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Before any major plumbing work, turn off the main water supply to the house.
Whenever you have a "wet wall," a wall with plumbing, typically there's an access panel on the opposite side of the wall. Remove the panel.
To cut pipes, use a close-quarter pipe cutter which is designed for tight spots (Image 1). Keep turning the cutter, tightening the wheel slightly after two rotations, until you cut through the pipe. Avoid overtightening the pipe so you won't crimp the pipe. Cut out approximately 12-inch sections, enough for access with a wrench.
Use an adjustable wrench and channel lock pliers (Image 2) to separate the copper supply lines from the galvanized iron pipes.
To ensure a good seal, clean off all the corrosion with a wire brush and peel off any tape from the galvanized pipes before you attach the shut-off valves. As well, preparation includes applying a pipe compound. There are several ways you can do that. For a tight seal and to prevent the shut-off valve from welding to the supply pipe, use plumber's tape (Teflon tape). Wrap the tape on clockwise and go about three-quarters of the way down.
When installing the shut-off valves, pay attention to the top and bottom of the valve body which typically use arrows to indicate water flow direction. Make sure the arrow is going up (Image 3) to indicate the direction water will flow through it.
Remove the remainder of the old pipes and old backerboards you encounter.
Cut new backerboards to replace the old ones. In our project, to avoid rot if the access areas get wet for any reason, we used 3/4" green-treated plywood for the backerboards. Three pieces were needed: one for the tub spout area, one for the valve body and another for the shower arm (Image 1).
Use an impulse nailer to secure the boards in place. After the first piece of backerboard is installed where the tub spout is, attach cleated nails or screws to the studs before you install the next piece. Tip: Use screws for added strength.
Drill holes for the fixtures with a hole saw using a chalk line to make sure you're on center. Dry-fit the fixtures in place (Image 2).
In the next phase of the project, the shower is replumbed.