Plumbing Trap Basics
Sewage disposal systems produce some pretty nasty odors 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.
**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. So that means every single plumbing fixture used to evacuate waste
from a building should be equipped with it’s own plumbing trap. However there are some plumbing
fixtures that sport traps that integral traps built in to the fixture itself such as a water closet, bidet or
urinal. 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)
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.

Fixture traps must be self cleaning by nature. Meaning that with each use wastes such as lint, hair
must be able to flush away unhindered.

No trap can be used that must depend on moving parts to retain it’s seal.
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No trap outlet can be larger than the fixture drain to which it is connected. So you can’t put an 11/2”
p-trap on a fixture with an 11/4” drain.
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Each fixture traps must have a trap seal of at least 2” but no more than 4” in depth.
You must install a trap level in relation to it’s trap seal. If the trap isn’t level it canbe susceptible to
back siphonage. (Figure #2)
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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 than go
through a second trap. (Figure #3)
Figure #2 P-Trap Level and Not Level
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 you
ask? Because the further the liquid waste drops the greater the velocity of the water. The greater the
velocity of the water rushing through the 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)
Figure #5 Horizontal Distance of Tailpiece Drop
When a water closet is flushed the velocity of waste water 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)
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
Figure #6 Important Part of a P-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 it’s 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 it depth it is less likely to lose it’s 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 seal trap would be ideal and all of the aforementioned advantage would apply here.
Here are a couple of reasons deep seal traps are not the norm for all applications.

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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
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Running Traps
Crown Vented Traps
¾ S Traps
Full S Traps
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Figure #7 Commonly Prohibited 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.
Figure #8 Drum Trap
A drum trap is a circular metal 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
trap 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 the ring or earring by sinking to the bottom, the likely hood that a
valuable would flow into the waste opening at the top of the trap is highly unlikely.
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 seriously risk to
the health and well being of the population especially in heavily populated areas.
Building Traps Come of Age
So to combat the issues above a house or building trap was 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.
Figure #9 Typical Building Trap
Reasons for Broken Plumbing Trap Seals
Trap Siphonage – This is described as 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 the waste water 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
waste water is released, the water rushing through the trap and waste pipe carrying some of the trap
water with it. 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 unvented. If the water vacates a fixture
into the trap in could build up enough velocity to drain the trap.
Back Pressure – So 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 and into the building.
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 within 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. What happens is the water in the trap evaporates when not used 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. Maybe a cleaning person runs out of clean water while mopping a large public toilet room.
They may not what 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 wick up the string and into the waste piping, draining the trap seal.
Figure #10 Capillary Attraction