How to Lay Tile on a Plywood Subfloor
Installing tile on plywood is possible. But you need to take a few precautions to make sure that the plywood does not crack the tile. Follow these steps for installing and using plywood as a substrate for ceramic, glass, stone, or porcelain tile.
Why Tile Must be Uncoupled from Plywood
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 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.
That is where plywood comes into the picture. Plywood and tile are dissimilar materials—one is wood and the other is clay—so they will expand and contract at different rates. Plywood is remarkably dimensionally stable. Even so, it's not stable enough for tile. Even concrete is not a suitable base for tile; concrete must be uncoupled from the tile.
Add an uncoupling material to the substrate to successfully install tile on plywood.
Uncoupling Membrane for Tile and Plywood
Schluter-Ditra is one popular brand of membrane. 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 the other. In addition, uncoupling tile membranes prevent moisture from infiltrating surfaces below.
One downside with uncoupling membranes is that they do add height to the installation, as you have two layers of thinset plus the membrane. But a positive of uncoupling membrane is that it adds less height to the stack than if you had used cement board as an uncoupling material.
What You'll Need
Equipment / Tools
- Cordless drill
- Notched trowel
- Rubber float
- Sponges
- Bucket
Materials
- Plywood sheets
- Modified thinset mortar
- Unmodified thinset mortar
- Plywood screws
- Uncoupling membrane
- Ceramic or porcelain tile
- Tile spacers
- Grout
- Grout haze remover
Instructions
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Install an Intervening Sheet of Plywood
Install an intervening sheet of thinner plywood on top of the subfloor plywood. Do not install tile directly on the plywood subfloor itself.
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Leave a Proper Gap Between the 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.
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Screw the Plywood Down
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. If the plywood flexes, add 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 2x10s or 2x8s is one way to strengthen joists.
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Trowel On the Modified Thinset
Use the notched trowel to apply thinset to the plywood. Be sure to use a modified thinset mortar for this first layer.
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Lay Down the Membrane
Press the uncoupling membrane into the wet thinset. Do not let the thinset dry; work quickly.
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Trowel on the Unmodified Thinset
Switch batches in the bucket from modified thinset to unmodified thinset. Use the notched trowel to apply the unmodified thinset to the top of the uncoupling membrane.
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Install the Tile
Apply the tile to the layer of unmodified thinset. Use plastic tile spacers to keep the tiles separate.
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Apply the Grout
Use the rubber float to apply the grout to the surface of the tile. Pull the grout in long sweeps diagonally across the tile. Remove as much grout as possible from the tile. Be careful not to dig into the grout between the seams.
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Remove the Grout Haze
Mix grout haze remover in a clean bucket. Apply the haze remover with a sponge to remove the haze from the tile.
Tips for Installing Tile on Plywood
Use High-latex Thinset
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.
Test the 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.
Sink the Screws Properly
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.
Cement Board as a Plywood Alternative
An alternative to installing an uncoupling membrane is to install tile on top of a cement backerboard such as Durock, Wonderboard, or HardieBacker.
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 cure 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 the 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.