As far as I can tell, none of my plumbing fixtures are vented. I have a double-sink in the kitchen, but no vent pipe. My washroom (photo here) similarly has an s-trap. Same thing on my washing machine and drain basin (photo).

I can see a large black pipe poking out of my roof, just about where the drain lines are going into the floor. So I assume I have a main vent stack, but what does it do in my case? Why do I have a main vent stack if none of my fixtures are vented?

If it helps to identify typical construction methods, my house is about 80 years old and located in Toronto, Canada.

  • 5
    Not everything needs its own vent if you employ wet venting.
    – Mazura
    Jan 28, 2016 at 3:40

5 Answers 5


Your system is almost certainly vented. Plumbing vents provide a path for sewer gases to escape so they don't bubble up through the P traps into your house under pressure (the sewer gases will expand and exert greater than atmospheric pressure), and the vents also keep water pressure from siphoning your traps dry, which would create an easy natural path for sewer gases to infiltrate your home.

So the comment about "wet venting" is right on target. As long as the combination of pipe diameter, the slope of the pipe and the distance between the trap and the vent ensure that the pipe doesn't run full of water (creating a siphon), a single downstream vent can be just fine.

  • 1
    That was the root of my question - wondering if old building codes did not require fixture venting. I did not realize that wet venting was acceptable for any fixture, so this helped me to understand. I can see that most of my fixtures are centred around the stack, so I think this makes sense. The exceptions are my kitchen sink (which I can sometimes hear gurgling, so I think it has some siphon issues), and the laundry (to which I had to add an AAV to solve a noticeable sewer gas problem)
    – Roberto
    Jan 28, 2016 at 17:25
  • Clothes washing machines eject water under a lot of pressure, especially newer ones. So it's not surprising that you had issues there. You could probably put an AAV on your gurgling sink, too. They make models meant for kitchen island dishwashers that might be a possibility. Jan 28, 2016 at 21:12

Depending on the size and age of your home, and the location of the plumbing fixtures. It's entirely possible that there's only a single vent stack.

If all the drains are within a certain distance of the stack, no additional vent pipes may be required.

According to Wikipedia's article on Drain-waste-vent system.

...Under many older building codes, a vent stack (a pipe leading to the main roof vent) is required to be within a 5-foot (1.5 m) radius of the draining fixture it serves (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof penetration as permitted by local building code, sub-vents may be tied together inside the building and exit via a common vent stack...

  • Yep. And the comment on the OP's question about "wet venting" is right on target. Jan 28, 2016 at 4:30
  • Thanks. From what I can tell, it sounds like this accurately describes my situation.
    – Roberto
    Jan 28, 2016 at 17:28

The one pipe you notice exiting from the roof is indeed a vent stack. Since there is only that single pipe projecting out it would be reasonable to guess it is the main stack that all the other drain lines connect into.

The simple reason for not seeing any smaller or secondary vents is because they are located out of sight behind the walls at each sink or tub or toilet. If you could remove the drywall and expose everything behind it you would see a maze of ABS (or cast iron) pipe. For every drain most vents start as 1 1/2 - 2 inch pipe. They will connect to the main stack which will increase in size depending on how much water it handles to about 3-4 inches in most residential homes.

  • Most of the time separate vents are not needed unless pipe runs are very long. Air admittance values are a more modem way of doing it in some cases.
    – Walker
    Jan 28, 2016 at 15:18
  • This is true, but you're most likely to encounter AAV's under (or close to) the respected drain line and not behind drywall. The photo's posted don't show any. Unless they are out of the frame of camera.
    – ojait
    Jan 28, 2016 at 17:02
  • 1
    Thanks. From what I can see, and the odd wall I have opened, I think this is more the case of wet venting that others have mentioned, as opposed to hidden vents. But I see your point that it might not always be possible to tell if a fixture has a separate vent without looking inside the walls. I thought maybe old codes did not require venting, but it sounds more like it was an a matter of wet venting.
    – Roberto
    Jan 28, 2016 at 17:28

(note: this answer is based on UK practices, the general principles are likely to apply everywhere but details of how the problems are mitigated may vary between jurisdictions).

Various nasty gasses can come off the contents of waste pipes and we don't want those gasses to be allowed into the house.

To block the sewer gasses from entering the house we have "traps" (the most common style being a U bend but there are other varieties). These are designed to remain full of water and hence provide a gas barrier between the room and the drainage system.

But traps only work for small pressure differentials. If the pressure in the waste system is too high (for example because of rotting waste producing gas) then gas from the waste system can bubble out through the traps. If the pressure in the waste system is too low (for example because of siphon affects) then it can suck the water out of the trap.

One common soloution to this is to have a vertical pipe (the "stack") running above roof level where it is vented. This provides a path for any pressure builup in the system to excape and as long as the fixtures are close to the stack and connected to it by gently sloping pipes this one vent at the top is enough to reduce siphon affects to an accepable level.

If the fixtures are a long way from the stack there may be a need for specific vent pipes returning from the fixtures to the stack (alternatively there may just be more than one stack so that there aren't any fixtures a long distance from their stack).

There are other soloutions too. Sinks, baths etc (but not toilets) in older houses in the UK often have an open pipe draining into a drain (for downstairs stuff) or hopper (for upstairs stuff) outside.

More modern systems may use air admittance valves which let air in but not out. Since they don't let air out they can be installed indoors. Such valves will let air in to break a siphon but they won't let pressure out. So they will reduce but not eliminate the need for "open" ventilation.

  • 1
    Maybe worth adding air admittance values are be used, then a stack is only needed at the end of the drain line.
    – Walker
    Jan 28, 2016 at 15:16
  • I've tried to keep this answer generic (partly because i'm not exactly an expert, partly because the OP is in a totally different jurisdiction to me. The system needs to be open vented somewhere but where exactly it needs to be vented and whether any of those vents need to be on your property is likely to depend on local rules and possiblly the type of drainage system outside your property. Jan 28, 2016 at 15:30
  • Thanks. I understand the need for venting, just did not know if that was a recent code issue or if the code 80+ years ago would have required it. I understand s-traps used to be considered OK, but did not realize this would have relied on wet venting to the main vent stack.
    – Roberto
    Jan 28, 2016 at 17:21
  • Just note that AAV's are mechanical devices that need to be replaced every few decades, where roof vents are much longer lasting.
    – Bryce
    Oct 2, 2019 at 7:52
  • In the US I think every trap needs its own vent of some kind., for followers, thank you for explaining why I see diagrams showing no vents on some fixtures at times :)
    – rogerdpack
    Jan 26, 2022 at 8:03

Of course your system is vented. If it was not vented it would not drain properly (if at all). The vent riser pipe is connected to your waste pipe system inside your walls, where you cannot see.

If you wish you could open the wall below where the vent riser is and educate yourself on the configuration. This would lead to significant wall repair so I would suggest going to your local library and checking out a book about residential plumbing design. Your taxes already pay for operation of the library, you might as well take advantage of the opportunity.

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