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How and Why to Vent Your Plumbing

Ron has been an online writer for over eight years. His articles focus on everything from philosophy to home repair.

Learn how to vent your plumbing system for a better sink.

Learn how to vent your plumbing system for a better sink.

I was a plumber, among other things, for 18 years as a private renovations contractor. During that time, I heard all kinds of noises from (and complaints about) drainage systems. Many had to do with inadequate venting.

2 Reasons to Vent a System

Most people think plumbing is all about drains and water pipes, and it is—but many people don’t know that those drains have to be vented. The reason for venting is twofold. First of all, noxious gas can build up in a system, and if there is no venting, these gasses will find an escape through drains and toilets. If you hear a bubbling noise coming from your toilet or sink, it is probably because the system is not adequately vented.

The second reason to vent a system is so the pipes remain at neutral pressure. This helps promote proper drainage. A lack of venting can make drains slow and can suck out the water from the P trap, leaving the house at risk of being flooded with noxious or even poison fumes. This often happens when a do-it-yourselfer forgets to vent the new bathroom they just finished in the backroom addition or basement or in very old houses that were plumbed before standards were in place.

What Is a Wet Vent?

New houses are all built with adequate venting, of course. The picture below shows a typical double drainage system. A typical system contains two kinds of vents: A wet vent and a dry vent. The wet vent is actually the stack. It is typically a 4-inch or larger pipe that runs from the sewer connection under the basement to the roof of the house.

The reason it is called a wet vent is because it obviously carries a lot of water and waste as well as being open to the outside air on the roof level.

All drainage pipes coming from all fixtures in the house—like sinks, basins, tubs, and toilets—are connected to the stack. Each fixture has its own size pipe. Toilets need 4-inch drainage pipes, sinks are connected with 1.5-inch drainage pipes, basins are typically connected with minimum 1.25-inch pipes, and stand-alone showers and tubs are usually connected with 2-inch pipes.

All drainage pipes are equipped with a P trap. You can see it if you look under your sink. Check the pictures below. The P trap is a water trap. The water in the trap prevents gases from the sewer from going into your house. But again, this can only happen at atmospheric pressure. If there is a pressure buildup due to inadequate venting, the gas can get through the water barrier, and as I said before, that water can also be sucked out by negative pressure, which can happen if someone upstairs rapidly drains a fixture.


Why Wet Venting Is Inadequate

Therefore, wet venting is inadequate on its own. You also need what is known as a dry vent. A dry vent does not carry water or waste products. It typically connects to the stack below the roof line and runs down the length of the building where it connects to the stack again just above the basement floor.1.5 to 2 inch dry vent stack is typical but codes in your area may vary depending on the material of the pipe. These days it is all PCV pipe but many houses still have cast iron and galvanized pipe in them.

Again, in the photos above, you will see a picture of a typical set up. A basin is connected to the stack well under the bottom of the basin. Another pipe runs off that one upward, well above the basin to the vent pipe. All your fixtures should be connected to it. Sometimes in bathrooms the toilet and tub or shower are wet vented through the basin drain which in turn is connected to the vent stack. Toilets and stand alone showers should be as close to the main stack as possible. But with adequate venting and slop longer runs are possible.


What to Do if Your House Has not Been Vented

So what do you do if your house has not been properly vented and you are getting strange smells and noises coming from your drain pipes? The only rational answer is to add proper venting. If the entire house needs to be done, then unless you have done this sort of thing before and don’t need this article, call a good reliable plumber.

You are in for a big expense where walls will need to be cut into and drywall replaced and repainted or refinished.

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But if you are just having problems with an addition, you can do it yourself if you can find the vent stack. It is a lot of work and still may involve the replacement of some drywall. It is a big job but not out of the realm of possibility for someone with a handyman bent.

And yet, for slow drains due to no vent, there is an alternative. It is called an air-admittance valve. There is a picture of it above in a typical application. It is a valve that allows air into the system but does not allow gasses to escape. Though it is not permitted under some codes and is not appropriate for all cases, it is widely used in Europe and is approved for use in some U.S. states and in most Canadian provinces.

Its only drawback is that it does not release pressure. Of course, we wouldn’t want it to. In most cases, that is not a concern, particularly if the rest of the house is properly vented. If your problem is with gurgling noises when no water is being run through the drain, you have bigger problems with venting than this valve can solve.

However, they are relatively easy to install, and they are definitely inexpensive at between 20 and 30 dollars, as compared to several hundred to several thousand. All that is required is that you have space to add a T to the system as shown in the photograph.

The Takeaway

In conclusion, problems you are having with improper or slow drainage may not have to do with your drains as much as they have to do with your venting. It’s always worth getting to know your drainage and venting system, particularly if you are buying a house or putting in an addition.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: How do you vent an under-counter oval basin that has no opening?

Answer: If you mean without a hole for overflow, that's not a venting issue, it just means your basin can overflow if you aren't careful. Obviously, it has a hole for drainage. Therefore if you are just replacing the basin the pipe you are attaching it to should already be vented. If you are doing a complete new install, like building a bathroom in your basement, and you have no previous experience, you will have to study and follow the local code or get a plumber. But in any case, all the basics of venting will be as I've outlined in my paper on the subject.


ChandrakantINDIA on September 26, 2016:


GW on August 15, 2016:

I just moved in a townhouse about a month ago which is a 3-story building plus a basement. There are two bathrooms in my house, one is on the top floor and the other one is on the first floor. I started noticing the musty smell from the first floor bathroom right after I moved in, and there were also other problems in this bathroom: a slowly leaking water supply line to the toilet tank and a leaking toilet (these problems have been fixed - the was ring was replaced and a new water supply line was installed). Since currently there is no water leaking in the bathroom that I can see, but the musty smell is still there, I am just wondering what could be the cause, could it be the mould still growing from the previous leak (as the old water supply line was very rusty and apparently the leak was there for quite a while but the previous owner never bothered to fix it), or could it be another leak from somewhere else in the bathroom as sometimes I can hear dripping sound from one corner inside the wall (I've read some previous posts saying this might not be a water leak). One thing to add is that the dripping sound appears every time I use the sink, but not the shower or the toilet, and the dripping sound does appears from the corner close to the sink. The musty smell is so strong every morning and it drives me nuts, I really want to get rid of it!! Thanks very much for any help!

Ron Hooft (author) from Ottawa on January 08, 2016:


You may have to get up in the attic to see if you have a stack or not, and see if it's blocked if you have one. If not, unless you can do the job yourself a plumber is definitely in order.

A metal roof may disguise it with a cap of the same material making it hard to see without close inspection. a bird may have made a nest in there or something, blocking it.

If there is no pipe in the attic that stack attached to your toilet ends somewhere. Finding where could be expensive and mean opening walls to correct the problem.

But unless your house was built pre-indoor plumbing I can't see modern builders not putting in a stack at all. Everyone these days has to follow code. But roofers don't get inspected by the municipality in most places when replacing shingles etc. Could they have blocked it off?Or was it always a metal roof?

Lots to consider and investigate before you spend money on either.

Ron Hooft (author) from Ottawa on January 08, 2016:


It could also be dangerous. You need to extend that pipe outside, asap.

Timeismine on January 03, 2016:

My roof has no outside vent stacks showing--it is a metal roof--and i have so much trouble with slow to completely stopped drains. I have always felt it was due to a venting issue but dont know if i need a plumber or a roofer!

RM on June 02, 2014:

Hi Mr. O'Brian, I have a question for you. Have you ever had experience with a home that had a venting system that vented directly into the attic and just stopped there, rather than the vents leading into the outdoors? I just bought a 25 year old home and discovered that the vents were never led outside of the attic; therefore the septic smell just fills up the attic. It quite disgusting, actually, so I'm not sure why it took 25 years to be addressed.

Ron Hooft (author) from Ottawa on February 01, 2014:

a plumber

That would depend on where you live and what the current code is..

a plumber on June 30, 2013:

since when do toilets have a 4 inch line?? most mains are 4 and all toilets are 3!!

Ron Hooft (author) from Ottawa on May 18, 2013:

Sounds like a pressure problem. If I understand you correctly either you need to change the entire speed supply line, which is the flexible pipe going from the half inch copper water pipe to your toilet because the pressure fitting is bad, or your water pressure is too high.

City water is usually no more than 60 psi. Speed supplies should have no problem with that.

If it pops off from the toilet itself it could be that the nut that holds it on is stripped, or the plastic threads on the flush valve are bad. In that case you have to replace the entire flush mechanism. Usually a 10 or 15 dollar part.

Bubbling can mean bad venting or blocked vents. in any case if you have bubbling in the toilet you have a problem. It should be attached with a 4 inch pipe going to the main stack directly leading down to the pipes under the house and vented through the main stack itself. See pic above.

Check to see that your main stack on the roof is clear.

My advice is to call a plumber if you can't isolate the problem yourself. You are obviously getting back pressure from somewhere.

sharrai on May 08, 2013:

my line keep popping off the toilet when flushed. We fix it then we go a few weeks with bubbling in the toilet when tub is drained or in tub when toilet is flushed, and then someone will flush a on the ground goes the toilet drain. What could be the cause of this?

Ron Hooft (author) from Ottawa on September 01, 2011:

The comment you made was fine. ;) Well I'm not that far from you then, I'm in Ottawa.

Yes there are many plumbing hubs. I'm actually surprised. ;)

Mr. Happy from Toronto, Canada on August 31, 2011:

Hello again Mr. O'Brian. Sorry for the selfish comment I left earlier. I had a buddy here last night - he works in the renovating business and he was the one to tell me that I was an asshole for leaving that comment. I now agree lol

In terms of where my sister lives - it's Toronto, not very warm but I like it (I live here too). I came across a blog today which made me think of you and this blog and I thought I would bring it to your attention. I am not sure if it's relevant or not but here you go either way:

All the best!

Ron Hooft (author) from Ottawa on August 29, 2011:

Hi Simone

All houses build in the last 70 years should have venting. Whether it is too corroded to work or not is another story. Depends on the material of your piping. If your house is relatively new you should be good depending where you live, of course. I can only talk about North America and most of Europe.

If your drain pipe is plastic and you live in North America you should be fine unless the house was built by an individual who didn't know what he was doing. ;)

But it is a good thing to look into. I'm not sure asking the land lord is going to get you a straight answer. In any case he or she may not know. If you have concerns or have slow drainage ask someone knowledgeable and reputable to give your house a quick check.

Estimates are often free so get a few opinions before you decide there really is something wrong and you need to spend money on it.

There are many noises toilets and other fixtures can make. Sometimes an old valve will rattle so loud you think the house is about to explode. It's a simple and cheep fix.

What you need to look for from vents is smell, no water in your toilet bowl after you did the dishes or drained the tub, bubbles in the toilet or other fixtures when they are not in use. That sort of thing may mean a venting problem. ;)

Simone Haruko Smith from San Francisco on August 29, 2011:

Gosh... I had never even heard of venting before reading this Hub. Do all houses have it? Or is it relatively uncommon? I just don't think I've ever seen anything of the sort before.

Well, this is a great explanation of why venting is really important... I'm going to have to ask my landlords of our plumbing system is well ventilated or not... I think that might explain some of the issues we've been having...

Ron Hooft (author) from Ottawa on August 29, 2011:

Mr Happy.

Don't know where you live and I don't do that kind of work much anymore, but if you are paying air fair and you live some where warm I would consider it for a winter job. lol...

Ron Hooft (author) from Ottawa on August 29, 2011:


Thank you. I would be happy to do other hubs along these lines if people ask me specific questions. I don't see an advantage to anyone if I just write about anything at all. And as you know I am more interested in philosophy these days. I'm also looking for new topics in that area.

Old houses are nice but unless you know enough about them you are at the mercy of contractors. Many are great. Others not so much. ;)

But you could do it yourself. I bet you could redo your entire drainage system if you wanted to. You would just need a little instruction first, and a local code book. ;)

Mr. Happy from Toronto, Canada on August 29, 2011:

lol My sister is renovating her house, maybe you should send me a business card : )

Ron Hooft (author) from Ottawa on August 29, 2011:

Mr Happy.

Not my usual hub to be sure. Just thought I would enter the contest since I have extensive experience in plumbing and all manner of renovations.

Xavier Nathan from Isle of Man on August 29, 2011:

For me the house is a symbol like any other symbol in this world of illusion. Whenever I dream of a house there is always a physical issue I need to address. If Slarty's hub was a dream I had then I would be facing some serious physical problems! LOL!

Mr. Happy from Toronto, Canada on August 29, 2011:

I will keep this in mind for when I ... who knows ... maybe for when I will be a cockroach and need to map-out my surroundings.

I have no clue what I am doing here rofl

Take care Mr. O'Brian - I will keep this in mind; if I ever need to learn it, I might be knocking on your door. (I'll send a pigeon first.)

Xavier Nathan from Isle of Man on August 29, 2011:

This is a very informative hub about something I know nothing about. We live in a very old house and we hear all sorts of noises but apart from the electrics tripping every so often we have not had any major catastrophes. The windows were never put in properly by the people we bought the house from so the natural ventilation from the windows is probably keeping us alive. LOL!

I think a series of hubs like this will be very popular especially when you make people aware of the problems and they choose to tackle it themselves or call in a professional. The next time I see our plumber I will ask him now about the venting. Thank you.

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