What is a P Trap & How Does it Work?

In the world of plumbing, the unassuming P trap plays a critical role in maintaining the cleanliness and comfort of our living spaces. 

Often hidden from view, it silently ensures that our sinks, bathtubs, and other fixtures remain free from foul odours and potentially harmful sewer gases. 

In this article, we will delve into the fascinating world of P traps, exploring what they are, how they function, and the benefits they bring to our plumbing systems. 

We will also shed light on their limitations, alternative options, and the reasons why they are favoured over their older counterpart, the S trap.

By the end of this article, you will have a comprehensive understanding of the humble yet indispensable P trap and its vital role in maintaining the cleanliness, hygiene, and safety of our plumbing systems. 

What is a P Trap?

The P trap, a cleverly designed U-shaped pipe, is a humble yet essential component of plumbing systems. 

Its primary purpose is to act as a barricade against the intrusion of foul-smelling and potentially hazardous sewer gases into our living or working spaces. 

The beauty of the P trap lies in its simplicity – a strategic bend in the pipe that forms a water seal, preventing these unpleasant smells from passing through.

How Does a P Trap Work?

P trap works by creating a water seal that prevents foul-smelling sewer gases from entering the living or working space. 

When wastewater flows down a drain, it enters the vertical section of the P trap and continues through the pipe system. 

As the water moves downward, a small amount of water collects in the bend of the U-shaped trap, creating a water seal that acts as a physical barrier. 

This water seal effectively blocks sewage gases from travelling back up the drain and escaping into the indoor environment.

The principle behind the P trap is simple but effective. 

The water in the trap serves as an impenetrable obstacle for sewer gases, ensuring they remain confined to the sewer system. 

By doing so, it helps maintain indoor air quality and protects occupants from potential health hazards associated with inhaling sewer gases.

Where Does a P Trap Need to be Fitted?

A P trap needs to be fitted at the exit point of various plumbing fixtures to create a water seal. 

Here are some common locations where P traps are typically installed:

Under Sinks

P traps are commonly found under kitchen and bathroom sinks. They are connected to the drain pipe to trap water and prevent sewer gases from entering the sink area.

Bathtubs & Showers

P traps are installed below bathtubs and showers to block sewer gases from entering the bathroom.

Washbasins

Washbasins, whether in bathrooms or utility areas, require P traps to prevent sewer odours from being released into the room.

Washing Machines

P traps may be fitted to the drain lines of washing machines to prevent sewer gases from escaping through the machine and into the laundry area.

Dishwashers

Similar to washing machines, dishwashers may also have P traps to keep bad smells out of the kitchen.

Floor Drains

Some buildings have floor drains in specific areas, such as laundry rooms or basements. P traps are essential in these drains to maintain indoor air quality.

Utility Sinks

In utility areas like laundry rooms or garages, where utility sinks are installed, P traps are used to keep the area odour-free.

What Are the Benefits of P traps?

P traps offer several key benefits, making them an essential component of plumbing systems:

Odour Prevention

One of the primary advantages of P traps is their ability to effectively block unpleasant sewer odours from entering your living space.

The U-shaped design of the trap creates a water seal that acts as a barrier, preventing foul-smelling sewer gases from travelling up through the drain and into your home or workplace.

This water seal remains in place as long as there is water in the trap, maintaining an smell-free environment.

Hygiene

P traps play a crucial role in maintaining a sanitary environment.

By blocking sewer gases, they also prevent bacteria, insects, and other harmful organisms from coming up through the drain.

This helps to ensure a clean and hygienic living or working space, protecting occupants from potential health hazards.

Safety

P traps serve as a safety feature in plumbing systems.

In addition to blocking sewer odours, they also prevent potential gas leaks.

Sewer gases, which include methane and hydrogen sulphide, can be harmful or even hazardous to human health if they accumulate indoors.

By creating a water seal, P traps effectively prevent these gases from escaping through the drain and entering your living or working areas, ensuring the safety of occupants.

Easy Maintenance

P traps are relatively easy to maintain.

Periodically adding water to the trap, especially in infrequently used drains, ensures that the water seal remains intact.

Regular maintenance prevents the water in the trap from evaporating, ensuring continuous odour prevention and safety.

Versatility

P traps are versatile and can be used with various plumbing fixtures, including sinks, bathtubs, showers, washbasins, and floor drains.

Their adaptability allows them to be installed in different locations throughout a building, providing consistent odour prevention and safety.

Cost-Effective

P traps are cost-effective solutions to prevent sewer smells and maintain a hygienic environment.

Their simple design and ease of installation make them a practical choice for plumbing systems in both residential and commercial settings.

What Are the Downsides of P Traps?

While P traps offer various advantages, they also come with a few downsides that should be considered:

Clogging

One of the main downsides of P traps is the potential for debris accumulation, which can lead to clogs over time.

As water flows through the trap, it can carry small particles and debris, such as hair, soap scum, food particles, and other materials.

If not cleaned regularly, these substances can build up and create blockages, causing slow drainage or even complete clogs in the plumbing system.

Drying Out

Another drawback of P traps is their susceptibility to drying out.

If a sink or fixture is not used for an extended period, the water in the P trap can evaporate, breaking the water seal.

When the seal is broken, sewer gases can travel freely through the drain and enter the living or working space, causing foul odours.

This issue is more common in infrequently used drains, such as in guest bathrooms or unused sinks.

Limited Space

In some situations, the available space for plumbing installations can be limited, making it challenging to accommodate a traditional P trap.

The U-shaped design of the P trap may require more vertical space under the fixture, which can be a constraint in certain plumbing configurations.

Maintenance

While P traps are relatively easy to maintain, some homeowners or occupants may find it inconvenient to keep track of periodic checks and cleaning to prevent clogs and drying out.

Neglecting maintenance can lead to unpleasant smells and potentially costly plumbing issues.

Freezing in Cold Climates

In regions with cold climates, water in P traps can freeze during the winter months, leading to blockages and potential pipe damage.

When water freezes, it expands and can crack or burst pipes, causing water leaks and other plumbing problems.

Trap Siphonage

In certain scenarios, a sudden change in water pressure within the plumbing system can result in trap siphonage.

This can cause the water seal in the P trap to be pulled out, allowing sewer gases to enter the living space.

To prevent trap siphonage, plumbing systems may require additional measures, such as the installation of air admittance valves or other ventilation devices.

What Are the Alternatives to P traps?

When considering alternatives to P traps, two other common options are S traps and bottle traps:

S Traps

S traps are an older design that was once commonly used in plumbing systems. 

However, they have become less prevalent in modern construction due to certain shortcomings, with many countries, such as the USA outright banning their usage.

S traps form a shape resembling the letter “S,” and they function similarly to P traps in creating a water seal to prevent sewer gases from entering the living space. 

The main difference is that S traps have a vertical outlet that connects directly to the drainage pipe, while P traps have a horizontal outlet.

Bottle Traps

Bottle traps are another alternative to traditional P traps, particularly suitable for small spaces or contemporary designs. 

As the name suggests, bottle traps have a shape resembling a bottle, with a visible curved section below the fixture that holds water to create the seal. 

The curved section is often transparent or made of clear material, which adds to its appeal in modern aesthetics.

Why Are P Traps Better Than S Traps?

P traps are considered better than S traps for several reasons, mainly due to their ability to maintain a reliable water seal and prevent drain odours. 

Here’s a more detailed explanation of why P traps are preferred over S traps:

Improved Seal Integrity

The U-shaped design of P traps is specifically engineered to create a water seal that effectively blocks sewer gases from entering the living or working space. 

This U-shaped configuration holds a small amount of water in the bend, which acts as a barrier against foul-smelling gases. 

As a result, P traps are less likely to lose their water seal and allow odours to escape.

Reduced Risk of Trap Siphonage 

One of the primary drawbacks of S traps is their susceptibility to trap siphonage, where a sudden change in water pressure causes the water in the trap to be syphoned out. 

This occurs when a large volume of water flows through the vertical outlet of the S trap, breaking the water seal. 

Once the seal is compromised, sewer gases can easily enter the room, causing unpleasant odours and potential health hazards. 

In contrast, the horizontal outlet of P traps reduces the risk of trap siphonage, as water flow is less likely to create the syphoning effect.

Compliance with Building Codes

In many regions, building codes and regulations have evolved to favour the use of P traps over S traps, with the USA making them illegal.. 

This is due to the concerns about the syphoning effect and potential health hazards associated with S traps losing their water seal. 

As a result, newer plumbing installations typically follow the P trap design to ensure compliance with safety standards.

Lower Risk of Water Damage

The potential for S traps to lose their water seal not only causes odour issues but can also lead to water damage. 

When the water seal is broken, water can leak out of the trap and cause damage to surrounding areas, such as cabinets or walls. 

P traps, with their more secure water seal, mitigate the risk of such water leakage.

Improved Plumbing Performance

P traps offer better plumbing performance compared to S traps, especially in preventing clogs and maintaining the integrity of the drainage system. 

With a consistent water seal, P traps effectively trap debris and prevent it from causing blockages in the pipes.

Conclusion

P traps are an essential plumbing component that helps maintain a hygienic and odour-free living space. 

They work by creating a water seal to prevent sewer gases from entering your home or workplace. 

While they have some downsides, their benefits in terms of hygiene, safety, and odour prevention make them the preferred choice over S traps. 

When properly installed and maintained, P traps play a vital role in keeping your plumbing system functioning effectively and your living environment comfortable and safe.