Laying a new floor over existing surfaces is a popular choice for homeowners looking to upgrade without the hassle of full demolition. When the subfloor is plywood, the process requires specific steps to ensure the tiles adhere correctly and the surface remains level for years. Success depends on preparing the old surface, selecting the right underlayment, and following a meticulous installation procedure.
Assessing Your Existing Plywood Subfloor
Before any tile goes down, you must evaluate the condition of the plywood beneath. The subfloor needs to be structurally sound, flat, and free of any issues that could compromise the tile work later. Ignoring this step often leads to cracked grout or loose tiles.
Check for Stability and Moisture
- Press down on the floor to check for any give or soft spots, which indicate rot or water damage.
- Look for signs of moisture intrusion, such as discoloration or musty odors, which can ruin adhesive.
- Ensure the plywood is securely fastened to the joists below; loose boards will cause the tile to crack.
Measure for Levelness
Tile requires a flat surface to adhere properly. Use a long level or a straightedge to check for high and low spots across the plywood. If the variation is more than 1/8 inch over a 10-foot span, you will need to self-level the floor or install a cement backer board.
Choosing the Right Materials and Underlayment
You cannot simply tile directly onto plywood with any type of adhesive. The combination of movement in the wood and the rigidity of the tile necessitates a proper isolation membrane or backer board. Using the wrong materials is the primary reason DIY tiling projects fail on plywood.
Membranes vs. Backer Board
For waterproof areas like bathrooms, a polymer-modified thin-set and cement board are standard. However, for dry areas, a flexible membrane is often sufficient. Ditra or similar uncoupling membranes are ideal because they allow the plywood to move slightly without transferring stress to the grout lines.
- Membrane: Creates a slip barrier and isolates tile from substrate movement.
- Cement Board: Adds rigidity and provides a solid surface for thin-set, but requires screws to plywood.
Surface Preparation is Key
Skipping surface preparation guarantees failure. Plywood is porous and uneven, which will cause the thin-set to dry too quickly or pool in low spots. You need to create a consistent, clean canvas for the adhesive to grip.
Cleaning and Stripping
Remove all dirt, dust, and old finishes. If the plywood was previously finished with oil or wax, you must strip it entirely. Lightly sand the surface to cut through any gloss and vacuum thoroughly to ensure no debris remains between the tiles.
The Layout and Dry Fit
Professional installers always start with a dry layout. This involves placing the tiles on the floor without adhesive to visualize the final pattern and ensure cuts are made in inconspicuous areas.
Finding the Center
Measure the room and mark the center point. Lay out two perpendicular chalk lines. This grid ensures your cuts end up against the walls rather than in the middle of the room, which looks unprofessional. Because plywood joints are usually oriented in specific directions, plan your layout to avoid small slivers of tile at the edges.
Mixing and Applying the Adhesive
The type of thin-set mortar is critical when bonding to plywood. You need an adhesive with high tensile strength to handle the movement of the wood. Using a standard "mudding" compound will result in the tiles popping up or shifting.
Notching the Adhesive
- Use a notched trowel to spread the thin-set.
- The size of the notch (1/4" or 3/8") depends on the tile size and the flatness of the subfloor.
- Apply the adhesive in sections to prevent it from drying out before you set the tile.
Setting the Tile and Managing Expansion
Wood moves with changes in humidity and temperature. Unlike concrete slab subfloors, plywood requires room to flex. If you lock the tiles edge-to-edge without movement space, the stress will cause the grout to crack or the tiles to buckle.
Installing Transition Strips
At doorways or transitions to different flooring types, use a metal or rubber transition strip. This covers the gap left by the expansion joint and protects the edges of the tile from chipping underfoot.
Grouting and Sealing the Surface
Once the adhesive has cured, you can begin grouting. Choose a sanded grout for joints wider than 1/8 inch to prevent cracking, and a non-sanded grout for narrow joints. When grouting over plywood, it is wise to use a flexible urethane grout or a high-quality polymer-modified grout to handle movement.
Maintenance Tips
After the grout sets, apply a penetrating sealer if you used natural stone. For porcelain and ceramic, sealing is optional but helps repel stains. Regular cleaning with a pH-neutral cleaner will preserve the finish and protect the underlying plywood for decades.
