how you plan on screwing them down? i suppose you could but if you want something that will last i wouldnt.
i have heard that you can rough up the surface of the tiles and skim them and then lay the tile directly over that but that might cause more problems then it would to take it all out and start over. and think of the extra weight on the floor going over it with more tile.
I second Danny's post----Rip those out --if they are less than 50 years old they are simply glued over a thin underlayment or glued directly to the subfloor.--the glue is not good any more and is not safe to go on top of.
If the mosaic is set in a thick concrete base (over 50 years typically)--then you can scuff the glaze with a grinder and tile right over the old mosaic (using a two part thinset like Kerabond and Keralastic)
Post a picture---lift up an air duct and see what the floor system is made of.--Mike---
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