No. The safest, most prudent course of action is to install tile on top of a cement backerboard, such as Durock®, Wonderboard®, or HardieBacker®.
Cement backerboard is made for tiling. By the way, we are referring to mortared, grouted tile, not vinyl squares. Backerboard does not shrink or expand when it comes into contact with water (mortar and grout both contain water.
To see how much plywood can shrink, consider a typical 4' x 8' sheet of plywood. When room humidity is cut in half, that sheet will shrink about a quarter of an inch lengthwise if unrestrained by fasteners. So even though plywood is considered to be dimensionally stable, it will still shrink/expand somewhat. Because tile is brittle, it cannot tolerate base materials that have any kind of movement.
If you want to do it right, lay down cement backerboard before the tile.
But Is It Even Possible to Install Tile on Plywood?You're looking for a more nuanced answer, I suspect, because there are instances when you really don't want to put down backerboard.
It could be that installing backerboard will raise the floor level that much higher, making the transition between floors more difficult. Or you're short on time and want to complete the project quickly. It could just be that you need to cut corners, reasons unspecified. Whatever the motivation, you want to determine if there are instances when you can lay down tile straight to plywood.
What do other voices say about this?
- Premium Thinsets: Some tilers recommend a high-quality, crack-isolating thinset mortar.
- Perimeters: Be careful to the perimeter edges of your tile. These are the areas where tile is most likely to experience forces which lead to cracking.
- Uncoupling Tile Membranes: Use a waterproofing, uncoupling membrane, like Schluter Ditra, between the tile and the plywood. A tight, unmoving bond between mortar and subfloor causes cracking. This membrane effectively uncouples the bond between plywood and tile mortar, allowing both surfaces to move independently of each other. Also, these uncoupling tile membranes prevent moisture infiltrating any surfaces below. Downside: they still do add height to your installation, as you have two layers of thinset plus the membrane. Keep in mind that, with Ditra, you still need a couple of layers of plywood. So, in the end, you're not saving much in terms of height.