Plumbing Traps: Everything You Need To Know

plumbing traps

Plumbers always seem to be talking about plumbing traps. If you or someone you know is a plumber, you’d know exactly what we’re talking about. It seems as though grease traps, p-traps, s-traps, drum traps etc. are present in almost every discussion among professionals. You may hear about them often but do you actually know what a plumbing trap does? Has a plumber ever stopped to tell you? Let us educate you so that it’s no longer a mystery.

 We’ll run through what a plumbing trap actually does, the different types of plumbing traps, when people use them and which traps are outdated and obsolete. Hopefully by the end of this page, you’ll have a wealth of knowledge. If you’re one to attempt any kind of DIY plumbing, you’ll want to ensure that you have a high level of precision.

Plumbing Trap Definition 

A plumbing trap is defined as a device that keeps a rather small amount of liquid whenever the fixture is used. The amount of liquid that is retained is called a trap seal. This prevents sewage system odours, gases, mice and insects from entering your home or work space. 

Trap seal is the maximum depth of liquid that a trap will hold, measured from the crown weir and the top of the trap dip. The most commonly used traps of them all is the p-trap. You can find this in kitchen sinks, bathrooms and laundry sinks.

leaking toilet cistern

Plumbing Trap Basics 

Not only do sewage disposal systems produce some very nasty odours, they can even come to a point where they are considered dangerous. To keep us protected, a barrier is placed between the plumbing fixture and the sewage waste system. Sewage disposal systems start in the production phase of the shower, bathtub or toilet. It then enters the disposal state, sending the sediment waste through a series of inline steps. This is to help make sure that the waste doesn’t overflow. 

A sewage system produces numerous gases, these include; 

  • Methane 
  • Hydrogen Sulifde 
  • Nitrogen 
  • Carbon Monoxide 

Any plumbing fixture directly connected to the sanitary drainage system must be equipped with a water seal trap. This means that every single plumbing fixture that is used to evacuate waste from a building must have its own plumbing trap.

Deep Seal P-Traps 

Deep seal p-traps have vertical depths of at least 10cm. These are only useful in a limited number of applications but they do have some advantages specific to their application.

  • A deeper trap means a deeper seal and that means more liquid and therefore takes longer to evaporate. This is useful when installing a floor drain in a remote location like a big warehouse. 
  • The deep seal trap has a greater capacity for resealing. Because it is quite larger than a normal trap, it can handle a greater flow of liquid. Therefore, the trap is less likely to lose its seal because of the extra amount of water. This is useful when a fixture calls for an indirect waste connection, such as an ice maker or a salad bar. 
  • Because of its depth, the deep seal trap is less likely to lose its seal due to the back pressure or trap siphonage. There are times where a fixture or trap may not be able to be properly vented. If this is the case, a deep primer seal trap would be ideal.


building a toilet trap

Deep Seal Traps Don’t Always Work 

If you’re working in a tight and confined space, deep seal traps won’t be viable as they are deeper and larger. Because they are deeper and have an increased capacity, they are more resistant to flow from a normal fixture. In turn, they will impede the drainage and cause the fixture to drain sluggishly. 

Building Trap 

Back before people really used or even understood system venting, mice and insects were free to move from building to building. Apart from the vermin, you would also have sewer gas odour which would be unbearable because of the back pressure and trap siphonage. This was obviously a serious health risk. To combat the issue, a building trap is now required in every property and provides a secondary line of defence against mice and sewer gas.

drain cleaning services

Trap Siphonage 

Trap siphonage is a low negative pressure within the plumbing fixtures drain. Picture it as a large amount of wastewater hitting the waste stack at any one time. As the wastewater goes past the other fixtures that are connect to the stack, it can pull water from the respective trap seals. 

Back Pressure 

There are times when water can be blown out of the trap into the fixture and in turn the water will enter the inside of the building. This can occur when a large amount of waste flows into the drainage system. The water will compress the air in front of it. If the fixture at the point of compression has no proper ventilation, it will blow out of the trap. 

Evaporation This is a fairly common occurrence, especially where a fixture or drain has sporadic activity. If the fixture/trap isn’t in use at least once a week, the water in the trap can evaporate. Hopefully you’re all the wiser after reading all of the above and can kind of understand how your system works and you won’t be absolutely clueless when professionals are talking about traps in your presence. If you have any questions or need any advice, don’t hesitate to contact us and we’ll do our best to assist you. Phone us on 1300 329 238.

Fawcett Group Team Photo