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I have just pulled up my old ceramic tiles from my bathroom an there is the thinset on the subfloor an some grout left on it. Do I need to clean that before putting new thinset on it?

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Is the old thinset a complete layer across the floor? If not, you will need to fill the areas flush with thinset, otherwise it will make a big mess of laying new tile. The last tile floor I pulled up and relaid with new, I cleaned the old mastic. I use thinset to relay the new. If it was thinset, I would have cleaned that off too. The mastic gave me a small break. –  Jack Feb 23 at 3:35
    
If you do take it out by hammering on a chisel you run the risk of cracking tiles nearby. I would use a tile cutting saw to make some rough cuts and work a screwdriver, or chisel, or putty knife in the cuts to pry up the old thinset. –  getterdun Feb 24 at 1:38

2 Answers 2

Yes, you should remove the thinset. You can usually chisel it up with a hammer and cold chisel.

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You don't have to remove it. If it is in good condition and not fragmenting and adhering well, you can apply a layer of fresh thinset over it. However, this will increase the height of the finished floor an extra 1/8 or 1/4 inch (depending on whether you use a 1/4 inch or 1/2 inch, respectively, notched trowel).

If the finished floor should be at the same height as the previous, then get that stuff out. Once you get the knack of getting it to separate from the substrate, it is fairly quick and easy to do. I use an old putty knife and hammer it to slide cleanly under a good edge. Once it is far enough in, large pieces come off.

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