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Why Your Home Needs a Plumbing Vent/Vent Stack

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 plumbing vent? 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

P-traps and Plumbing Vents

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.


Problems with Plumbing Vent Systems

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.

Can I Fix a Clogged Plumbing Vent?


There are several things that can cause a clogged plumbing vent which you don’t want because it will cause your whole plumbing system to stop functioning correctly. Here are 3 signs that indicate you may have a clogged plumbing vent.

1) Strong Sewer Smells

2) Slow Drains

3) Gurgling toilets or drains

If you believe your plumbing vent is clogged, you can climb onto your roof to check. Clear any debris around the pipe and shine a light down the pipe. You can use a plumbers snake to retrieve something near the surface.

According to Expert Plumbing Utah, if you don’t pull anything out or the problems persist, contact a professional. It may involve opening the walls to examine the plumbing system.


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! 

© 2020 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.

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