The parts of a shower valve that you can see are the tip of the proverbial iceberg. Hidden behind the shower wall are the pipes and mechanisms that actually make the shower work. You need access behind the wall to install the plumbing and to service these mechanisms if anything ever goes wrong. Contractors often design shower stalls to give you this access, especially if the shower has tile walls. If your shower has no rear access, though, you have to install the valve before you finish the walls.
Step 1 – Turn off the water to the main water supply lines running underneath the shower. Cut into these lines with a pipe cutter at a location as close as possible to the wall behind the shower, and let the water drain out.
Step 2 – Solder tees onto the supply lines that face upward toward the shower. If the lines are 3/4-inch copper, use 1/2-inch by 3/4-inch reducing tees. If the lines are 1/2-inch copper, use 1/2-inch copper tees. Clean the ends of the pipes with a wire brush, then spread flux on the pipes and on the insides of the tees. Fit the tees onto the pipes, then heat each joint in turn with a propane torch until the flux smokes. Remove the heat and touch a coil of lead-free solder to the joint. Move it quickly around the pipe while the flux wicks into the joint.
Step 3 – Drill 1/2-inch holes through the bottom plate of the shower wall over the tees, drop 1/2-inch copper pipes through and solder them to the tees. These pipes should extend about a foot up into the wall behind the shower.
Step 4 – Solder male adapters onto the ends of two 1-foot sections of copper pipe. Wrap plumbing tape around these adapters and screw them into the inlet holes of the shower valve. Use two wrenches to do this — one to hold the pipe and the other to hold the valve.
Step 5 – Position the valve in the center of the shower wall at the correct height and recessed about 1 inch from the edges of the wall studs. Measure and cut the pipes so their ends are directly above the stub-outs coming up from the floor.
Step 6 – Solder 90-degree elbows onto the ends of the pipes, and then measure and cut two more lengths of pipe that extend from the elbows to the stub-outs. Slip couplings onto the stub-outs and connect the elbows to them with the lengths of pipe that you cut. When the pipes are assembled, take them apart again and spread flux on the outsides of the pipes and the insides of the fittings. Re-assemble them and solder all the joints.
Step 7 – Install a 2-by-4 block of wood between the studs at the height of the showerhead. The face of this block should be recessed 1 1/2 inches from the front edges of the studs. Measure and cut a length of 1/2-inch pipe that extends from the shower valve to the midpoint of this block.
Step 8 – Solder a male adapter onto this length of pipe, wrap plumbing tape around it and screw the pipe into the outlet on the top of the valve. Solder a brass 90-degree female adapter onto the other end of the pipe, and screw the adapter into the block of wood.