If I be lay tile surrounded by a bathroom , do I hold to use underlayment ?

Do I use underlayment or set on floor already there? Im doing clay tile

Answers: The floor needs to be stiff, no flexing or flaccid. It also must be clean for tile glue to stick. If in doubt put down a plywood or concrete board underlayment.

I agree near morris. Also, use grout sealer after the job is done. You don't want to gain the floor wet below the tile.