Reinforcing a Bathroom Subfloor


If you plan to install a new shower, bathtub or toilet in your bathroom, you must make sure the floor is properly reinforced to support the installation and for future security. Older homes especially are liable to have water damage and rot. Strong floors provide a more stable surface to mount a toilet to keep if from rocking back and forth when used. Stable flooring also gives support to shower pans and tubs, which require a perfectly flat surface in order to drain properly.

Things You'll Need

  • Crowbar
  • 2-by-6 boards
  • Hammer
  • Nails
  • Plywood flooring
  • Floor mortar
  • Trowel
  • Cement tamper
  • Cement board
  • Masonry screws
  • Screwdriver
  • Test all flooring boards for strength, cracks or rot damage by lightly pushing on them from a safe position on the floor. If you detect any weakness, rip out the flooring with a large crowbar. Pull up all boards with holes or cracks and any board that appears to have mold or pockets of rot.

  • Build a chase sleeve for the toilet and bath drain. A chase sleeve is a box that encapsulates the drain. To do this, cut two 2-by-6 boards to the same length as the width of the two joists the drain pipe is to run beween. Place the boards between the joists, 12 inches apart, on either side of where you plan to install the toilet drain. Nail through the joists and into the boards to secure both boards to the joists. In essence, you are creating a box that surrounds the drain pipe, with the two joists and two boards forming the four walls of the box. Perform the same task for the bathtub and shower drain lines.

  • Install solid plywood flooring boards on top of the floor joists with hammer and nails. Space the flooring so that four corners never intersect. This adds strength to the flooring and is a standard method of installation.

  • Install a mortar bed on the flooring beneath the tub or shower pan by spreading sand-mix cement over the floor with a trowel. Tamp down this cement with a flat tamper until you have formed a solid, 1-inch thick floor. Let it dry for 24 hours. Or, if your shower pan or tub calls for it, install the shower or tub into the wet cement. To reinforce a floor without wet cement, install cement board over the plywood with masonry screws and a screwdriver.

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