Rotten flooring can damage more than just the floor of a house. It can make wall construction weak where it meets the floor. From there, it can even cause structural damage to the ceiling and roof. It is always best to replace any flooring that is rotten or beginning to rot. The area where this is most likely to happen is where the main water line comes into the house. It can also begin in the wall wherever water lines travel.
Determine where the floor is rotting and clearly mark where it needs to be cut. Recommendations are that the area cut out should extend beyond the rotted area by one floor joist. This allows for a smooth transition from the existing floor to the new one.
Set the depth of the skill saw blade to 1 5/8 inches deep. This will allow a clean cut without damaging the floor joists. Cut out the marked area. Use a reciprocating saw in close areas.
Remove the rotten area first with the hammer and nail pullers. Next, remove the area that went beyond the rotted area. Be certain to remove all nails or screws.
Inspect the floor joists. Since the joists are solid lumber, they will resist water damage for extended periods. However, if some of them are clearly compromised, go on to the next step, if not, skip to Step 6.
Cut a 2 by 8 long enough to clear the compromised area of the floor joist at least 48-inches on either side. Secure the 2 by 8 to the existing joist with 2 rows of 16d nails on 8-inch centres. Repeat this for each damaged floor joist.
Place a sheet of the OSB on the floor joists. Be certain it fits tightly to the existing floor and secure it to the floor joists with 8d nails on 8-inch centres. Repeat this until the area is closed in, cutting the sheathing as needed. OSB Seams must meet on the floor joists.
Repeat Step 6 for the ¾-inch plywood. An "AC" grade of plywood is recommended for smooth finishing. Be certain that all the seams are tight.
Always wear safety glasses.
Be certain to keep the area marked for cutting out as square as possible. This allows for a smooth installation of the new flooring.
In some cases, it may be necessary to add pier supports under the floor joists where new joists are secured to older joists. The overall length of the joist is the determinate factor.
Use a reciprocating saw for areas of the flooring around doors and next to walls.
An alternative to securing the flooring with nails is to use deck screws. Just be certain to countersink the heads.
Do not set the blade of the skill saw any lower than necessary to make the cut.
Do not leave power tools unattended.
Clearly mark all waterlines and/or electrical conduits that may be under the floor.