Replacing plank subfloors with tile is common in renovations, but success depends on the substrate, stiffness, and proper preparation. This article explains when tile over a plank subfloor is feasible, step-by-step preparation and installation methods, and best practices to avoid cracks and failures. Key takeaway: Proper structural stiffness and a suitable underlayment are essential for long-lasting tile.
| Situation | Feasibility | Recommended Approach |
|---|---|---|
| Plank Subfloor With Adequate Stiffness | Possible | Plywood Overlay + Backerboard Or Uncoupling Membrane |
| Loose Or Gapped Planks | Not Recommended | Repair Or Replace Planks First |
| Water-Prone Area (Bath, Shower) | Conditional | Waterproof Membrane + Proper Slope |
| Thin Planks On Wide Joist Spans | Not Recommended | Add Rigid Plywood Structural Layer |
Why Tile Over Plank Subfloor Requires Special Care
Tile is rigid and brittle; it does not tolerate movement in the substrate. Plank subfloors—especially older, softwood tongue-and-groove boards—can flex, gap, or settle, causing grout to crack and tiles to pop. Addressing stiffness and movement is the priority before any tile work.
Evaluate The Existing Plank Subfloor
A thorough inspection determines whether to proceed, reinforce, or replace. Check for rot, insect damage, soft spots, excessive gaps, loose boards, and joist spacing. Measure deflection by walking across the area; noticeable bounce indicates insufficient stiffness.
Joist Span And Deflection Requirements
Tile installations generally require a deflection limit of L/360 for ceramic/porcelain and stricter for natural stone. Calculate based on joist span and spacing. If the structure fails to meet L/360, add structural layers or sister joists.
Repair And Reinforce Before Tiling
Fix loose boards, replace rotted planks, and tighten fasteners. Use galvanized screws to secure planks to joists. Fill wide gaps with appropriate materials or replace affected boards. Ensure a continuous solid surface before overlaying.
Overlay Options For Creating A Tile-Ready Surface
Several overlay methods convert plank subfloors into suitable tile substrates:
- Plywood Overlay: Install 1/2″ or 3/4″ exterior-grade or CDX plywood perpendicular to planks, glued and screwed to meet thickness/stiffness requirements.
- Cement Backerboard Over Plywood: After plywood, install 1/4″ or 1/2″ cement backerboard with thinset and screws; butt joints and tape joints per manufacturer.
- Uncoupling Membrane (e.g., Schluter-DITRA): Mounted over a proper plywood or smooth plank surface; helps prevent cracks due to minor movement.
- Thick Rubberized Underlayment or Fiber Cement Panels: Specialty panels can replace backerboard in some cases; follow manufacturer guidance.
Plywood Overlay Thickness Guidelines
| Existing Plank Thickness | Plywood Overlay | Resulting Stiffness |
|---|---|---|
| 3/4″ Plank On 16″ Joist Spacing | No Overlay Needed If Stiff | Often Acceptable With Fastening |
| 3/4″ Plank On Wide Spans (>16″) | Add 1/2″ Plywood | Meets L/360 In Many Cases |
| 1/2″ Or Thinner Planks | Add 3/4″ Plywood | Recommended For Tile |
Surface Preparation Steps
Prepare a clean, stable, and level surface to ensure proper tile adhesion. Remove paint, old adhesives, and debris. Sand areas with high spots. Vacuum and damp-mop to remove dust before applying adhesives or membranes.
Fastening And Glue Practices
Use construction adhesive between layers and screw every 6–8 inches into joists when installing plywood overlay. For backerboard, use manufacturer-recommended screws and fill joints with alkali-resistant mesh tape plus thinset for a continuous surface.
Choosing The Right Underlayment Or Membrane
Select materials based on movement control, waterproofing needs, and tile type. Cement backerboard gives a direct cementitious surface. Uncoupling membranes offer crack control and are easier to install over minor imperfections. For wet areas, choose a waterproof membrane system.
When To Use Uncoupling Membranes
Uncoupling membranes are beneficial when minor substrate movement is expected or when avoiding thick backerboard. They require a firm, flat substrate—often plywood overlay—and provide lateral movement control to protect tiles and grout.
Waterproofing Considerations For Bathrooms And Wet Areas
Waterproofing is non-negotiable in showers and tub surrounds. Apply a bonded waterproofing membrane (liquid or sheet) over the backerboard or use integrated systems like Schluter-KERDI. Ensure proper slope at shower floors and proper detailing around drains and penetrations.
Tiling Techniques Over Prepared Subfloors
Use full-bed thinset mortar and proper tile-sizing techniques. For large-format tile, follow the manufacturer’s recommendations for mortar coverage (often >95%). Use a notch trowel suited to tile size and back-butter large tiles for full contact.
Grout, Movement Joints, And Perimeter Isolation
Install movement joints at transitions, long runs, and at perimeter edges using flexible caulk or expansion materials. Grout should be chosen for expected use (sanded vs. unsanded) and sealed if necessary for porous tiles.
Tools, Materials, And Cost Estimates
Typical tools: circular saw, drill, cement trowel, notched trowel, tile spacers, level, tile cutter, mixing paddle. Materials include exterior-grade plywood, thinset mortar, backerboard or membrane, screws, grout, and tiles.
| Item | Estimated Cost Range (USD) |
|---|---|
| 1/2″ Exterior Plywood (Per 4×8 Sheet) | $30–$50 |
| Cement Backerboard (Per 3×5 Sheet) | $10–$20 |
| Uncoupling Membrane (Per Sq Ft) | $3–$6 |
| Thinset Mortar (Per 50 lb) | $15–$40 |
| Tile (Per Sq Ft) | $1–$30+ |
Common Mistakes To Avoid
Skipping structural assessment, failing to add stiffness, and ignoring movement joints are leading causes of tile failures. Other mistakes include inadequate adhesive coverage, installing over dirty or unstable planks, and neglecting waterproofing in wet areas.
Code And Manufacturer Requirements
Follow local building codes and tile manufacturer instructions. Many tile warranties require that substrate deflection and installation follow industry standards such as ANSI A108/A118/A136 and TCNA guidelines; noncompliance can void warranties.
Timeline And Project Planning
Plan for at least several days for substrate prep, overlay installation, curing times, and tile setting. Plywood adhesive and thinset require cure time; grout sealing may add additional drying time. Complex waterproofing or structural repairs will extend schedules.
When To Call A Professional
Hire a contractor if structural reinforcement, joist repair, or extensive waterproofing is required. Professionals can assess deflection, sister joist if necessary, and ensure code-compliant waterproofing for wet areas.
Summary Of Best Practices
Ensure structural stiffness (L/360), repair or replace damaged planks, add plywood overlay when needed, and use appropriate backerboard or uncoupling membranes with correct thinset application. Include movement joints and waterproofing where required, and follow manufacturer and code guidelines for a durable tile installation over a plank subfloor.