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

You lost me with.
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. .
By stand pipe do you mean the stack? Please explain, in detail, what these pipes connect to. If there is no trap then what's the pipe coming from the shower? The pipe with no threads sounds like they were cut off when the shower was disconnected. One other thing. If the shower connects directly into the stack where's the shower vent to? You say PVC's outlawed id S.F. Will you be pulling a permit and call a inspection on this job? If not then remove all the galvanized piping and convert to PVC. Sound like a plan? Tom

The 2" pipe coming from the shower is the drain.

The 2" galvanized pipe may have been cut, not sure. It connects to a pipe coming from the toilet in the same bathroom.

PVC is not allowed in San Francisco, and although no permit is being pulled, I don't want to use it. It would be much easier if I could.

In theory I could remove the 20" long pipe and replace it with another 20" with threds. That is what I will do unlesss there is an easier way. I am not sure how easy to remove a pipe placed there in 1935.

I don't think there is a vent for the shower. Do I need one with such a short connection?

The 2" pipe coming from the shower is the drain.
You say you have no "P" trap and raiser. What's coming down from the shower drain itself? This 2" pipe's just hanging there not connected to anything?

PVC is not allowed in San Francisco, and although no permit is being pulled, I don't want to use it. It would be much easier if I could.
I try to stay within code but if a repair works and staying within code presents a hardship I go with the easy way. Unless you're planing on selling I would take the easy way out. Growler would disagree but I believe some codes are goofy and since this job won't be inspected, why not?

In theory I could remove the 20" long pipe and replace it with another 20" with threds. That is what I will do unlesss there is an easier way. I am not sure how easy to remove a pipe placed there in 1935..
If I were you I would remove the galvanized pipe from the sanitary tee on the stack, install a PVC male threaded adapter and convert to PVC. This is the easiest but if you wish to stay within code you can redo with galvanized or convert to copper at the san tee.

I don't think there is a vent for the shower. Do I need one with such a short connection?
Again, code mandates a vent for every fixture that has a trap. However, I'm,a great believer in, "if it ain't broke, don't fix it." Your shower's been draining since 1935. Adding a vent would be a major undertaking at this time.
If you're really concerned you could add a AAV, (air admittance vent) uust past the trap but I presume that S.F. has those outlawed also. Good luck, Tom