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
    Commented Oct 1, 2019 at 23:50
  • Exactly. Clarified question.
    – Bryce
    Commented Oct 2, 2019 at 0:29
  • Don't forget your laundry room vent.
    – JACK
    Commented Oct 2, 2019 at 0:42
  • 2
    And the clothes lines didn't need a vent..
    – JACK
    Commented Oct 2, 2019 at 12:03
  • 1
    @CarlWitthoft let's see. If the vent is active, I will see nothing because the water trap in any active fixture will block the smoke. If the vent is inactive, ending in a wall, the house catches fire. Yeah, not so much.
    – Bryce
    Commented Oct 4, 2019 at 4:53

3 Answers 3


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
    Commented Oct 2, 2019 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
    Commented Oct 2, 2019 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. Commented Oct 2, 2019 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
    Commented Oct 3, 2019 at 6:17
  • @bryce I personallly could live with re-patching every 10 years :-) . Commented Oct 3, 2019 at 14:23

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 Commented Oct 3, 2019 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.
    Commented Oct 3, 2019 at 16:32

I came here looking for ideas. Here’s one. If you have an access point on the vent that’s in the basement (to rule out anything above and because hot air rises), stick a colored smoke “bomb” cylinder.

Some safety notes:

  • Do this at your own risk
  • Don’t use this method on ABS of plastic pipe that can melt under heat. My pipe was all galvanized.
  • Most smoke bombs release smoke from the side that you light then color with a 2-3 torch. Let that torch burn out then stick it in the hole.

Obviously this is better with 2-3 people but it can be done alone.

First, close all the doors to any room with a drain. This helps you isolate smoke since a small smoke bomb only burns for about 20 seconds.

Second, have a friend on each floor and the most important is outside to make sure the smoke exits the roof. If you don’t see smoke coming out of the stack on the roof, that’s a whole other set of questions.

Third, light your smoke bomb and let the magic happen.

If you’re alone like I was, get ready to dash around. Maybe plan on doing this 2-3 times.

In the end, I found my vent wasn’t used which was a big relief.

Please comment if you see any scientifically problematic here.

  • The water in the traps means you should not get any smoke in any rooms with plumbing connected to the vent in question. Smoke should go out the roof, and only out the roof, even if every trap in the house is connected to it. So this is not a useful method to determine if a vent pipe is in use.
    – Ecnerwal
    Commented Jul 10, 2023 at 2:51
  • Not sure how I managed to overlook that little detail! Sheesh. Ok, but theoretically, if I took the traps off my sinks that are potentially connected to this vent, smoke would come through if they were connected, right? Commented Jul 11, 2023 at 4:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.