Replacing 15+ year old vanity sinks

Hi

I am replacing 4 vanity sinks that are 15+ years old. My question is what under the sink should I replace, p trap, supply lines, shut off valve???

If you are going to the trouble of replacing the basins, why wouldn't you go ahead and replace everything? Replace it all.

What smelly said. Once you look at the P traps and smell them, you will want to replace them, supply lines are probably copper and hard as a rock, stops are probably corroded and hard to operate, and with the basins out of the cabinet you've got lots of room to work. Biggest issue will be removing the old stops -- depends on how they're attached -- soldered, threaded, or compression, each method has its quirks.

sinks

It is a lot easier to make the connections using new parts than it would be to modify the old ones to make them work.

Yes! Replace the whole entire thing. I ran into this problem when replacing just a faucet and cleaning a slow drain in an old house that I bought. The drains were old, the water lines in were old and NOTHING was available for purchase. I started out trying to replace a faucet and clean a slow drain and ended up with an $800 job when it was complete. When trying to disconnect the drain line from the main line in the wall...it turned in the wall! We ended up having to remove the part of the wall, in case the pipes turned in there...leaking water without our knowledge.
Like I said, replace it all. Much easier to do. And..the drain lines are more than likely going to be a mix of metals, such as galvonized and copper..which corrodes over time and will be a bigger problem that you will have to address again later.

http://www.brasscraft.com/PDF/Brass%20Craft%20-%20Solution%20for%20Missing%20or%20Seized%20Stops%20Flier.pdf