There were driplets of condensation on the tile backs.

I know we didn't approach this project like we should have and I will certainly be preparing the thinset properly and spreading it, tile by tile, after the tiles are cut next time. Outside of that, what's up with all the condensation?
  1. I was told that this next time around we should "rough up" the black adhesive layer on the floor. Is this part of the problem?
  2. I'm trying to think about the fact that the floor gets hot and cold as the heater goes on and off - would this hurt the reaction of the thinset? Would it create the excessive condensation?
What are our issues here? Is tiling even a good idea in this room?

Recommendations/advice is most appreciated.


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Old 12-29-2009, 12:56 PM   #2
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I guess you didn't like the excellent advice to got at the other forum? I love the internet.

You can start by knowing that black adhesive is cut-back, and you should have scraped it away until only a film remained and you could almost see concrete.

It sounded like the thin set may not have been mixed correctly. Sounded like you did not "burn" the thin set into the slab with the flat side of the trowel first. Sounded like you did not use the proper trowel and so did not apply the mortar thick enough. Sounded like you let the thin set skin over, and you set the tiles over dried mortar. Any one of these would cause you trouble.

Who told you to rough up the black stuff? Have you been hanging out at those big stores where the associates wear vests?



TILE GUY - retired- TROY, MI - Method & Product suitability consulting.

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Old 12-29-2009, 02:42 PM   #3
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Thanks JazMan. I didn't realize the same folks were on both forums.

Because I don't know jack about what I'm doing (as is clearly obvious), I'm hedging my bet with the answers I receive. Additionally, my post at the other forums was not very well constructed initially and I had to add several replies to myself and I wasn't sure that would get a full response, or if it would simply receive a response to my first post only.

I've taken notes on what you and Bud both had to say.

And to answer your question, I got the tile at FloorsUSA - they only have 2 stores... I got some other disinformation from an ex-mason at the big box H.D., where I was told immediately that Drylock was absolutely necessary. The kitchen contractor didn't note the black coating at all when I told him I was laying tile either. So, in the end I went into this completely blind.

This time around I'm going to the forums (here I am!) and will be following the advice, along with the book advice, and being thorough and completing the job myself to avoid shoddy workmanship. Problem solved, but money and time lost due to first mistakes. C'est la vie.
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Old 12-29-2009, 02:45 PM   #4
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Oh, JazMan, how do I prevent the thinset from "skinning over"? Is that different than just letting it sit too long drying before laying the tiles?
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