Caution 1: Because you are torching amongst the studs and drywall, be very careful. Buy a heat-resistant cloth at a plumbing store or at The Home Depot -- in the tool area where they sell propane torches -- that you can drape over the studs. Caution 2: Soldering at the valve can damage the ceramic disk inside the valve.Usually, you have a threaded fitting that solders onto your water pipe and screws into the valve.You'll see it in the way a good plumber arranges pipes and valves for ease of maintenance, avoids wasting material or weakening framing, and takes care to wipe joints clean."Too bad so much of our good work gets buried," Richard laments.
Very likely, you will need to make changes both on the supply side and the drain side.
An undersize, improperly sloped drain will continually clog.
And supply pipes that aren't anchored every 6 feet or so will rattle and thump every time a faucet is turned off.
But whether the pipes are plastic, metal, or a combination of the two, it's the plumber's skill that determines how well the system works.
A lot is at stake: One leaky joint can cause thousands of dollars worth of damage.
Replace it with new pipe when you reconnect the fittings.