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I'm doing roofing, and looking at a roof vent pipe (e.g. a plumbing drain waste plumbing penetration). It is 1.5" galvanized pipe original to the construction. I see it emerge from the top plates and up to the roof.

I can match up all the other roof vent pipes with their fixtures below, but not this one. I think it might be abandoned.

How can I determine reliably where it goes, and if it's still part of the home's plumbing system? All I can think of is to smell for sewer gases.

  • Could it possibly be a vent pipe? – JACK Oct 1 at 23:50
  • Exactly. Clarified question. – Bryce Oct 2 at 0:29
  • Don't forget your laundry room vent. – JACK Oct 2 at 0:42
  • Not the laundry room vent. House is 93 years old, so no laundry machines were present at construction time. – Bryce Oct 2 at 7:48
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    And the clothes lines didn't need a vent.. – JACK Oct 2 at 12:03
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One thing you can do regarding the question if it an active part of your plumbing is to turn on your water sources one by one. While water is running down the respective drain put your ear to this unknown vent pipe top and listen for the sound of running water.

One way to make this process efficient is to have two people talking on cell phones. One person is on the roof to listen at the pipe. The other person operates each sink, toilet or washer in turn while communicating with the person on the roof.

Of course use common sense safety strategy climbing to the roof, especially if it is particularly steep of high from ground level.

  • I am wondering how this could be done without actually having to access the vent end. For instance, my vents are 30ft high and thhe roof is SLICK standing seam metal. No truly safe way for me to ever be up there sans access to a boom truck/cherry-picker. A quiet drone with microphone to hover at the vent and transmit the audio? – peinal Oct 2 at 15:53
  • @user68825 - The drone idea might work to place the microphone but I would think after lowering it into the pipe that you land the drone nearby and shut it down. Even a quiet one will make local noise that could completely drown any faint sounds in the pipe. It does sound like an excellent reason to acquire a drone and some electronic playthings to go with it. – Michael Karas Oct 2 at 16:07
  • Can you access the pipe inside your attic? If so, cut a hole in the side and try this test. There's no pressure involved, so patching the hole with metal tape is more than sufficient. – Carl Witthoft Oct 2 at 19:16
  • Um, metal tape? That won't last, after 10 20 no more than 30 years you'll have a sewer gas hole in the attic. – Bryce Oct 3 at 6:17
  • @bryce I personallly could live with re-patching every 10 years :-) . – Carl Witthoft Oct 3 at 14:23
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This helped. I tried my handy dandy electronic tracing tool, meant for CAT5 wiring. This required some sandpaper to get a clean electrical connection through the rust. I was able to follow the pipe a good long distance, though not quite far enough:

Tracing plumbing pipe the electronic way

  • This should probably be a comment, since it doesn't provide a full answer – Carl Witthoft Oct 3 at 14:22
  • @CarlWitthoft, pictures can't be included in a comment, and it does provide a possible answer that would work for some people, even though it didn't completely work for OP. I think it's fine. – Nate S - Reinstate Monica Oct 3 at 16:32

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