Hey all, I need to expand the plumbing on the old post office building we own. Right now, we have a sink and toilet and I want to add a shower and washing machine drain. I've ripped out the drywall and have exposed the copper cold water line that feeds the sink and also have exposed the pipe that the sink drains into and also connects to the vent. The drain pipe is metal and goes into the slab. the sink drain line goes into the side. For drainage is it possible (or even a good idea) to cut out the metal drain pipe near the slab and use pvc to connect to the vent? That way I assume that I could use pvc couplings for the shower, washing machine, and replacement sink drains. Are there 'one way' couplings that would prevent a backed up sewer line from coming up the shower? As for the water intake, I was thinking of cutting the existing copper cold water line and putting on a compression coupler to 't' off lines to the shower, washer, sink, and a water heater. I've been thinking about getting an electric on-demand water heater. Any advice or warnings? Thanks!