the old has to be in good condition or I mean fastened down in all areas. Any old tile that is loose you can take that square up and fill in with a dummy tile, the same thickness, and then go on.
I have found that the key is on going over a good old floor is to make sure its cleaned good. Any grime, sticky stuff, or whatever will contaminate the glue of the new flooring.
I do this quite alot and it will work IF......
Also, make sure that you do not place the new square directly over the old square, stagger the new tile so that it sits over the intersection of the four old ones.
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