What kind of subfloor do I need under bathroom tile on the second floor?

For second-floor bathroom tile, you'll need a proper subfloor system that can handle moisture and provide adequate support for ceramic or stone tile. The key is creating a rigid, waterproof assembly that meets New Brunswick's National Building Code 2020 requirements.

Your subfloor system should consist of 3/4" tongue-and-groove plywood or OSB subflooring over your floor joists, followed by a cement backer board like Hardiebacker, Durock, or similar fiber-cement product. The backer board is critical - standard drywall or even moisture-resistant drywall isn't adequate under tile in wet areas according to NBC requirements.

Deflection is your biggest concern on upper floors. The NBC requires that floor assemblies supporting ceramic tile deflect no more than L/360 under live loads (where L is the span length). For a 12-foot joist span, that's maximum 0.4 inches of deflection. If your existing joists are undersized - common in older New Brunswick homes from the 1970s-1990s - you may need to add blocking, sister joists, or install a beam underneath to stiffen the floor.

Before installing anything, check your joist spacing and size. Joists 16" on center are preferred for tile floors, though 24" spacing can work if the joists are adequately sized (typically 2x10 minimum for longer spans). Many post-war bungalows in areas like Riverview and suburban Fredericton have 2x8 joists at 24" centers, which often need reinforcement for tile.

The installation sequence matters: subfloor, then cement backer board screwed every 6-8 inches with appropriate fasteners, then waterproof membrane over the backer board, then tile. Don't skip the waterproof membrane - New Brunswick's freeze-thaw cycles make water penetration especially damaging.

For a typical 8x10 bathroom, expect to budget $800-$1,500 for subfloor materials including backer board and waterproofing, plus $1,200-$2,500 for professional installation if structural work is needed. If you discover undersized joists during the project, reinforcement could add another $1,000-$3,000 depending on accessibility.

This is definitely professional territory if you're unsure about joist sizing or structural adequacy. A building permit from your municipality or Regional Service Commission may be required if structural modifications are needed, and you'll want the floor assembly inspected before covering it up.

Start by having a contractor evaluate your existing floor structure - many tile failures in New Brunswick homes stem from inadequate subfloor preparation rather than poor tile work.

---

Find a Bathroom Renovations Contractor

New Brunswick Construction Network connects you with experienced contractors in the directory:

• R.e.j Construction ✓
• 3Tone Construction Ltd ✓
• M&L Commercial and residential services ✓