First you'll need to ask yourself why you want to go right onto the plywood instead of the more conventional path of installing cement board over plywood.
Cement Board Fairly InexpensiveConvenience? Cost? Given that most bathroom floors are well under 100 square feet, it's not all that expensive or difficult to lay down cement board.
Assuming a bathroom floor of 75 square feet and pricing Durock cement board at $10 per 3'x5' board, you're only looking at a cost of about $50 to lay down Durock.
Reasons for Cement BoardThe cement board won't mitigate floor deflection (thankfully you have a solid floor). But it will provide an optimal surface for tiling:
- Bonding: Cement backerboard is the perfect bonding surface for the thinset.
- Fill: When you lay down cement board on plywood, you will trowel thinset between the two. The cement board smooths over imperfections in the plywood, and the thinset below the cement board will fill in voids.
If You Want to Install Tile Directly on Plywood...Plenty of pro tile installers do install tile right onto plywood. Bill Vincent of Creative Ceramic and Marble, based in Bridgton, Maine, says that you can indeed tile over plywood--and he does this all the time.
Vincent mentions at least two conditions that must be met in order to do this:
- Proper Thinset: He notes that "a quality unmodified thinset be used and mixed with a latex additive, such as Laticrete's 317 thinset mixed with their 333 liquid latex additive." The reason for this is that it will give you a higher latex content, essential for bonding to plywood.
- Proper Plywood Subfloor: This one is a little trickier, and may not be within your control if the subfloor has already been laid. Vincent recommends two layers of exterior grade or better plywood, with the top layer screwed down every 8" on the edge and every 6" in the field (interior). Make sure the joints of the top layer are well away from the joints of the bottom layer. Leave a 1/16" gap for the joints. But here's the most important part: Vincent stresses that "when screwing down the top layer...you're going no further than the bottom layer of plywood." This, he says, "...negates the effect of double layering the floor by transmitting the movement from the joists right to the top layer of plywood."