According to backer board manufacturers and tile setters, installing backer board directly over an old, solid-wood subfloor is risky for a couple of reasons: First, changes in humidity make solid wood shrink, swell and cup more than plywood. Second, the strength of wood boards isn't always consistent. Splits and knots create weak spots that flex and lead to cracking tile.
Here are two steps that compensate for solid wood's shortcomings:
- Screw the old floorboards to joists with 2-in. screws. Place the screws no more than 3 in. apart. Screws have better “hold-down” power than nails and will help prevent boards from loosening or cupping.
- Add a layer of 1/2-in. BC plywood over the old subfloor, using construction adhesive and driving 1-1/4-in. screws every 6 in. The plywood adds stiffness and stability to the floor.
With the plywood in place, install the backer board as you normally would. To avoid building up the floor too high, you can use 1/4-in.-thick backer board instead of 1/2-in. backer.