What Are Plumbing Traps And The Proper P-Trap Configuration?

You likely know P-traps serve an important function and are a necessity when it comes to your home’s plumbing. But have you stopped to think why these traps are so essential? Furthermore, why does there seem to be so many different kinds of plumbing traps?

Many plumbing traps are no longer used or very limited, like bell traps, running traps, and drum traps, among others. Today, P-traps are the most common plumbing trap, typically used in bathrooms, kitchens, and laundry rooms. Plumbing codes place strict restrictions on the use of P-traps, including outlet size, maximum vertical drop, and self-cleaning qualities.

A plumbing trap is a critical component of plumbing systems, keeping things flowing (literally) and keeping you safe. They serve many functions, including preventing deep clogs and keeping unpleasant odors, insects, and sewer gases out of your home.

Many of these gases can reach a point where they become dangerous. For example, sewage systems create things like carbon monoxide, methane, hydrogen sulfide, and nitrogen. If these gases back up into your home, it can cause hazardous consequences.

These gases can build up and cause severe health problems like headaches, nausea, memory loss, vomiting, and worse. In extremely serious cases, if left unchecked or unaddressed, they can even be fatal.

Every time you use a sink, toilet, or other such fixture, a plumbing trap retains a small amount of the produced liquid. This liquid is called a trap seal. Specifically, a trap seal is the maximum vertical depth of liquid that the trap can hold.

P-traps are the most common type of seal. A P-trap forms a barrier between the plumbing fixture, like a toilet or sink, and the sewage waste system. When installed correctly, the trap effectively blocks the gases.

A properly installed P-trap also helps catch debris that goes down the drain. This can be things like hair, dirt, and other items that would generally cause significant problems for your plumbing. Basically, the seal keeps the debris from traveling farther into the system and clogging the pipes.

Basically, a P-trap consists of three parts: the inlet, the downward bend, and the outlet. Wastewater enters through the inlet, and water fills the downward bend, forming an airtight seal. Lastly, wastewater exits through the outlet and heads to the public sewer plant.

Typically, residential homes feature P-traps consisting of PVC or steel pipes. However, they can also be copper, cast iron, brass, and some feature recycled materials.

Mostly, you find P-traps in kitchens, laundry rooms, and bathrooms, including appliances like dishwashers and washing machines. However, you need a trap wherever a plumbing fixture connects to the sanitary drainage system.

You can have one P-trap for up to three fixtures, such as a two or three-compartment sink or three lavatories. The drains, or waste outlets, must be in the same room and 30 inches apart or less. Also, flood level rims must be at the same level. The flood level rim is the point where when you fill the sink, the water spills over the edge.

Typically, when using one P-trap for multiple fixtures, pipes would come down from each waste outlet. Then, a pipe would cross to the adjoining unit, meeting together at the single p-trap. If it’s a three-compartment sink, pipes from the outside units will meet under the central unit.

The vertical drop is the length of the pipe that comes from the fixture outlet, or drain, to the water seal. The shorter the vertical drop, the more efficient the trap. This is because the longer the drop, the greater the water’s velocity, making it more likely to compromise the seal.

Vertical drops for toilets, sinks, showers, bathtubs, etc., are usually between 18 and 24 inches. Some may be 15 inches or perhaps less; the International Residential Code (IRC) does not stipulate a minimum vertical distance. However, the maximum allowable vertical drop is 24 inches, including all pieces of the pipes, fittings, etc.

P-traps should be self-cleaning, which is pretty much exactly how it sounds. The P-trap essentially cleans itself, so if you were to take it apart, the inside would be nice and clear.

In other words, it is made of a smooth material that makes it difficult for water particles to build up. Additionally, the P-trap is self-scouring, pulling dirt and debris through without clogging the trap.

Self-cleaning traps typically feature pipes made from brass, copper, or cast iron. But, this doesn’t mean you can’t have a P-trap made of other materials, like PVC. However, just know these are usually not self-cleaning. In these situations, you would likely need to clean the P-trap manually every now and then to avoid clogging. Luckily, this is a relatively simple process.

After reassembling the trap, run the faucet for roughly 30 seconds to check for leaks. If you notice any, ensure the washers are correctly set and tighten the couplings. Finally, rerun the water and recheck for leaks.

Of course, if you’re not comfortable with DIY plumbing tasks, you can always call a pro. A plumber might charge anywhere from $50 to $150 or more, depending on the severity of the issue and the trap’s location. For example, a shower trap will typically cost more than a trap under the bathroom sink.

However, it’s definitely a job worth learning how to do yourself. Especially since the typical recommendation is to clean a P-trap three to four times a year. If you have more than six people in your household, it’s preferable to clean often-used traps monthly.

Next, slowly start to pull the trap arm out of the wall opening. Keep pulling it until you can line it up with the top part of the trap. (The U-shaped portion.)

You might need to make some adjustments in this step, as well, depending on the length of the trap arm. If the arm comes out of the wall entirely before you can attach it to the trap, it’s too short. You will need to purchase a longer pipe for the trap arm.

Alternatively, if the trap arm goes into the wall fully and still extends past the trap, grab the hacksaw. Cut the trap arm so that you can adjust it to line up with the U-shaped portion of the trap.

Once you get the trap arm to align with the trap, thread on the slip nut (but don’t tighten it). After joining all of the trap parts, you can tighten the slip nuts. If something is wrong with your existing P-trap and you don’t have much plumbing know-how, call a pro first. A professional can assess precisely what is wrong with your trap and determine what your specific needs are.

Possibly, there might be more going on beyond the P-trap, and trying to fix it yourself could cause more damage. If a plumber replaces or installs the P-trap, expect to pay between $200 and $300 or possibly more.

Related Questions

What is an S-trap, and when is it used?

An S-trap is a plumbing trap found in many older homes. Named for its “S” shape, it is basically a normal trap with another trap installed right against it, upside-down. The Uniform Plumbing Code (UPC) prohibits S-traps because they siphon water from the trap and allow dangerous gases into homes. Therefore, if you are doing new construction or remodeling and find an S-trap, you must replace it. If not, you would violate the plumbing code. 

What is a building trap?

Nowadays, building traps are not used, and in the majority of cases are considered a code violation. However, builders once used them as a sort of secondary line of defense against vermin, sewer gases, and odors. These traps were usually a larger diameter S-trap, placed on top of a house. People basically used them before understanding the importance of venting plumbing fixtures.

What is a deep seal trap?

A deep seal trap is a plumbing trap that features a trap depth of more than four inches. Although permissible, you won’t find them in many applications, although they do have several advantages. Because the trap holds more liquid, water in it doesn’t evaporate as quickly. Therefore, it is often suitable for locations that are remote and used infrequently. Also, it can typically handle a greater water flow, making it less likely to lose its seal. This quality makes a deep seal trap a good choice for an indirect waste connection. For example, fixtures like a salad bar or commercial ice maker may use a deep seal trap.  Furthermore, they are also less prone to issues like back-pressure or siphonage. If a fixture can’t be adequately vented, a deep seal trap can be a good solution. However, deep seal traps are larger and, therefore, more challenging to fit in small spaces. They can also cause fixtures to drain slowly due to their increased depth and capacity. Therefore, they are not often used in residential applications and are certainly not the norm.