How To Tell If Your Plumbing Vent Is Clogged

The inner workings of your house, the pipes and fittings hidden behind walls, in ceilings and underground are important pieces of the puzzle that makes up your home.

Your plumbing system is comprised of drainage to take your waste away from sinks, tubs and toilets as well as venting that connects to the drainage of your house, allowing waste to flow freely.

How can you tell if your vent is clogged? Issues in slow drainage, self-flushing toilets, rotten sewer smells are all ways that may indicate there is an issue with your venting. When venting is clogged, proper air circulation is cut off, making it difficult or impossible for your drainage to work.

It’s true, your home plumbing relies as heavily upon a ventilation system as much as the actual drainage itself.

Clogs can show up in a number of ways and can create a slew of different problems that this post aims to address. Stick around to learn more about clogged vents in your home and what you can do about them!

Where Do Plumbing Vents Go & Where Are They Located?

A plumbing system, no matter where it is, whether it be an office, your home, a movie theater or a school will require vents in order for them to work effectively (or even at all).

Any and every piece of drainage that runs through your house requires the ability for it to breathe.

This breathing is actually just the inside of the pipes trying to maintain atmospheric pressure of 14.69 pounds per square inch.

I just wrote another article on “do all drains need vents?” that highly relates to this topic I am writing about here. I invite you to check it out by clicking here:

This venting occurs wherever a fixture is present an follows the guidelines of:

Based on the slope of the pipe and the inside diameter of the pipe, it will get measured out into feet.

So if you have a 1 1/2″ diameter pipe (common size for sinks, showers, vents) at 1/4″ per foot of slope (or grade) then you are looking at requiring venting every 6 feet.

  • An easy way to calculate it would be to divide the diameter of the pipe (1.5″) by the slope per foot (.25″) to equal the distance in feet before the next vent is required
  • You can figure out the slope of a pipe by simply placing a level on it and determining where the bubble is in relation to the center which is in between both lines (or completely level)

This is all just a fancy way of saying that the vent piping is all through out your house and mostly hidden to you behind dry wall.

The vents will head in an upwards direction through your house and eventually into your attic.

Once they are in the attic, they eventually will join together, increase in size to 3″ or 4″ depending on your local code and then exit through the roof with a layer of neoprene (rubber) flashing that acts as a sealant , so as to not allow rain water to slip in beside the pipe into your home.

If you would like to learn more and get a much more in depth look on how your plumbing vent pipes work, head over to my other blog post by clicking here:

How Your Plumbing Vents Can Become Clogged

There are a few different scenarios I can think of that would cause venting to become clogged, and you’re first going to have to put yourself into one of two categories.

Are you someone who has owned your home for a while? Or are you someone who just moved into a home?

If you have lived in your home for a while, and all of a sudden you are having issues with your drainage, it may be because you experiencing a clog somewhere in your system.

The same is true if you are new to where you live, however you may also be experiencing things like improper installation of pipes or drainage issues that were never disclosed to you.

This however, is unlikely as most homes have to go through a plumbing inspection in order to gain occupancy.

I have also been guilty of leaving a test cap on one of the vents in a home that caused it to not work until the cap was removed.

When we test the drains, we cap off all of the ends to the system to make it a closed system and pump it full of air pressure (5 p.s.i) to check for leaks. If the system can hold that pressure for 24 hours, then it is leak free and will pass inspection.

So if you have lived in your home for a while and have just started experiencing issues with things like drainage in your home, it may be because a portion of your venting is blocked off.

You may be asking, “What could cause vents to get clogged? I thought only air passed through them?”

You’re right, however the biggest culprit for a vent clogging is located on the roof of your house: The vent stack.

The diameter of this pipe poses both problems and solutions. Because it is at least 3″ (where I live), the pipe allows for snow and rain to enter inside of it without the worry of clogging it up.

The size doesn’t allow an accumulation to occur and will end up just draining the melted snow or rain down the drain and into the sewer.

That’s the good part of it, the bad side of it is that it is large enough for rodents, birds, tree roots, leaves among other objects to get caught inside of it.

Birds have been known to set up a nest right over top of this vent because of the heat that is sometimes let off from inside of the plumbing system.

All of this is a recipe for clogging to occur. It doesn’t take much to fall down inside of that drain to clog it up, especially since it’s sometimes only a total of 5-6 feet of this size pipe.

The majority of the vents that allow your sinks and drains to breathe will be 1 1/2″ to 2″ in diameter all the way up until they are about to exit the roof.

How To Tell If Your Plumbing Vents Are Clogged

Depending on how and where the clogging occurs will determine what kind of issue you’re facing.

Since the most common clog will occur closer to where the vents exit your house through your roof, you are probably experiencing slower drainage and possibly even sewer smells.

Because the main supply of air is cut off, your drains tend to slow right down.

This can actually pose even greater problems as certain waste can get left behind in your drain pipes, dry up and then build up over time. This will actually cause your drains to clog as well.

Sewer Smells

In your home, you have a bunch fittings known as p-traps. These p-traps are located at every fixture (besides the toilet) and prevent sewer gases and bugs from crawling in.

So how does clogged vents leave you with sewer gas? It’s the same exact principle as how a toilet works.

Toilets are purposely un-vented until they reach the drain and are designed with an s-trap pattern.

This s-trap pattern will create a vacuum inside the curve of the S that will suck the water from the toilet bowl down as the water in the toilets tank gets flushed.

When a p-trap is not vented, it will act the same way. As your waste travels down the drain, it can actually pull some or all of the water located inside of the non-vented p-trap with it.

This will leave your house exposed to all of the lovely aroma’s of the sewer.

This can be the same effect on the toilet as well, since the vent that allows the drainage below the toilet to breathe gets clogged up, whenever waste passes by the toilet drain, there’s potential for the water in the toilet bowl to go with it. This may also just result in some minor gurgling or movement in water.

Recommended reading: – In this post I dive into everything plumbing trap related and give a full breakdown on the different types of traps, their purpose and much more.

How To Clear The Clog

This pipe can range in different colors and materials from black to white, cast iron, PVC or ABS.

Go Up On The Roof

The next way is to go right up to the source, and check out your vent pipes on your roof.

I don’t recommend you do this unless you have experience climbing on a roof as there is definite potential for danger. If you do decide to go up, ensure that you are securing yourself with the proper safety harness and tie-off system to prevent falling and don’t climb onto a slippery surface! Otherwise, a quick call to a licensed plumber will be your best course of action!

If you decide to go this route, take a flashlight with you and a drain snake. You can find drain snakes, flashlights and anything else recommended on this post by heading over to my resources page by clicking here:

Once you are at the pipe, flash your light down the vent pipe and look for any kind of debris, birds nest, leaves that can be removed.

Next, insert the snake down the vent pipe and begin the snaking process by other hand cranking it on hand held ones or turning the power on on the powered version.

Make a few passes down the pipes and back up to you to ensure you get everything. This whole process could take you a while to do, so running the snake up and down the vent a few times could save you from climbing up and down the ladder more than necessary.

Have your spouse, family member or friend check on the status of the drainage while you’re up there. If there still seems to be a problem keep going until there isn’t.

Recommended reading – I have a full article on how to snake a drain, which highlights the different types of snakes on the market, what their use is and how to operate each one, check it out here:

Another method (as shown in the video above) is to put a garden hose down the drain. The most effective way to utilize your garden hose is to actually hook it up to a source of hot water.

You can do this by heading over to your washing machine, turning the valve off to the hot water side, hooking up your hose to it, turning the valve back on and then running it up to your roof.

Ensure your hose is long enough to reach. You can buy hoses at lengths of 25 ft, 50 ft, 75 ft and even greater. If you would like to check out my garden hose buying guide to help you find the right one, you can do so by simply clicking here:

Head Into Your Attic

If climbing up on your roof doesn’t sound too appealing to you, you may also be able to access your vent from your attic space.

You are definitely going to need to grab a flashlight to see your way around the attic.

Find the vent pipe where it is about to exit through your roof. Grab a sawzall or handsaw and cut into the pipe.

I should mention that it is recommended to call a plumber if you are unsure of how to do anything mentioned in this post.

From here, you will be able to access the inside of the pipe where you can begin snaking. You may also want to try and snake upwards as well, as it is very much possible that the clog is closer to the top of the pipe.

So a recap on the supplies/tools you’ll need:

  • Sawzall or handsaw
  • Flashlight
  • 3″ (or 4″) PVC, ABS or cast iron coupling (depending on the type and size of pipe)
  • PVC or ABS glue & PVC primer
  • or MJ coupling for cast iron with nut driver
  • Screws and screwdriverThe pipe where it heads in a vertical direction through your roof will most likely be supported by metal strapping and screws
  • Drain snake
  • Bucket
  • Gloves


Tyler Takacs

My name is Tyler, I live in Ontario Canada and enjoy learning about common plumbing issues in the household. I have spent just over three years in the trades as a plumbers apprentice, but am now onto a less physical job. I still enjoy studying and learning about my own house's plumbing as well as finding ways to help others with their issues.

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