Can You Install Tile With Thinset on Concrete Floor That Has Mastic?

Man Installing Tile on Concrete Floor

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Question: I am tiling (mosaic and small pieces) over what once was a concrete basement floor, which on occasion, in [my city] with all the rain can get flooded. What was there when I bought the house 2 years ago was an asbestos (?) linoleum 1' square very thin tile over a thin black mastic.

I removed the tile but not the mastic and began to lay my 2 " tiles and mosaic. I went to a local tile shop...and they sold me their best (Ultraset?) thinset mortar which I am mixing with 447 primer and also Weldbond and water. When I walk on what I have tiled, sometimes I pop up a tile or a couple tiles, but over most of the area I can walk fine.

I began using the 447 primer over the mastic and that helped some. I don't want to go further till I know what I need to do to the mastic (it's a very thin coat as if applied with a brush or roller, it's not at all lumpy). The thinset I am using now is M Ultrabond with the additives.

Answer: I'm not clear how far you are into the project; this makes a difference to me with regards to my advice. It sounds like you did a little bit, but now are having reservations?

Are you having reservations because some of the tiles pop up?

It is a good thing if they are popping only because you are walking on them prematurely. If that's the case, then you're in luck--it's just a simple matter of waiting until the tiles are fully set.

A pop sounds to me more like a problem of properly distributing the thinset than it does with having mastic as a base layer. As you say, the mastic is paper-thin, no lumps. Mastic should not be the cause of the pops. If you have any problems with mastic as the ultimate base, these are problems that would develop over time--not within the space of a few weeks.

You probably already know that mastic is difficult to remove from concrete. When it's as thin as you describe, it's practically embedded in the concrete's pores.

The best solution (if you have ceiling height to spare) is give yourself a fresh surface to lay tile on. Lay cement board across the entire surface. This will give you a surface that's better than a concrete floor.