If you have wood flooring or other wood surfaces in your bathroom, this area is especially susceptible to wood rot. Wood rot is generally caused by excessive exposure to moisture and humidity, both of which can be common occurrences in a bathroom. The time it takes to treat wood rot depends on how extensive the damage is, but the basic process will be the same, regardless of the amount of damage.
Cut out all areas of wood in your bathroom that are rotting. These areas cannot be repaired and need to be removed.
Replace the areas that you cut out with wood that has been treated with a chemical stain to resist future moisture damage. If the wood has not already been treated, you can paint on the water-resistant stain yourself; give the stain a few hours to dry before using the wood.
Fill in any holes in the wood surface with epoxy filler. Follow the directions on the epoxy filler packaging for drying times.
Sand the epoxy filler so that it's even with the surrounding wood area in the bathroom.
Paint the repaired wood area so that it matches the rest of your bathroom if necessary.
Fix any moisture issues that caused the wood rot in the first place, such as a leaking pipe or poor ventilation. Installing an exhaust fan can greatly reduce high humidity levels in a bathroom.