When installing tile, having a solid and tight substrate (or base) that will not shift, expand, or contract is critical.
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.
So, follow these tips for installing and using plywood as a substrate for ceramic, glass, stone, or porcelain tile:
Use an Intervening Sheet of Plywood
Tile can be laid on plywood. But do not install tile directly on the plywood subfloor itself. Use an intervening layer of a sheet of thinner 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.
Use the Right Type of Plywood
Exterior grade or plywood should be used for the substrate. Under no conditions should you use veneer-bond plywood. Veneer plywood 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 releasing the veneer, creating an unstable base for the tile.
Space 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. Make sure that all screw heads are screwed flush with the level of the plywood.
Space Out 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 Between Boards
Leave a 1/16-inch gap for the joints to allow for expansion and contraction.
If you leave no gap between boards, the edges of two adjacent sheets of plywood may press against each other and deform when the plywood expands. Leaving a gap allows for this inevitable expansion.
Test Plywood For Flex
After you lay the plywood, check it for deflection with this simple test. Set a dinner plate on the plywood. Set a cereal bowl on the plate. With a separate container, fill the bowl to the rim. Then, walk on the plywood around the bowl. You should see some ripples from vibration, but the water should not spill onto the plate.
If it does, add some more screws to the boards. If that doesn't do it, then the floor joists may be faulty. Shoring up the joists from below by sistering them with two-by-tens or two-by-eights is one way to strengthen joists.
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.
Use 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.
Consider 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) as opposed to 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.