Assuming you are using wall tiles which are generally lighter than floor tiles, you can use an appropriate tile mastic. They shouldn't fall off. I have done alot of ceilings and aside from a very stiff neck, have never had any problems. Make sure that you firmly press each tile into place to set it into the adhesive. I wouldn't pound on it though! Good luck.
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - TileProductions.com
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