Bamboo Flooring Over Tile

Q: When installing floating Bamboo flooring over uneven concrete (Old tile mortar and slightly uneven) What would be the best material to use underneath?
A: When installing a floating floor over old tile or just remaining mortar, it is always suggested to make this floor as flat as possible. Now, if you still have the tile laid down and your worried about grout lines, you should have little issue as long as you use a vapor barrier and a good underlayment. I would suggest using 6mm cork or 3mm cork at a minimum.
If you have removed the tile and have only the mortar remaining you must grind this down. Similar to the discussion on removing thinset, Try using an electric chipper or a grinder. This will be dusty, so wear eye protection and a dust mask. Remember with any floor, it is important to have the subfloor clean, dry and flat.
After you have prepared your subfloor, I would still suggest using a 3mm cork underlayment for your floor. Cork is dense, so it will provide a better feel under foot, and its natural sound dampening and insulation qualities will be great. Good luck!