Replacing a subfloor under a wall may be necessary if the subfloor is damaged or no longer structurally sound. The subfloor is an important component that provides stability to the floor and walls in a room. While replacing a subfloor under a wall can be a challenging task, it is possible to remove parts of the subfloor without taking down the wall entirely. Here are the steps involved in replacing a subfloor under a wall:

1. Remove the baseboard: Gently remove the baseboard and any quarter-round trim near the area where you want to remove the subfloor. Set these pieces aside.
2. Remove sections of drywall: Cut away sections of drywall to the required width and about 1-foot high. Dispose of the removed drywall in a heavy-duty trash bag.
3. Remove the floor covering: Remove the floor covering directly above the subfloor that you want to replace. Some floor coverings can be easily lifted off, while others may require cutting. Ensure you create enough space to work comfortably.
4. Remove any underlayment: Some floors have a thin underlayment between the subfloor and the floor covering. Cut away a section of the underlayment using a utility knife or adjust the circular saw to cut only into the underlayment board without damaging the materials beneath it.
5. Cut away the nails: Use an oscillating multi-tool with a metal blade to cut away any nails that run downward from the bottom wall plate. Alternatively, a nail puller tool can be used to remove the nails completely.
6. Cut away the subfloor: Set the circular saw to the proper depth and cut the subfloor without damaging the materials below it. Cut as close to the wall as possible, usually within an inch or two. Use a hand saw to make additional cuts along the side of the square until you reach the bottom plate of the wall.
7. Remove the subfloor from under the wall: If the subfloor is heavily damaged, you may be able to pull it back and remove it by hand. If not, use a drill with a 3/4-inch auger bit to drill underneath the wall bottom plate on both sides of the damaged area. This will allow you to remove the section of subfloor.
8. Remove subfloor from the other side (if necessary): If the subfloor extends beyond the wall, repeat the previous steps on the other side of the wall to free up the subfloor and remove it.

It’s important to note that if you need to replace the entire subfloor under an exterior wall or a load-bearing wall, it’s recommended to hire a professional. They will ensure proper support is provided to prevent any sagging or structural issues with the walls while the subfloor is being replaced. For smaller sections of the subfloor, up to about 14 inches wide between joists, you can cut out and replace without needing extra support.