Does Every P-Trap Need A Vent?( Yes, See Why)

Does Every P-Trap Need A Vent?

Note: As an amazon associate I may earn a small commission from qualifying purchases if you click to amazon from my site and choose to make a purchase.You can read my complete affiliate disclosure for more details

Does Every P-Trap Need A Vent?

P-traps are designed to hold water to block sewer gases from going into the house through the plumbing system.

P-traps are attached to every drain in your house, or rather that’s how each drain should be if they do not have traps.

The only place with no p-trap is the toilet, and that’s why they have their inbuilt trap that works differently from a p-trap.

Sewer gas can get into your house through open drainage from as far as the sewage treatment plant; this is a cause for foul smell.

But the sewer gas can contain methane which is harmful to human health. The sewer gas can make you sick, and if it’s highly concentrated, it can go to the extent of killing you.

Sewer gas is also flammable and so having a p-trap in your drainage system is very important. 

Yes, p-traps need vents. As p-traps are built to hold water and keep the sewer gases out of your house.

Vents provide the sewer gases with a ventilation area to avoid them from building up pressure in the sewer plumbing system.

Vents also help in stopping water siphoning from the p-traps.

Siphoning happens when, for instance, there is a controlled form of siphoning in the toilet, which helps in flushing and refilling the toilet with water again to act as a trap (S-trap).

Unlike if siphoning happened in a p-trap without a vent attached to it.

There are chances of the trap being left empty because its siphoning method is not controlled as that of the toilet.

So, every p-trap needs a vent to ensure the trap is filled with water at all times.

This will ensure that there is no sewer gases building pressure in the sewer lines as it gives them a place to vent.

If your p-traps are all vented, you might consider yourself safe from exposure to sewer gases.

Will A Sink Drain Without A Vent?

No, a sink will not drain efficiently without a vent. A vent is a very vital part of the drain system when it comes to plumbing.

Vents equalize the pressure in the pipes and stop a vacuum from developing during the drainage process.

This is crucial to the sink’s drainage system and every other drainage system in the house.

You might come across scenarios where your sink does not drain faster, or you hear some weird sounds emitting from your drains.

Or even notice stagnant water in your sink, which could all be issues caused by a blocked vent.

You should follow some distinguished measurements when installing a vent between the sink p-trap and the other opening where the waste ends up in the main sewer. 

A waste line that has 1 ¼ inch in diameter requires to have the vent as close as no more than 2 ½ feet from the trap.

And if the waste line measures 1 ½ inch, then the distance should be 3 ½ feet.

Does Every P-Trap Need A Vent?

The installation distance of the vent from the trap will be determined by the size of the trap in inches and the slope of the pipe in inches per foot.

There are times when you will feel the smell of sewage in a specific area in the house, such as the kitchen sink area.

That sewer smell is caused by water in the trap siphoning out due to lack of a vent to regulate the air escape.

So, installing a vent in the sink’s drains after the p-trap will save you many avoidable issues like water clogs in the sink or sewer smells in the house. 

Can A P-Trap Be Installed Backward?

Yes. You can install a p-trap backward. Well, installing a p-trap backward will work but will come with problems that are not worth the time and risk.

According to the building codes, the set minimum trap seal is 2 inches, whereas the maximum is 4 inches.

If the trap seal is less than 2 inches, it will fail to provide sufficient protection from having gas pressure penetrate the seal.

On the other hand, if it’s more than 4 inches, there are possibilities of it clogging. So, a properly installed trap pipe should have its outlet end being lower than the inlet.  

P-traps are designed to be self-sufficient in terms of cleanliness, and that is why they have that ‘P’ shape.

In a p-trap, we have the u-shaped pipe under the setup, which is the “trap,”

This is where water is held or trapped after draining the sink to keep the various sewer gas from rising through the drain system into your home.

Then there is the “trap arm,” which extends from the trap into the wall giving the whole setup the sideways P-shape hence the name a p-trap.

Now, if the p-trap is installed backward.

The drained water will have difficulties draining as the distance for it to overcome to get into the “trap arm” will be higher than when it is installed typically.

If the p-trap is installed correctly.

Drainage will be smooth as the distance from the sink drain to the “trap” will be high, creating enough force for the water to climb the “trap arm.” 

In summary, you can install a p-trap backward, but it is not something recommended; this is because of how the p-trap is designed to work.

Installing a p-trap backward could lead to clogging, which is not suitable for your drainage system and safety as well. 

Can A Toilet Shower And Sink Share A Vent?

Yes, a toilet shower and a sink can share a vent. It can be made possible through wet venting.

Wet venting is a method used to protect a drain pipes trap on several plumbing fixtures. Here is how this works;

The sink’s p-trap is connected to a vertical drain pipe that connects with the shower p-trap.

In this case, the sink’s drainpipe acts as the vent for the shower. Wet venting comes in handy as it saves on space and time and saves money on buying fewer materials. 

Can Two Toilets Share A Vent?

Yes. Two toilets can share a vent. How is this possible?

Some homes have more than a toilet in them; if the toilets are located on one side, the toilets can share one vent.

In a scenario where two toilets share the same vent, the waste lines tie into an individual stack.

If the toilets are on opposite sides of the house, each will require its stack.

Each stack needs to be vented, so having two toilets with different stacks will mean having two vent openings on your roof.

A vent stack is a pipe that leads to the main roof vent and is always required to be within a range of a 1.5-meter radius of the specific draining fixture, e.g., the toilet, sink, or shower.

We should also note that plumbing air vents should always be located on roofs, somewhere far from the air conditioning units installed or the windows.

Such that the gases can dissipate without a problem in the open air. 

Can A Plumbing Vent Run Horizontally?

Yes. A plumbing vent can run horizontally.

There is no problem having your vent running horizontally as long as you always keep in mind that you should give the pipes a clearance of 6 inches minimum above the spill line.

A spill line is a level at which water starts to overflow from specific plumbing fixtures like toilets and sinks.

Giving the vent pipes the clearance will allow them not to be water plugged.

The water that happens to get into the vent pipe opening should move freely down to the sewer without it creating a form of “trap” in the waste line.

The vent pipes can run horizontally for a distance of 20-25 feet across the attic connected to a vertical pipe that goes via the roof.

You might have heard of a sanitary tee or seen it in use; it is used to connect a horizontal run in that the waste arm is linked to the vertical riser via a sanitary tee.   

Can You Re-Route A Vent Stack?

Yes. You can re-route a vent stack. Re-routing the vent stack requires a little plumbing skill; all the plumber needs to do is avoid any sharp bends as possible.

Usually, vent pipes are installed to exit through the roof, where it’s sporadic for them to be interfered with.

Before re-routing your vent stack, some planning will need to be done before the re-routing.

It’s advisable for the person re-routing the vent stack to always consult with a plumber or a professional before making the changes.

Consulting with a professional will help you avoid getting disappointed or frustrated due to improper changes or even mismatch of the pipes and links. 

What Can I Use Instead Of A P-Trap?

There are times when one cannot install a p-trap in the drain system because the space is small.

In such a scenario, the alternative to using a p-trap would be using the new invention called HepvO Waterless Valve.

HepvO Waterless Valve has been ranked as the new solution to problems where homeowners cannot install p-traps in their drains.

Inside the device is a silicon tube placed there to seal air but permit water in a single direction; it does all this without the need for water or bulky compared to a p-trap.

A HepvO Waterless Valve can be installed horizontally, making it the perfect instrument for tiny spaces; you will not need to create more space by digging out the ground for it to fit.  

This is a normal p-trap, and as you can see, the ground has been dug to create room for the trap to fit. 

 HepvO Waterless Valve is connected to the drain of the sink. It’s not bulky and does not require a lot of space to fit correctly. 

Can A P-Trap Be Higher Than The Drain Pipe?

Yes, a p-trap can be higher than the drain pipe.

As much as installing a p-trap above the drainpipe is not advisable because doing so will force water out using water gravity instead of having the water flow naturally, downwards.

A p-trap is designed to work with the use of gravity from a constant downward flow of water.

So, as we all know, gravity drains do not go upwards unless you intend to pump the liquid up. 


P-tarps are installed in every drain pipe in the house; they usually retain water in the bottom curve, hence labeled “p-trap.”

The slight amount of water trapped in the p-trap helps prevent sewer gases from coming up your plumbing fixtures into the house.

As we have seen, sinks cannot function appropriately without a p-trap that is not correctly vented.

The vents help equalize the pressure in the pipes to prevent air from creating a vacuum in the trap.

P-traps can also be installed backward, but it is not advisable as this will interfere with how the device is designed to work.

Hence it will cause some problems like clogging and slow water drain in plumbing fixtures.

When installing a plumbing vent, you can choose to run it horizontally or vertically, but it must end up going through the roof to dispose of the sewer gases.

Pipe vents are usually installed a few inches after the p-trap, and in so doing, plumbing fixtures can share it through a method known as wet venting.

For instance, a toilet sink can share a vent with a shower through wet venting; it also saves on money, space, and time.

Generally, p-traps play vital roles in our drains and should not be installed without proper guidance from an expert. 


Hi! I' am Tom. I was a manager in one of the biggest stores for over 10 Years, am also an SEO by night. I don't like to call myself a blogger; they are very analytical, do email marketing, and know all SEO stuff. I faced many questions from customers about different products, and there was hardly any help on the internet. After learning all the things about these products as a manager the hard way, I decided to start a blog and help other people.

Recent Posts