When installing tile, having a solid and tight substrate that will not shift, expand, or contract is all-important. The most skillfully laid tile floor or wall can be ruined with a base that is not dimensionally stable. Many other types of floor coverings such as laminate or luxury vinyl tile have more flex and are more forgiving. But ceramic, porcelain, and glass tile, along with their service components of thinset and grout, are far too fragile to withstand even the slightest substrate movement.
A number of different types of substrate are used for tile, including plywood. While most floor tile these days is installed onto a layer of cement board or similar backerboard, it is possible to lay tile directly over plywood. But a number of precautions must be taken to ensure a successful installation:
For Flooring, Use Intervening Sheets of Plywood
While tile can be laid on plywood, do not install tile directly onto the plywood subfloor itself. Use intervening layers of two sheets of plywood.
Use Thinset With a Higher Latex Content
Thinset is the wet base that you first trowel on the plywood to make the tile stick. Higher latex content is essential for bonding to plywood. A quality unmodified thinset should be used and mixed with a latex additive. One example would be to use Laticrete's 317 thinset mixed with Laticrete 333 liquid latex additive.
Use the Right Type of Plywood
Exterior grade or better plywood should be used for the substrate. Under no conditions should you use veneer-bond plywood. This is interior-grade plywood that has a top and bottom layer of very thin hardwood bonded to it. Laying tile to veneer plywood risks loosening and unbonding the veneer, creating an unstable base for the tile.
Space the Field and Edge Fasteners Correctly
The top layer of plywood should be screwed down every 8 inches on the edge and every 6 inches in the field, or interior, section.
Space Out the Board Joints
Joints of the top layer should be well away from the joints of the bottom layer. Avoid having two joints on top of each other.
Leave a Proper Gap Distance
Leave a 1/16-inch gap for the joints to allow for expansion and contraction. Leaving no gap means that the edges of two adjacent sheets of plywood may press against each other and deform when the plywood expands. Leaving a gap allows for expansion.
Achieve Proper Screw Depth
When screwing down the top layer, make sure that you are screwing no farther than the bottom layer of plywood. Driving screws all the way down to the joists negates the effect of double layering the floor by transmitting the movement from the joists directly to the top layer of plywood.
Consider an Uncoupling Membrane
One popular way to augment plywood substrate is with a waterproofing, uncoupling membrane between the tile and one layer plywood. Schluter Ditra is one popular brand of a membrane. Schulter expressly notes that Ditra replaces that second layer of plywood.
A tight, unmoving bond between mortar and subfloor causes cracking. An uncoupling membrane does exactly what the name 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.
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 with uncoupling membranes is that they do add height to the installation, as you have two layers of thinset plus the membrane.
Plywood Substrate Alternatives
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 and filling. First, cement backerboard is the perfect bonding surface for the thinset because of its porous texture. Second, cement board helps with 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.