Does Every Drain Need A Vent?

If you’ve ever been to the kitchen sink to prepare your favorite vegetables for that nights dinner, or hopped in the shower because the dinner was important and then used the washroom after the dinner because that meat was just a little bit expired, then you can understand the importance of the drain.

Drains are a vital part of any home’s plumbing system and without them, plumbing would cease to exist.

But what makes them work? Are they the same as a slip n’ slide at your local park? Or do they require something else to function effectively?

So, does every drain need a vent? The answer is yes, every drain from your kitchen sink, to toilet, shower, laundry, floor drain and more all require venting. Vents are the allowance of atmospheric pressure in drains to prevent airlock from occurring.

The plumbing in your home is a lot more intricate than you may have initially thought. Venting is as important as the drainage itself and can lead to a whole slew of problems without it.

There’s lots more to share about the topic of venting, so continue reading!

Why Drains Need Vents

If you can think back to when you were a kid in science class, did your teacher ever give you the experiment of putting a straw inside a glass of water, covering the end of it and then taking it out of the glass?

The water stays inside of the straw for as long as your finger covers it and this is because atmospheric pressure can’t stay consistent through out it.

In plumbing, this is known as “air-lock” and, fun-fact, it is exactly the principle the allows a toilet to work.

A toilet is a perfect example of utilizing a principle of that traditionally does not work to be the sole reason it works.

The S-Trap Of The Toilet & Venting

A toilet forms what’s known as an s-trap. This s-trap in any other form is illegal based off of plumbing code, but works for the design of the toilet.

The bowl of the toilet holds the water which is the “trap” portion. There is then a sideways “s” formed in the porcelain of the toilet that when water gets flushed from the tank into the drain, it literally sucks the water in the bowl down with it.

The reason for this simply is because the toilet itself is not vented until it hits the drainage pipe below.

The space between where the water flushes directly down the drain from the tank and the water in the bowl is empty and an airlock is created.

As the big wave of water rushes down the drain it has no choice but to try and grab atmospheric pressure from anywhere it can, thus resulting in the flushing of the toilet.

Toilet Drainage Pipes

Although the toilet itself requires no drainage to operate, the drain pipes below are another story.

Arguably the most important of the drains in ones house is the toilet drain for the simple matter that is the one that takes the largest volume and size of waste through it’s pipes.

Typically, toilet drains require a 2″ vent pipe which must remain consistent all the way through the house until exposed to open air and can only increase in size.

What Happens When A Drain Is Not Vented?

In the example of the toilet, that deals with larger forms of waste, not having a vent hooked up to the pipes below can cause major problems and back ups.

Intuitively, we think that if a pipe has some slope to it, and there is water running through the pipes that it should just take anything inside along for the ride.

If a drain lacks a proper vent and can’t receive enough air, air-lock occurs and can drastically slow down the drainage process to almost nothing.

When a toilet drain isn’t vented, there simply isn’t enough air involved to keep the flow of waste going.

A lot of the time, there isn’t just a complete stop in movement, but rather a major reduction.

This reduction causes the waste to not move to where it needs to be before everything inside of the pipes dries out. When everything dries out, the waste will stay in it’s place and begin to buildup on the inner walls of your pipes.

It won’t take very long at all before you’ve flushed a couple of times for this buildup to completely block passage way in your piping. When this happens, the water and waste have no choice but to head back up your toilets s-trap, into your bowl and all over your floor. Gross.

Bathroom & Kitchen Sinks

What happens when there is lack of venting on smaller pipes, such as the kitchen and bathroom sink, for example?

A brief description of household plumbing pipe sizes:

  • Toilets require the second largest drain in your house at 3″ diameter
  • Bathroom & Kitchen sinks require 1 1/2″
  • Tubs & Showers typically utilize 1 1/2″ drains, but it’s also common to find 2″
  • Washing machine for laundry is 2″
  • Dishwasher is hooked up to your kitchen sink
  • Main building drain is typically 4″ in diameter, and is the thing all of the waste from your home will eventually drain into

When there’s a lack of venting in places like your kitchen sink or bathroom sink, an s-trap can actually be created whether you have the traditional p-trap or not.

This is because a lot of the times the drain of the sink when it goes out of visibility (behind drywall) it typically heads straight down into either the floor if your in a basement, or into the joists of the room below.

As water travels down this leg of pipe that’s not vented, it creates that air-lock and pulls water from the p-trap out with it.

Thus resulting in potential for a useless p-trap as well as a slow drain. Exactly as the toilet drains don’t allow fast enough movement of waste before it gets to where it needs to, the same happens with all other drains.

Why P-Traps Need Venting As Much As The Rest Of The Drains?

P-traps are also a vital part of any home’s drainage system and will require venting as well.

As much as you may want to slap vents anywhere convenient to allow for atmospheric pressure, you need to have an understanding of why vents are placed where they are as well as their importance.

For a p-trap to operate, there cannot be any possibility of water exiting the “U” portion of the pipe.

A p-trap’s purpose is to create a seal with water so as to not allow sewer gases and bugs to pass through your drains from the sewer and into your household, unless that’s what you’re into (but I don’t recommend it).

Traps are located anywhere in your home where there is potential for the bugs and gases to enter. Examples of traps in your home:

  • Toilet – Is an s-trap
  • Kitchen & bathroom sink
  • Tub & showers
  • Laundry
  • Floor drain

If you would like to learn more about the different kinds of traps and their role in your home, then head over to my other blog by clicking here.

How Far Can A Vent Pipe Be From The Drain?

A common question I find asked is how far exactly can a vent pipe be from the drain? There isn’t a one-size fit’s all measurement for this, but there is a way to figure out the maximum distance you need.

Whether you are dealing with a p-trap drain, or the rest of the drainage in your house, you’re going to need to vent it.

Let’s first talk about drain slope. A drain requires slope in order for waste to follow the correct gravitational pull to effectively get it into the sewer.

Typically for drains in your home that are above ground, you are looking at 1/4″ of slope per foot.

If you take an 1 1/2″ kitchen sink drain and measure out 1/4″ per foot, you’ll get 6 feet by the end of the run.

The reason this is important is because this is the exact point at which the top of the pipe at the high end (obvert) of the drain and the bottom (invert) of the pipe at the low end of the drain become level on a horizontal axis.

This is the first point in which your drain becomes cut off to atmospheric pressure and will become air-locked.

  • For 2″ pipe it would be 1/4″ per foot for the 2″ diameter. So you calculate that it takes 8, 1/4″s to get to 2″, therefore the distance in feet is 8 feet.
  • 3″ pipe at 1/4″ per foot of slope would be 12 feet.
  • For the main building drain that’s run underground at a 4″ diameter for 1/8″ per foot of slope would equal to 32 feet.

Keep in mind that codes may vary from state to state and province to province, so be sure to consult your local code book for your locations exact specifications.

If you would like to learn more about how plumbing vent pipes work in your home, than you might like my other blog post that I wrote that you can find by clicking here.


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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