Does a toilet need a ‘P’ trap? Find out why you should never fit one!
I am currently using my DIY skills to help me renovate my home and the project for this quarter is the family bathroom. Just like every DIY project I undertake I make project plans and consider what option we have and how I can improve things,
This time I am considering integrating the toilet and bathroom which means relocating the toilet soil pipe and changing the waste pipe connections
This got me wondering if toilets need a P trap or not? All the other connections to the waste pipe from the sink, shower, and bath all have P traps.
After some research and fact-finding here’s what I was able to find out
A porcelain toilet does not require an additional ‘P’ trap to keep the smells and odors from the sewer rising into the room. Modern designs of toilets have a trap built-in which allows waste to be flushed away and clean water then collects in the bowl preventing bad smells from entering the room.
I found the answer to my question, I also found that there is much more to why you don’t need an additional waste trap for a toilet, and fitting one can even cause serious problems.
I have included more details in the following post, I found some great DIY information and I tell you why drains need proper venting and why you should never ever use a ‘P’-trap with a modern toilet.
Why do toilets have a trap?
Modern toilets which are often made from porcelain and fitted to the bathroom of most houses have a bowl that holds water at the bottom and is refilled every time the toilet is flushed.
The part of the toilet behind the water is called the ‘trap’ and the water provides a seal between the air in the room and the air in the waste pipe behind the toilet.
The waste pipe is connected to the sewer and will be exposed to effluent that will give off methane gas. If the water was not present in the bowl of the toilet the sewer would be directly linked to the room and your home, allowing the smelly methane gas from the sewer inside, methane is a combustible gas and can be a health risk if allowed to collect inside for too long.
So the function of the trap in the toilet bowl is to prevent the sewer gas from entering the room where the fixture is installed, this is also the same for any other p-trap, fitted between the waste outlet and the drain on a sink, bath, or shower for example.
Under normal conditions, the pressure of gases in the sewer cannot push past the water and escape the drain, which keeps the gas trapped behind the toilet bowl.
Does a toilet need a ‘P’ trap?
Modern toilets do not require an additional ‘P’ trap to keep the smells and odors from the sewer rising into the room.
Toilets have been developed over several centuries starting as simple holes in the ground to becoming the sophisticated efficient fixtures fitted into most modern dwellings.
Modern designs of toilets have the trap built-in, which allows waste to be flushed away, and clean water then collects in the bowl preventing bad smells from entering the room.
This means that unlike sinks, Shower, or bath wastes, toilets do not need to be fitted with a P-trap as they already have one built-in.
Why you shouldn’t fit a P-trap to a toilet.
Because the design of a modern toilet includes the correct shape to hold water in the bowl when it has been flushed creating a built-in P trap adding another P-trap to the outlet will cause serious problems
When you flush a toilet, which is fitted with a second P trap, an air pocket forms between the toilet and the P-trap. The water from the toilet bowl creates a seal on one side and the water in the P-trap creates a seal on the other.
This means that when you next flush the toilet, the water and waste becomes trapped by the air pocket between the two seals.
And because air can be compressed it will squeeze down rather than push things along which stops the water and waste from moving down the drain.
You probably won’t notice this for a couple of flushes but after a while, the toilet will become blocked and stop working, and every time you clear the toilet it will quickly block back up and so on and so on.
Does a toilet need a vent?
From the previous details about toilet designs and waste connections for Sinks, showers, and baths you can see why they have a trap fitted to prevent nasty sewer gases from entering your home.
But simply closing off all the connections to the sewer and plugging the waste pipes with water will the plumbing will also end up failing.
Waste pipes, including the toilet, need to be vented to provide the correct atmospheric pressure inside the system which allows air to move and prevents vacuums or air pockets from forming in the pipes which will block the pipes.
If a drain system does not have a vent, is poorly vented or the vent becomes blocked, the air trapped in the drain will cause any toilets, sinks, and baths connected to drain slowly, make gurgling noises or the water could siphon out completely, allowing sewer gas to enter the room.
Blocked or poor drain vents can cause damage to the pipe and should be clear as soon as possible.
This means that any fixture either a toilet, sink, shower, or bath with a wastewater drain will need to be properly vented to allow them to work correctly.
Can a toilet, shower, and sink share the same drain?
Most modern bathrooms will have several fixtures that need to be connected to the drainage systems.
A fitted toilet, shower, sink, or bath will often share a drain line and have a single vent stack to reduce the amount of plumbing and pipework that needs to be fitted to reach the main sewer line.
This type of drainage system is called ‘wet venting’. If any of the connections to the drain line or if the vent stack becomes blocked, air can be trapped in the system and be pushed either up or down your toilet, or to one of the other waste connections, this can make loud gurgling noises, and poor drainage ending up in blocked drains.
Correct drain design requires good venting to avoid these issues and is covered by building regulations to ensure that poor venting is avoided.
What is Ghost or Phantom flushing?
Ghost flushing is an issue that can occur with modern toilets which can cause drains to block for different reasons to trapped air and venting.
Ghost flushing which is often called Phantom flushing occurs when water slowly leaks out of the tank over a long period of time.
Leaking of this sort is normally caused when the flapper seal of the toilet siphon becomes ripped or damaged.
The damaged seal allows water to seep past, and down into the toilet bowl, this means the water level in the toilet water tank will constantly need refilling and waste good, clean water.
The reason that the toilet waste becomes blocked in this situation is that the damaged flapper seal does not create enough suction and when the toilet handle is pulled or pressed to flush it the siphonic force is too small to pull all the water from the tank.
A lower flow and reduced volume of water moving from the tank to the toilet bowl means that the waste solids are not fully flushed away, and will be left behind causing the toilet to become blocked.
The best way to fix this is to replace the flap seal, which usually means buying a new toilet siphon assembly and replacing the whole unit, this is not a huge job but it can be awkward as the toilet water tank will need to be disconnected from the toilet base to allow access to the fixings.
This article was written by: Richard Quinton – The DIY Help Desk Owner, Engineer & technical specialist.
Richard is one of the key partners in The DIY Help Desk team. He is a qualified Engineer, writer, and publisher, educated to Master’s level. He is a keen advocate of DIY and home improvements.
Richard enjoys helping others to learn new skills and reach their goals and believes that passing his knowledge and experience on through his writing is an effective way to positively impact the lifestyles and well-being of others on a larger scale.
