UPDATED: July 8, 2017

Plumbers always talk about them, it seems grease traps, p-traps, s-traps, drum traps etc. come up in the discussion of most plumbing repairs, right? How many times has a plumbing tech stopped to explain what a plumbing trap actually does? Are the installation, products, and supplies a secret of the trade? Well it certainly shouldn’t be and the rest of this piece will explain what a plumbing trap actually does, what types of plumbing traps are available, in what situations they are used and which traps are no longer used, so you can become a plumbing trap master too. When doing anything plumbing related you want to make sure you have a high level of precision. Don’t fear, we can help build your plumber profile! Let me bottle up some tips and tricks for you.

Plumbing Traps Basics

Sewage disposal systems produce some pretty nasty odors (like the odor of pee) and some can even be dangerous, in order to protect us from those dangers a barrier is placed between the plumbing fixture and sewage waste system. Sewage disposal systems start in the production phase, in the shower, bathtub, or toilet. Then it enters the disposal stage, sending the sediment waste through a series of inline steps. This helps ensure waste doesn’t overflow.

**FAST FACT***Common Gases that Are Produced in a Sewage System

  • Methane
  • Hydrogen Sulfide
  • Nitrogen
  • Carbon Monoxide

Having said that, any plumbing fixture directly connected to the sanitary drainage system must be equipped with a water seal trap. That means every single plumbing fixture used to evacuate waste from a building should be equipped with its own plumbing trap. So by definition, a plumbing trap is a device that keeps a small amount of liquid every time the fixtures is used. The amount of retained liquid is called a trap seal. This trap seal prevents sewage system odors, gases and vermin (mice, insects, etc.) from entering the living or work space.

The most common of all plumbing traps is the p-trap it is used on kitchen sinks, lavatories, and laundry sinks. (Figure #1)

plumbing traps

Figure #1 Tubular P-Trap

Most Plumbing Codes place restrictions on how a p-trap is used and manufactured. Here are some of those limitations, restrictions, and clarifications.

  • No trap can be used that must depend on moving parts to retain its seal.
  • No trap outlet can be larger than the fixture drain to which it is connected. The sizes of the trap outlet matter. Don’t be flexible with this. So you can’t put an 1 1/2 inch p-trap on a fixture with an 11/4 inch drain.
  • Fixture traps must be self-cleaning by nature. Meaning that with each use wastes, such as lint or hair must be able to flush away unhindered.
  • plumbing traps

    Figure #2 P-Trap Level and Not Level

    You must install a trap level in relation to its trap seal. If the trap isn’t level it can be susceptible to back siphonage. Check out the diagram in Figure #2.

  • As we’ve said before every fixture must be trapped separately except for fixtures with integral traps i.e. water closets, urinals, and bidets.
  • No plumbing fixture can be double trapped. This is best illustrated using a water closet as an example. Put simply, waste cannot discharge through the integral trap of a water closet then go through a second trap. (Figure #3)
plumbing traps

Figure #3 Double Trapped Water Closet

Most codes allow double bowl kitchen sinks, triple compartment sinks, and laundry sinks to be furnished with one p-trap connected by a continuous waste. The compartments must be next to one another and each compartment cannot be spaced more than 30” inches apart center to center of the drains. (Figure #4)

Figure #4 Two & Three Compartment Sink Spacing

There is usually a maximum allowable vertical drop from the fixture outlet to the water seal. Why is that so? Because the further the liquid waste drops the greater the velocity of the water. The greater the velocity of the water rushing through the plumb fixture drain the more likely the drain will siphon the trap seal. The vertical drop for sinks, lavatories, showers, and bathtubs is between 18” and 24” but not more than 24”. The shorter the vertical distance the more efficient the trap functions. (Figure #5)

plumbing traps

Figure #5 Horizontal Distance of Tailpiece Drop

When a water closet has flushed the velocity of wastewater completely removes the trap seal. The trap seal would remain compromised if the trap seal wasn’t restored by the refill tube on a tank type toilet.

Concealed traps used for bathtubs, showers etc. cannot have integral cleanouts.

Here are the Different Notable Parts of a P-Trap (Figure #6)

plumbing traps

Figure #6 Important Part of a P-Trap

Trap Seal – Is the maximum vertical depth of liquid that a trap will retain measured from the crown weir and the top of the dip of the trap.

Explaining Deep Seal P-Traps

Deep seal traps have vertical depths of 4″ or more and are used in a limited number of applications however there are some advantages to using deep seal traps. These advantages are specific to their respective application.

ADVANTAGE: A deep trap has a deeper seal with more liquid, therefore the seal doesn’t evaporate as quickly.

APPLICATION: When a floor drain is installed in a remote location i.e. a large warehouse and receives very little water to reseal the trap a deep seal trap is recommended.

ADVANTAGE: the deep seal trap has a much greater capacity for resealing. Because the trap is usually quite a bit larger than a traditional trap it can handle a much greater flow of water and the trap is less likely to lose its seal because of the extra amount of water.

APPLICATION: When a fixture or piece of equipment calls for an indirect waste connection, like a commercial ice maker, salad bar or in some cases a triple pot sink a deep seal trap is used.

ADVANTAGE: Because of its depth it’s less likely to lose its seal because of back pressure or trap siphonage.

APPLICATION: There are instances where a fixture or trap cannot be properly vented. In cases such as these, a deep primer seal trap would be ideal and all of the aforementioned advantages would apply here.

Here are a couple of reasons deep seal traps are not the norm for all applications.

  • The traps are deeper, therefore, larger and tougher to fit into confined spaces.
  • Because of their increased depth and capacity, they are more resistant to flow from a standard fixture, therefore, they impede drainage and make the fixture drain sluggishly.

What are the Types of Plumbing Traps that are Prohibited by Most Plumbing Codes?

  • Bell Traps

    plumbing traps

    Figure #7 Commonly Prohibited Traps

  • Running Traps
  • Crown Vented Traps
  • ¾ S Traps
  • Full S Traps
  • Drum Traps- I think drum traps need to be explained a bit because they aren’t exactly prohibited they’re just not used in new construction anymore. Drum traps were used when installing bathtubs and lavatories. A trap adapter is used in residential and commercial drain, waste and vents systems.
  • plumbing traps

    Figure #8 Drum Trap

    A drum trap is a circular metal barrel canister with the inlet near the bottom of the trap and the waste outlet is at the top with a removable cover. Ideally, the drum trap is installed in a place with easy access so you can remove the cover to clean. Unfortunately many drum traps we’ve encountered are buried behind a wall or the cover is unable to be removed, fused together by time. So when drum traps stop they are extremely difficult to clear by rodding because of their inlet and outlet configuration. Drum traps were installed so people could find jewelry or valuables if lost down the drain. The trap would catch and guard the ring or earring by sinking to the bottom, the likelihood that a valuable would flow into the waste opening at the top of the trap is highly unlikely. A sand trap helps also separate sand, and oxidized organics found in water supplies.


What is a Building Trap?

Back in the day before system venting was used or understood vermin like rats or insects could move freely from building to building, house to house. Not to mention the sewer gas odor could be unbearable because of back pressure and trap siphonage. Health officials knew that this could be a serious risk to the health and well-being of the population, especially in heavily populated wet and dry areas.

Building Traps Come of Age

So to combat the issues above a house, a building trap is required in each building. The building trap provided a secondary line of defense against the vermin and sewer gas. Most times the building trap was a large diameter S-trap. Today most codes do not require a building trap if fact in most cases they are against code.

plumbing traps

Figure #9 Typical Building Trap

Reasons for Broken Plumbing Trap Seals

Trap Siphonage – This is described as a low negative pressure within the fixture drain. However, it’s most easily understood by describing it like this, picture a large amount of waste water hitting a waste stack at one time. As the wastewater goes by the other fixtures connected to the stack it can pull the water from their respective trap seals.

Another scenario the could cause trap siphonage is a fixture using an S-trap. If the fixture is full and the wastewater is released, the water will rush through the trap, with the waste pipe carrying some of the trap water. What happens is not enough liquid is left to form an adequate trap seal. A similar scenario can occur if a fixture is installed on a long run of piping that is not vented. If the water vacates a vent fixture into the trap in could build up enough velocity to drain the trap.

Back Pressure – In the above we’ve described instances when a trap is compromised by being pulled out of the trap. However, there are instances when water can be blown out of the trap, into the fixture and subsequently into the building. This is caused when a large amount of waste flows into the drainage system compressing the air in front of it, if the fixture at the point of compression is not vented properly it will blow out the trap.

Wind Effect – Have you ever been in your bathroom on a windy day and seen the water level in the toilet bob up and down? This same action can compromise the traps in your home. The pressure or suction caused by the strong winds can cause the water to rise and fall into the trap. If the fluctuations are big enough a small amount of trap seal may spill into the waste system causing the trap to be compromised. As you can imagine the seal is more susceptible to both back pressure and trap siphonage.

Evaporation – This is a very common occurrence especially in instances where a fixture or drain is only used sporadically. The water in the trap then evaporates when not in use, at least once a week. Laundry room floor drains, remote floor drains, fixtures and all traps in a summer home are all more vulnerable and evaporation will occur.

Capillary Action – This doesn’t happen often but if there is a constant issue with a fixture loosing it’s trap seal this is something you can rule out. I think an example will serve this concept better than a lengthy explanation.

plumbing traps

Figure #10 Capillary Attraction

Maybe a cleaning person runs out of the clean water while mopping a large public toilet room. They may not want to fill up the bucket again, so they flush one of the water closets a few times to make sure the water is clean. They rise the mop off in the toilet and one of the strands of the mop dislodges and gets stuck inside the trap seal but stretches to the fixture branch and into the waste system. The water from the trap will pick up the string and into the waste piping, draining the trap seal.

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