What thickness of plywood subfloor is required under ceramic tile in a second-floor NB bathroom?

For a second-floor bathroom in New Brunswick, you need a minimum 1-1/8 inch (28mm) combined subfloor thickness under ceramic tile, typically achieved with 3/4 inch plywood subfloor plus 1/2 inch cement board backer.

The structural requirements for tile installation are more demanding than other flooring types because ceramic and porcelain tiles are rigid and will crack if the subfloor deflects under load. In NB's older housing stock, many second-floor bathrooms have 5/8 inch or 3/4 inch plywood over 2x8 or 2x10 joists spaced 16 inches on center. While this meets building code for general flooring, it's often insufficient for tile without additional reinforcement.

Proper subfloor assembly for NB bathroom tile starts with assessing your existing structure. If you have 3/4 inch plywood in good condition over adequately sized joists (2x10 minimum for 16-inch spacing), you can install 1/2 inch cement board directly over it using appropriate screws. This creates the 1-1/4 inch total thickness that provides adequate stiffness. However, if your existing subfloor is thinner or shows any flex when you walk on it, add a second layer of 1/2 inch plywood with construction adhesive and screws, staggering the seams from the original layer.

Moisture protection is critical in NB bathrooms due to our Maritime humidity levels. Before installing cement board, apply a waterproof membrane (like RedGard or Kerdi) over the plywood, paying special attention to corners, penetrations, and the tub/shower area. The cement board goes over this membrane, then gets its own waterproofing treatment before tile installation. This redundant moisture protection prevents the subfloor rot that's common in older NB homes where bathroom moisture has penetrated inadequate vapor barriers.

Deflection testing is essential before committing to tile. Walk across your proposed tile area and feel for any bounce or movement. Place a 4-foot level across the floor and look for gaps that indicate dips or humps exceeding 1/8 inch over 3 feet. Tile installations fail when the subfloor moves, creating stress cracks in grout lines and eventually cracked tiles. If you detect any flex, add blocking between joists or install additional plywood layers until the floor feels rock-solid.

When to hire a professional: Subfloor assessment and reinforcement requires construction knowledge beyond surface-level flooring skills. If your bathroom shows signs of previous moisture damage, has undersized joists, or needs structural modifications to achieve proper stiffness, consult a contractor experienced with NB's older housing stock before proceeding with tile installation.

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Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

• M&L Commercial and residential services
• Forever Epoxy Inc
• RenoMe
• Thirty Four Renovations
• Arctic Fox Construction Inc.