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I am replacing a vanity in my bathroom. The brass p-trap from the old sink was connected to the galvanized stub out as seen below. While there haven't been any leaks that I'm aware of, the slip nut that attached the p-trap is broken so I want to replace the whole p-trap and connection. I realize galvanized drains are bad news but I don't have the resources to replace them right now so I'm focusing on what I can control.

My question is, what is the best means to connect a new PVC p-trap to the existing galvanized stub out?

My plan is to use a 1-1/2" to 1-1/4" zinc slip nut with a reducing washer to replace the broken one (assuming I can get the old one off cleanly) and attach the new p-trap. Will this work?

Alternatively, I read about using a brass waste connector which screws on to the stub out further than a slip nut and reduces the chance for issues related to corroded threads. Would this be better?

Should I use pipe dope or teflon tap when installing the new slip nut/connector? How clean from the old pipe dope does the stub out need to be?

Update 3/4/2020: After attempting a new zinc slip nut and washer, I simply couldn't get a good enough seal to prevent a very slow drip. The bottom of the pipe had a small notch that I think was to blame.

Ultimately, I ended up installing a brass waste connector from Home Depot with teflon tape, then connected the trap arm with the nut that came with the brass waste connector and a new rubber washer. The nut did extend the trap a little further making it difficult to re-install. Time will tell if this is a lasting solution.

galvanized stub out

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what is the best means to connect a new PVC p-trap to the existing galvanized stub out?

The plan you are proposing is best (with some things to consider, see below).

...Would ...a brass waste connector ...be better?

YES, for the reasons you (and the manufacturer) state.

...pipe dope or teflon tape ...?

I prefer tape because it is cleaner, and dope tends to harden over time and when (not if) it needs to be disassembled in the future, old hard dope is a pain sometimes.

How clean ...does the stub out need to be?

You should wire brush all old/hardened dope from the threads.

I am wondering why you are selecting an 1 1/4" trap (and wall adapter) for this job. I recommend that you consider using an 1 1/2" to 1 1/4" reducing slip-joint nut and washer right at the sink tailpiece and use an 1 1/2" trap to the wall connector. Bigger is better, unless there is some type of space limitation.

I do not recommend a clamp-on rubber boot; I have seen many of them leak. The "easiest" way is not always (in fact rarely) better.

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  • And yes, I have seen many traditional lav waste assemblies leak too... but the rubber boot trick is a shortcut to doing it right, IMO... – Jimmy Fix-it Feb 20 at 5:39
  • Jimmy Fix-it, I chose the 1-1/4" p-trap because that was what was there before and I assumed it was standard (plus it would be one less "conversion" of size.. so easier?). Makes sense to use the bigger one though so I may do that. – Andrew Feb 20 at 21:06
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It looks like you have 1.5" pipe reducing to 1.25" (1 1/4"). The easiest way to handle this is with a reducing coupler (example).

Reducing coupler

This should be less hassle than the reducing ring (no messing with gunky threads) and you should get a better seal overall

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  • Can that be used over the galvanized stub out threads? Wouldn't I still have to clean them? This kinda feels like it would be cutting corners (not that I'm opposed) but I want to make sure I'm doing it the "right" way – Andrew Feb 19 at 20:37
  • It should be safe, yes. That rubber gasket is quite thick. You'll need to remove the old reducing ring first, to put the clamp past the threads – Machavity Feb 19 at 20:42
  • This looks like a pedestal situation where the plumbing is exposed. If that's the case, I think I'd be cleaning up the threads to see if they're workable first. – isherwood Feb 19 at 21:23
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    If the threads are workable, go with them. I've used the rubber coupler before and they are a good alternative. – JACK Feb 19 at 22:44
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    From a functional perspective the rubber coupler should work well enough but if this is a pedestal sink with exposed plumbing I would go with the slip nut/brass adaptor approach as @isherwood suggested just for the aesthetics. Just wire-brush the threads on the stub, cleaning it up as best you can and go with it. – HoneyDo Feb 19 at 22:46

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