Dry rot is caused by fungi getting into the wood of a home. When dry rot gets into the floor joists, damage to the joists can cause heavy objects to fall through the floor. To get rid of the fungi, all moisture must be removed from the area underneath the floor joists. Have a professional help with this task, as well as the task of removing the fungi from the wood. Sistering the floor joint and then cutting out the area affected by the rot is often necessary.
Measure the length of the floor joist that requires a sister joint. Take the measurement to a lumberyard and have a piece of lumber ripped to the correct size.
Lift the sister joint into position and clamp it to the rotted piece of wood. Place a nail at each side of the joist to attach the new piece to the old joist.
Add a 6d nail every 4 inches, except where the old joist is rotted.
Set a jigsaw to the depth of the rotted piece of wood and cut out the rotted area. Make the cut as square as possible. Measure the area.
Cut a piece of lumber the same size as the piece you took out. Add wood glue to one side and pound it in place with a hammer. Add a 4d nail on each corner to keep it stuck in place.