Laying tile over concrete floor?

alstAugust 29, 2007

Hi, another bathroom tile question.

The bathroom we are finishing is in the basement of our house, and the floor is concrete. Can we install tile directly over the concrete, or does it need some kind of backing? I've never worked with tile before, so apologies in advance if this is a dumb question.

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fnmroberts

Has your concrete been painted or sealed? If yes, it needs to be removed.

Are you planning ceramic tiles or "peel and stick"? If you plan on using ceramic, the mastic will be sufficient. If "peel and stick" I suggest a spreadable (via a notched trowel) adhesive, let it cure until tacky, then peel the backing from the tiles and install them. My reasoning is that your concrete floor will have some unevenness which will cause "peel and stick" flooring to have hollow spots when walking upon it and the adhesive will fill these.

Hope this helps.

    Bookmark   August 29, 2007 at 9:48PM
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mongoct

Before you do anything else, try a very rudimentary test to see if moisture is coming through your slab.

Tape a 2' by 2' square of clear plastic to the floor. Seal all four edges of the plastic to the slab, duct tape is fine.

Leave it there for several days, a few weeks is better, and best is to have it there a few days after it has rained.

If water vapor is coming through the slab, the plastic will trap it and cause it to condense into liquid.

Once we get past that step, we can move forward.

However, I'll caution against mastic and instead recommend a modified thinset. Mastic is water soluble, and in the preceeding example, water vapor coming through the slab could essentially dissolve the mastic.

But again, let's address the possibility of water vapor first.

And by the way, this is not a dumb question.

Asking questions before you proceed is the smarter thing to do.

Best,

Mongo

    Bookmark   August 29, 2007 at 10:47PM
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pete_p_ny

Mongo, can you set tile on a painted floor. My basement was tiled over a painted surface using modified thinset. I recall the contractor saying he added a milk additive, which sounded funny, but I guess that was more latex. The painted surface was intact and has been there since the house was built in the 1960s. I mentioned my concern about the painted floor, and he said it was not a problem. No water test was done either.

If there is water vapor coming throught the floor, can tile be set using thinset...? Or is this a condition of a NO can do installation.

    Bookmark   August 30, 2007 at 7:27AM
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shaughnn

Pete,
Tile should never be set over a painted surface. The correct method of installation would have been to mechanically remove the paint(grinder with vacuum) before installing the tile with a modified thinset.
"Mastic" and "pre-mixed thinset" should NEVER EVER EVER be used to set floor tile.
Shaughnn

    Bookmark   August 30, 2007 at 9:13AM
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pete_p_ny

Well that is bad news..I guess my tiles will pop off in the future. How do you grind a large basement floor??? Is there some sort of machine for this other than a small hand held angle grinder.

    Bookmark   August 30, 2007 at 9:42AM
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mongoct

Pete,

"Milk" is a nickname of sorts for latex additive.

Yeah, the floor should have been scarified for the best installation.

No need to worry about it now, as it's already tiled over.

You can rent walk-behind machines that will prepare a concrete surface for tile. A lot easier and faster than an angle grinder.

Mongo

    Bookmark   August 30, 2007 at 10:53AM
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alst

Hi again everyone, and thanks for the help.

The concrete has not been painted or sealed. It is a newly poured basement floor. I believe it was poured approximately three weeks ago.

I don't know if there is moisture seeping out or not, we aren't in the home yet and don't have full-time access to check it.

We are planning on ceramic tile... we will pick it out and then the contractor will install it before the purchase of the home is final.

I hope that helps! Thanks again. You are all lifesavers!

    Bookmark   September 2, 2007 at 3:24PM
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popollie28_frontier_com

we just finished removing old tiles on our basement floor there is still some black adhesive but its dried up . we would like to just paint the floor should we be aware of any problems with the dried old glue . could we just tile over the floor if we decided not to paint thanks

    Bookmark   January 15, 2011 at 12:27PM
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