Moisture trapped under a building's walls often rots the subfloor. Left alone, the moisture eventually destroys the joists below, and the wall's plate above, the subfloor. Usually a builder uses 3/4-inch-thick plywood or OSB particle board as the subfloor, which is mounted directly against the building's floor joists and gives the finished floor covering a solid base on which to rest. After identifying and repairing the water leak, a contractor removes and replaces the rotten subfloor before installing a new floor covering.
Cut out a 12- to 18-inch-tall section of the drywall covering each side of the wall above the rotted area of the subfloor. Use a utility knife to slice through the drywall; remove the cutouts.
Inspect the area inside of the wall. If electrical wires or plumbing pipes run through the wall's base plate, use a local building code-approved method to disconnect and remove the wires or pipes.
Lay a reciprocating saw's blade flat on the base plate and cut through the nails holding the wall studs to it, using the top of the base plate as a cutting guide.
Cut through the base plate on both sides of the rotten subfloor, using the reciprocating saw.
Slide a nail puller's V-shaped groove under one of the nails holding the cut section of base plate to the subfloor and use a hammer to force the V-shaped groove under the nail's head. Remove the nail, using the nail puller's handle for leverage. Remove all of the nails in the cut section of base plate using the same procedure. Slide the cut section of base plate out of the wall.
Remove the nails holding the rotted part of the subfloor against the building's floor trusses using the nail puller and hammer. Carpenters drive nails into the subfloor every 6 to 8 inches along each floor truss.
Adjust a circular saw's blade to the 3/4-inch position using the saw's blade depth guide as a reference.
Cut out the rotted section of the subfloor with the circular saw, using the nail holes in the subfloor as a cutting guide.
Lift the rotted section of subfloor from the floor trusses using a pry bar for leverage. Set the removed section of subfloor on a new sheet of 3/4-inch plywood.
Trace the outline of the rotted piece of subfloor on the new plywood's surface with a pencil. The rotted piece of subfloor acts as a template.
Cut through the new plywood with the circular saw along the outline you traced.
Run a 3/8-inch-wide bead of construction adhesive across the top of each floor truss using a caulking gun to spread the adhesive.
Set the plywood cutout on the floor trusses. Step on the plywood, pressing the patch into the adhesive.
Secure the plywood to the floor trusses with 1 1/2-inch deck screws using an electric drill. Place a deck screw every 6 to 8 inches along each floor truss.
Slip the removed section of the base plate into its position under the wall studs. Secure the base plate to the plywood subfloor with 3-inch deck screws using the electric drill to place a screw between each wall stud.
Secure the wall studs to the base plate with 1 1/2-inch deck screws. Use, the electric drill to run the screws at an angle through the bottom of the wall studs down into the top of the base plate. Running screws, instead of driving nails, reduces vibration-caused damage to the drywall along the top portion of the wall studs.
Connect any removed electric wires or plumbing pipes, using a local building code-approved method.