Tiling over an old, solid wood subfloor is dicey, even with a layer of backer board. Avoid tile and grout cracks in the finished floor with proper preparation and lots and lots of screws.
Wooden subfloors in older houses need to be strengthened and stabilized before backer board and tile are installed. The best method is to refasten the subfloor, then add a layer of plywood.
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:
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.
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.