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Plumbing Vents/Vent Stacks

You’ve certainly seen pipes sticking up a few inches above the roof. These are typically called vent stacks or plumbing vents. They are part of your home’s drain, waste, vent (DWV) system. What is the purpose of a vent stack? Their purpose is twofold: to allow your plumbing fixtures, tubs, sinks, and toilets, to drain properly; and to keep sewer gases out of your house. They do this by giving a path for sewer gases to escape and by keeping the pressure in your plumbing system the same on both sides of the P-trap.

plumbing vent/ vent stack

What is a P-trap?

sink with p trapA P-trap is the “U”-shaped pipe underneath the sinks in your home. It is “P” shaped lying on its back. This is where it gets its name. Your tubs, showers, and toilets also have traps – they’re just not as visible. Their purpose is to maintain a water barrier between your home and the sewer system. As long as your P-trap stays full of water, sewer gases cannot get into your home because they are blocked (trapped) by the water in the trap.

How does a home’s plumbing venting system work?

As mentioned above, the venting system works by maintaining equal pressures on both sides of the P-trap. This allows the drains to flow well, and the sewer gases to exit the drain pipes through the vent pipes on the roof.

diagram of how p trap worksHave you ever taken a straw out of a glass of water or Coke while holding your finger or thumb over the top of the straw, and seen how the water or Coke stays inside the straw rather than running out of the bottom of the straw? Well, without a proper venting, the sinks, tub’s, and toilets in your home also cannot drain properly. With proper venting, as the water flows down your drain pipes, air is pulled in through the vents behind the exiting water to maintain pressure and not allow a vacuum to form. This keeps the drains flowing properly. 

Let’s talk briefly about a plumbing system with traps, but without proper venting. With this setup (called an S-trap instead of a P-trap), it is likely when you drain a sink or tub or flush a toilet that the trap would be sucked dry when the sink, tub, or toilet finished draining. This is due to the siphoning effect – water flowing through a closed pipe pulls the water that is behind it until the siphon (or suction) is broken by air in the line. This is the problem with S-traps. See my blog about S-traps for more on this. On a properly vented plumbing system, the air in the vent breaks the siphon and prevents the water in the trap from being sucked out.


Your home’s plumbing vent system is very important for the health of your home. It keeps nasty sewer gases out of your home, and it helps the drain lines to drain properly. A clogged or inadequate venting system can cause poor draining which can allow debris to slowly build up leading to clogs. If you notice any gurgling, this may be an indication that a vent is becoming clogged and needs to be unclogged. It is not uncommon for a small animal to get into a vent pipe up on the roof and cause it to clog. If you smell sewer gases or notice gurgling, it may be time to call a plumber.

You learned what P-traps are, learn more about S-traps here! 

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