The short answer to the first question: Yes. Before you go too far, ask the opposing question: Why not use cement board over the plywood?
Veteran Tile Man Tells the Best Way To Do It--If You Do It
Plenty of professional tile installers do install tile directly onto plywood.
Bill Vincent of Creative Ceramic and Marble, based in Bridgton, Maine, set his first tile in 1967 and has been in the tile trade since 1980
However, it is important to know that you should not install directly onto the plywood subfloor. You must have an intervening layer of one sheet of plywood.
Several conditions must be met in order to do this:
1. Proper Thinset: Higher Latex Content
The first condition is to have the right type of thinset. Thinset is the wet base that you first trowel on the plywood to make the tile stick.
Vincent notes that "a quality unmodified thinset [should] be used and mixed with a latex additive, such as Laticrete's 317 thinset mixed with their 333 liquid latex additive."
Higher latex content is essential for bonding to plywood.
2. Proper Plywood Subfloor: Five Conditions That Must Be Met
Ensuring a perfect subfloor may be difficult and may not be within your control if the subfloor has already been laid.
However, if you are laying your own plywood, Vincent recommends these conditions:
- Correct Plywood: Two layers of exterior grade or better plywood
- Field and Edge Fasteners: The top layer of plywood should be screwed down every 8" on the edge and every 6" in the field (interior).
- Board Joints: Joints of the top layer should be well away from the joints of the bottom layer.
- Gap Distance: Leave a 1/16" gap for the joints for allow for expansion and contraction.
- Screw Distance: As for the real trick to getting this right, Vincent stresses that "when screwing down the top layer...[make sure that] you're going no further than the bottom layer of plywood." Driving screws all the way down to the joists, he says, "...negates the effect of double layering the floor by transmitting the movement from the joists right to the top layer of plywood."
Alternative: Use Uncoupling Tile Membrane Like Ditra
Or you can use a waterproofing, uncoupling membrane between the tile and one layer plywood. Schluter Ditra is one popular brand of membrane. Schulter expressly notes that Ditra replaces that second layer of plywood.
As mentioned, a tight, unmoving bond between mortar and subfloor causes cracking. Uncoupling membrane does exactly what it says: it uncouples, or releases, the bond between plywood and tile mortar, allowing each surface to move independently of each other.
In addition, uncoupling tile membranes prevent moisture from infiltrating surfaces below.
Shulter mentions that tile uncoupling is not a new practice. Shear release interfaces have been used for thousands of years, beginning with the Romans and Greeks.
One downside: they do add height to your installation, as you have two layers of thinset plus the membrane.
Schulter Ditra is a pricey product when used over large expanses of flooring. A 54 sq. ft. roll currently runs about $86 at Home Depot. For an average kitchen sized at 150 square feet, you would spend $260 just for the Ditra.
Or Go the Cement Board Route
The safest, most prudent course is to install tile on top of a cement backerboard, such as Durock®, Wonderboard®, or HardieBacker®.
Cement backerboard is made for tiling. Backerboard does not shrink or expand when it comes into contact with water (mortar and grout both contain water).
Even though exterior grade plywood is designed to stand up to harsh weather, nothing can tolerate moisture better than a product that is entirely composed of minerals (cement board) vs. an organic product (plywood).
Cement board will not mitigate floor deflection, but it will provide an optimal surface for tiling, by:
- Bonding: Cement backerboard is the perfect bonding surface for the thinset; and by
- Filling: When you lay down cement board on plywood, you will trowel thinset between the two surfaces. The cement board smooths over imperfections in the plywood, and the thinset below the cement board will fill in voids.