Plywood subfloor can swell when wet.

How to Fix a Wet Subfloor in a Bathroom

by Wade Shaddy

It happens all the time: Water from the sink, tub or toilet overflows or splashes onto the floor. If the water is excessive or isn't wiped up promptly, it seeps through seams on the floor covering, penetrates down through the particleboard overlay, and soaks the plywood subfloor. The swollen plywood lifts the overlay and flooring into a bump, bulge or worse. If the damage is not too bad, you can sand it flat again. If it's excessive, replace the subfloor where it's damaged.

Insert the tip of a putty knife between the baseboard and the wall. Pry out gently and insert a screwdriver behind it. Use the screwdriver to pry off the baseboard. If the baseboard is wooden, remove the nails using diagonal pliers. If it's vinyl, set it aside.

Insert the putty knife under the floor covering and pry it up. When you have enough to grasp, pull the floor covering back to expose the damaged area.

Use a tri-square to draw a rectangular box around the damaged area. Set the guard on the blade of a circular saw to cut 1 1/2 inches deep. Use the circular saw to cut the rectangular box drawing out. Pull the particleboard overlay and the plywood subfloor out of the hole.

Measure the cutout hole and cut new pieces of particleboard overlay and plywood sublfoor to fit using a table saw.

Insert the new plywood subfloor piece into the hole. If there are no floor joists, or floor joists on one side only, measure and cut two-by-four studs, using a miter saw, to fit inside the hole where needed to brace the new pieces of subfloor. Screw the pieces directly to the sides of the existing floor joists to create a shelf, or lip, to attach the new piece of subfloor. Use a drill/driver and 3-inch screws.

Place the new piece of plywood subfloor into the hole. Screw it to the two-by-four braces around the perimeter using a drill/driver and 2-inch screws.

Place the new piece of particleboard overlay on top of the sublfoor. Screw it down around the perimeter using the drill/driver and 2-inch screws.

Sand the fixed area smooth with an orbital sander using 100-grit sandpaper. Clean thoroughly with a brush to remove dust and debris. Add vinyl floor adhesive and press the floor covering back in place. Nail the baseboard back on using a hammer and 1 1/4-inch finish nails for wood, or glue the baseboard back on with vinyl floor adhesive.

Items you will need

  • Putty knife
  • Screwdriver
  • Diagonal pliers
  • Tri-square
  • Circular saw
  • 5/8-inch particleboard
  • 3/4-inch CDX fir plywood
  • 1 two-by-four stud, 96 inches
  • Miter saw
  • Drill/driver
  • 3-inch screws
  • 2-inch screws
  • Orbital sander
  • 100-grit sandpaper
  • Vinyl floor adhesive
  • Hammer
  • 1 1/4-inch finish nails


  • Water can find a way to penetrate any kind of floor covering you have, resulting in damage or swelling to the subfloor. Consider replacing the floor covering for best results. If your damage is extensive, remove the toilet and vanity cabinet to make the job easier.


  • Find out where or why the water was leaking in the first place. Fix it before repairing anything. Wear safety glasses when working with wood.

About the Author

Specializing in hardwood furniture, trim carpentry, cabinets, home improvement and architectural millwork, Wade Shaddy has worked in homebuilding since 1972. Shaddy has also worked as a newspaper reporter and writer, and as a contributing writer for Bicycling Magazine. Shaddy began publishing in various magazines in 1992, and published a novel, “Dark Canyon,” in 2008.

Photo Credits

  • Hemera Technologies/ Images