I have a three story house. The second story (main level) has one small bathroom. It has tiles floor (probablyencased in cement and not removable) with a drain. I have never used this shower and I am in the plan to renovate it. All is well in the bathroom itself, but the bottom floor (basement, but not under ground) has exposed ceilings, so I can get to the drain from the shower. It has been disconnected prior to my moving in. I want to install a p-trap, if that is what it needs. The remaining plumbing consists of a 1 1/2" drain from the shower, it is galvanized pipe with 1 1/2" threds. Then there is a 20" long 2" galvanized pipe, (no threds) coming from the standpipe. There is enough gap to put a standard p trap in there, but not sure if that is what I need. Can you describe to process so it works correctly, assuming I cannot use PVC (live in San Francisco). Do I need venting? Do I need to replace the entire system with 2" p trap, or is the 2" pipe going to the stand OK? Anxious to get this finished. If I can use a 1 1/2" brass p trap, which fits inside the 2" pipe. How do I connect the two? Thanks