How to Lay a Subfloor in a Bathroom

Clear the Bathroom Floor Area

• 1

  Shut off all water supplies to the bathroom. Flush the toilet to empty the bowl of water.

• 2

  Remove all existing surface flooring, such as tile or carpeting. Remove appliances that rest on the existing floor, such as the toilet, sink and tub.

• 3

  Check the stability and condition of the existing subfloor surrounding the tub. If it is stable, work around the tub.

Replacing Existing Subfloor

• 4

  Examine the condition of the existing subfloor. Look for spots that have rotted, warped or have mildew. Determine which areas can be saved, if any.

• 5

  Draw a chalk line square around the damaged areas.

• 6

  Put on safety glasses and remove as much of the area as possible with a circular saw. Set the depth of the circular saw about 1/8-inch deeper than the thickness of the existing plywood. Switch over to the hammer and chisel to remove remaining pieces. Sand rugged edges so the new plywood will fit snugly.

• 7

  Put a piece of newspaper over the square and trace the opening with a marker. Cut the newspaper and use it as a template for cutting the new plywood replacement piece. Glue it down with contact cement.

Installing the New Bathroom Subfloor

• 8

  Follow the flowing manufacturer’s recommendations for the type and thickness of the subfloor material to use, such as plywood, OSB or contact cement backer board.

• 9

  Review your floor plan measurements for the bathroom to determine how much subflooring material you need. Add 20 percent to the layout measurements to make sure you purchase enough subflooring material.

• 10

  Use newspaper to draw a template for the bathroom subfloor. Cut the template and lay it on the diagonal of the subfloor material. By cutting the subfloor pieces on the diagonal, you strengthen the load-bearing capacity of the subfloor and reduce stress and pressure on the floor joists. Use this technique if you are installing the subfloor on a concrete foundation.

• 11

  Apply the adhesive type recommended by the subflooring material manufacturer to adhere the subfloor material you selected. In most cases, this means contact cement.

• 12

  Drive corrosion-resistant screws with a drill through the plywood and into the floor joists for upper floor installations. This additional step will make the subfloor firm and steady. Drive the screws about 1/8-inch below the top surface of the plywood.

• 13

  Fill in the area above the screws with a lightweight gypsum leveling compound. Also do this for low spots or uneven areas. This helps prevent tile applications from cracking; it creates a smooth and level subfloor for any type of final surface flooring material.

• 14

  Allow the subfloor adhesive to dry for at least 24 hours before moving on to install the top surface flooring material.