How to Repair a Bathroom Floor & Subfloor

Written by myra smith
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Water damage in the bathroom can occur so slowly that the floor weakens without anyone noticing. A toilet that seeps slowly around the wax ring or a sink drain that has a slow drip can do significant damage. The floor becomes spongy or the floor covering begins to crack and lift. The toilet may rock when the subfloor the toilet flange is attached to begins to rot. This type of problem must be addressed immediately because the damage will progress

Skill level:
Moderately Challenging

Things you need

  • Wrenches
  • Screwdrivers
  • Large pan
  • Plywood
  • Circular saw
  • Toilet flange
  • Wax toilet seal
  • Flange bolts

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    Widespread Water Damage

  1. 1

    Remove all fixtures from the bathroom and remove the floor covering. Pay careful attention to the fixtures and drains when removing the fixtures. The source of the leak must be located and corrected to avoid damage to the replacement subfloor and floor.

  2. 2

    Cut the floor out along the joist as close to the walls as possible. Use an old saw blade in the circular saw when cutting out old flooring; embedded nails will ruin saw blades. Remove the subfloor in the same manner. Replace the subfloor and floor with materials of the same thickness as the original. Remember to cut the openings for all drains and water supply lines before nailing the subfloor and floor in place.

  3. 3

    Reinstall the fixtures and reconnect the water supply lines and drains. Always use a new flange and wax seal when reinstalling the toilet. Replace washers and gaskets in drains before retightening. Do not overtighten drain and water connections to avoid damaging washers and seals. Caulk around all fixtures after reinstalling. Replace the floor covering.

    Leaking Toilets

  1. 1

    Turn off the water supply and drain as much water as possible from the toilet. Remove the toilet from the bolts that attach it to the floor flange and loosen the water supply line from the toilet. Lift up on the toilet and place it in a large pan to contain any water not removed. Inspect the floor flange for rust and damage. Test the flooring around the toilet flange with a small screwdriver for softness. Check for water stains running underneath the edge of the floor covering. Remove the floor covering if water damage has spread.

  2. 2

    Locate the floor joist on each side of the toilet, mark and cut both the floor and subfloor in the centre of the joist. Mark a straight line horizontally across the floor in front and behind the toilet. Cut carefully with a circular saw set to the depth of the floor. Use an old saw blade because you will encounter nails. Remove the section of floor and then cut the subfloor in the same manner.

  3. 3

    Replace the subfloor by cutting a new section of plywood that is the same thickness as the surrounding subfloor. Cut the patch piece of plywood flooring to fit as tightly into the opening as possible. Nail the patch in place. Remember to cut the opening for the toilet drain before nailing in place. Use the same procedure to replace the floor.

  4. 4

    Install a new toilet flange by screwing it to the floor. Align the flange with opening carefully. Install a new wax ring to the flange and replace the toilet. Torque the bolts that fasten the toilet in place by alternating tightening from side to side. Do not overtighten. Reconnect the water supply line. Check for leaks when water is turned back on. Replace the floor covering.

Tips and warnings

  • Visually inspect floors beneath bathrooms as often as possible.
  • Replace caulking as needed
  • Have help available to remove and replace tubs and fixtures from the bathroom

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