How to Waterproof a Bathroom Plywood Subfloor

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A subfloor is installed on top of a structure's construction floor and provides a flat, level surface for finish flooring such as tile or linoleum. Often the subfloor consists of plywood that is nailed or screwed to the construction floor. Before finish flooring is installed on top of a plywood subfloor, you must waterproof the subfloor, especially in a bathroom. If you do not waterproof the subfloor, moisture can make its way under the subfloor and cause mold growth or other damage.


Step 1

Cut the end of the silicone caulk nozzle at a 45 degree angle using a utility knife. Poke the foil at the bottom of the nozzle using the metal rod attached to the caulk gun that is designed for this purpose.

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Step 2

Install the silicone caulk tube into the caulk gun and push the plunger into the back of the tube. Turn the plunger handle 180 degrees and push the release button.


Step 3

Apply a bead of the silicone caulk around the entire perimeter of the subfloor. Place the angled part of the nozzle flat against the crevice between the subfloor and the bottom of the wall and squeeze the trigger slowly. Move the caulk gun evenly along the crevice to apply a bead of caulk. Drag the side of your thumb along the bead of caulk to press it into the crevice after every 3 to 4 feet. Wipe the excess caulk off your thumb with a damp rag.


Step 4

Caulk any seams where two pieces of plywood are butted together. Dab caulk onto nail or screw heads to seal the potential leak point they provide. Caulk any splits in the wood or any other areas that may provide a point for water to seep under the subfloor. Allow the caulk to dry for 24 hours before installing the finish flooring.



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