It should be 18mm thick at least for the fixing to be correct. Make sure the joints are well glued together and secured well to the floor.
If you find the floor is not solid to the joists or is 'bouncy' after the fixing, then you need to ply out with at least 15mm plywood.
In any case, you will need to prime the surface with acrylic primer (as directed by the brand on the label) and use a corresponding brand of flexible adhesive & grout.
Use a bead of colour corresponding silicone around the floor edge to act as a movement joint
