There are a number of reasons to consider installing a subfloor before installing adhesive tile, all of which have to do with how the new tile adheres.
1. Adhesive Residue
If you have removed the old flooring to apply the adhesive tile, the residual adhesives used to attach the old flooring may prevent the adhesive on your new tile from adhering properly to the floor. It can result in buckling. Even if you try to remove the old adhesives, there may be enough residue to prevent the tiles from sticking correctly. The installation of a new subfloor ensures a clean surface that will give your new adhesive tile a solid grip.
2. Uneven Floors
Floors may be uneven, especially in older homes. A new subfloor, properly installed, will even the surface and facilitate strong adhesion of your new tiles.
3. Surface Problems
If the old floor has old stains or imbedded soil on its surface, the new tiles may not stick. Once again, a subfloor eliminates this problem.
4. Previous Finishes
If the old floor has been oiled as part of its maintenance, it will interfere with the adhesion of new tiles. Installing a new subfloor eliminates the necessity of stripping it and eliminating every trace of oil.