In the realm of second homeowner woes, I had a P-Trap assembly under the bathroom sink. It was a straight connection to the pipe connected to the main sewer line. (least I think that was the case). It was not vented and it worked just fine. But after dealing with a clog the corroded bottom of the P-Trap (which was made of chrome plated metal) broke and left a 1/2 inch long hole in it.

Suffice it to say, I had to replace the entire assembly. As with most projects, more pieces needs replacing. I replaced the sink flange and tail pipe and the P-Trap now made of plastic.

new p-trap assembly

As you can see in the image above that I used the original back plastic pipe leading down the wall.

When water drains now it backs up and eventually gurgles. Also when it backs up it leaks where the tail pipe and the P-Trap meet. My research tells me this is because this assembly is not properly vented. Is this true?

I ask only because the former assembly did not have venting problems. Is it possible that the former parts were venting through bad seals?

When does an under the sink P-Trap assembly need venting?

  • 1
    I'm not sure if you can tell just from the picture if it is vented or not...
    – Steven
    Nov 3, 2012 at 15:05
  • The assembly in the picture is not vented. However there is a roof vent connected to the sewer line under the floor. That means the vent is below the sink's assembly.
    – Sukima
    Nov 3, 2012 at 17:06
  • 1
    google: plumbing air admittance valve Nov 3, 2012 at 19:28
  • 6
    It's hard to tell from the picture, but are you sure you have a proper slope on the pipe leading from the trap assembly to the drain pipe in the wall? It looks to me like the pipe is sloping towards the trap, instead of towards the drain.
    – Tester101
    Nov 3, 2012 at 23:23

4 Answers 4


All traps need venting in all cases. Without a vent, the water seal in the trap gets sucked out and sewer gases can enter the room. There's also all sorts of examples where someone thought they were providing venting, but done in such a way that clearing the water seal was still a possibility. Some of these methods even used to be accepted practice, such as S traps. There's still several appropriate ways to achieve proper venting. For example, in island sink situations, a loop or foot vent is acceptable.

It's hard to say why the previous installation worked without venting. For sure, one way or another, the siphon action was disturbed by the entrance of air somehow, either from oversized pipe or leaking fittings. Regardless, where do you go from here? The downward facing ell in wall needs to be replaced with a sanitary tee for starters. The top of the new tee needs to tie into a vent somewhere, without travelling downwards. Is there a vent pipe nearby, such as from a toilet or shower? Or a pipe could perhaps be extended up into the attic, where it should be much easier to make a run to a vent pipe?

If all that sounds like too much, get a mechanical vent such as jberger suggested. They are not accepted by many codes, but they do work. Just be sure it's attached to a long enough pipe so that the valve is above the flood level of the sink. No doubt this all sounds like a real PITA right now. Once it's all done, you can take pride and sleep well knowing you have a properly functioning and completely sanitary sink drainage.

  • 1
    I added a air introducer vent (mechanical vent) as suggested. Although it does not stand above the sinks flood level it does work. I have found another problem much further down the line as the former owner jerry rigged a PVC pipe to a cast iron sewer pipe. Which after wiggling the PVC during installation broke the seal in the basement and now leaks. It is also possible that the jerry-rigged connection has partially clogged causing the back flow problem in the first place.
    – Sukima
    Nov 4, 2012 at 3:33
  • 3
    Oh my! I feel for you. The main problem with a mechanical vent installed too low is it can leak inside the wall in the event of a clog and you will not see it. If it is high enough, the sink will overflow before it can leak. As long as you're informed, you can leave it as it is, and let the next owner curse you :)
    – bcworkz
    Nov 4, 2012 at 17:21
  • I remember seeing in older homes various types of s-trap-ish things that looked as though they were designed to avoid having all the water siphon out. Did those designs not work, or what led to the shift away from them? I would think that in cases where adding a vent to an existing fixture would be impractical an anti-siphon arrangement should be an improvement over a simple S pipe.
    – supercat
    Nov 13, 2015 at 4:14
  • 1
    This sounds reasonable on paper but I can say from experience I've never seen it be an issue. I've owned several older homes and none of them have had vents after the p-trap. And the p-traps most certainly are full of water at all times (whenever I've cleaned or replaced them they were full). In addition, the cheapo DIY flange / p-trap kits they sell in hardware stores do not include vents. I suspect venting would only really be an issue for very small diameter pipes (1.25 - 1.5 inches is what I've always seen in my homes).
    – MgSam
    Jan 28, 2020 at 21:30

Every sink needs a sink vent. A plumbing fixture like a sink can not function properly without a sink vent. When a drain pipe drains it creates a negative pressure in the sink drain; the negative pressure will pull the water out of the p-trap of the sink that's draining and sewer gases can enter your home if a sink vent is not installed. There are several different ways in which to install a sink vent at home even if you want to install a sink vent on an existing sink. A sink needs a vent because the pressure put on the drain pipe when water flows down the drain pipe needs to equalize. The air needs to escape the drain pipe in order for the sink to vent.

The best way to think about a sink vent is if you were to hold a pop bottle upside down and dump it out, it glugs out. Add a hole to the top of the bottle and it pours out, that is a sink vent.

  • 1
    Right answer, but the soda bottle analogy is completely wrong, although it's often repeated. Your toilet and sink always have a hole at the top, no matter how badly you mess up the installation - it's where you wash your hands/do your business. The vent prevents P-trap suction, not slow draining. Sep 23, 2019 at 19:50

cut the tailpiece about 1" to help with slope toward drain and add a tee with a pro vent. Fit check it on 1 and 1/2" pvc about 2' long and slide it up inside the wall...you may have to cut the pvc down...once you get the correct fit glue it together.


I'm no plumber, but the drain pipe from the sink looks like 1-1/4" in diameter, then the p-trap looks like 1-1/2" in diameter then the short piece into the wall looks like 1-1/4" in diameter again, you should be consistently getting bigger diameter pipe as you get further from the sink, not going back and forth. Yes plumbing needs to be vented, you could install an air admittance valve.

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