Trap Plumbing or P Traps: What Every Homeowner Should Know

Plumbing systems are built to make your life easier, cleaner, and safer—but there’s one small, curved piece of pipe doing a bigger job than most people realize. The Trap (plumbing), also known as the P trap, is one of the most important parts of your home’s drainage system.

At DrainRooter Plumbing, we see clogged, misaligned, or poorly installed traps every single week. In fact, traps are one of the most common sources of bad odors, slow drains, leaks, and service calls.

This guide is designed for homeowners in North America who want to understand their plumbing system better, prevent problems, and know when to call a professional. Let’s break it all down.

What Is a Trap in Plumbing?

A Trap (plumbing) is a curved section of pipe found beneath sinks, bathtubs, showers, laundry drains, and other fixtures. It’s shaped like the letter “P” on its side, which is why it’s called a P trap.

The trap holds a small amount of water at all times. This isn’t a mistake or a design flaw—it’s intentional. That water acts as a seal that keeps sewer gases, pests, and unpleasant smells from rising up through your drain and into your living space.

Think of it as a one-way gate: wastewater flows down, but sewer gases can’t come back up.

Unfortunately, traps are also one of the most commonly misinstalled components by inexperienced contractors. If the slope is wrong or the venting isn’t correct, your trap can siphon itself dry or clog easily.

Do All Drains Need a Trap?

Yes. By modern North American plumbing code, every drain requires a trap—no exceptions.

Why? Because without a trap, your plumbing would act like a direct vent from the sewer line into your home. That means:

• Nasty odors
• Dangerous sewer gases like methane
• Easier access for pests like drain flies or roaches
• Higher risk of bacteria entering your space

Even floor drains in basements, garages, and laundry rooms need traps. If you ever smell something foul coming from a little-used drain, it’s often because the water in the trap evaporated and the seal was lost.

Trap vs. Drain: What’s the Difference?

A lot of homeowners confuse these terms, but they’re not the same.

• Drain: The straight line that carries wastewater from your fixture into the plumbing system. Think of it like a highway where everything flows out of your house.
• Trap: The curved pipe sitting right below your fixture. It’s the “gatekeeper” that prevents odors and gases from entering.

Without a trap, your drain is incomplete. Without a properly sloped drain, your trap won’t work as intended. They’re a team, but they serve different roles.

Why S-Traps Are Illegal in Modern Plumbing

If you’ve ever seen an S-trap, it looks similar to a P trap, but instead of turning toward the wall, it drops straight down into the floor like the letter “S.”

Here’s the problem: S-traps can siphon themselves dry. Every time you run water, the flow pulls the water seal out of the trap, leaving nothing to block sewer gases.

This is why building codes across North America no longer allow S-traps. They were common in older homes, and if you have one, it’s worth replacing with a P trap. A properly vented Trap (plumbing) ensures that water remains in the bend, maintaining that critical barrier at all times.

Quick tip: If your bathroom or kitchen has a mysterious sewer smell and your home was built before the 1980s, there’s a good chance you might still have S-traps. It’s worth getting a professional inspection.

What a Drain Trap Looks Like

A drain trap is easy to spot under most sinks. You’ll see a short, curved piece of pipe connecting the tailpiece (from the sink) to the wall drain.

There are typically three visible components:

1. Tailpiece – the vertical pipe from the sink drain.
2. P trap bend – the curved section that holds water.
3. Trap arm – the horizontal section that leads to the wall and connects to the vented drain line.

Most modern P traps are made of PVC, but you may also find metal versions (chrome, brass, or cast iron) in older installations. PVC is lightweight, affordable, and easier to maintain.

Common Contractor Mistakes with P Traps

Even though P traps are simple, they’re often installed incorrectly. Some of the most frequent mistakes we see at DrainRooter Plumbing include:

• Improper slope: The trap arm must have the correct downward slope. Too flat and water won’t flow. Too steep and it will siphon the trap dry.
• Wrong height: If the trap is too far below the fixture, the seal may not hold properly.
• Double trapping: Installing two traps on one line can cause poor drainage and venting problems.
• Unvented traps: If a trap isn’t connected to proper venting, it can’t maintain the water seal reliably.
• Using S-traps in areas where they’re no longer permitted.

Small errors during installation can lead to big headaches later on: constant clogging, gurgling, or unpleasant odors.

How Long Does Water Stay in a Trap?

A Trap (plumbing) isn’t static. The water in the bend evaporates slowly over time—especially in unused fixtures.

In a normal indoor environment, it can take anywhere from a few days to a few weeks for the water to fully evaporate. That’s why you’ll often notice a musty or sewer-like smell coming from a basement floor drain, spare bathroom, or laundry sink that hasn’t been used in a while.

A quick fix is simple: just run water for a few seconds to refill the trap. For rarely used drains, some homeowners pour a few drops of mineral oil on top of the water in the trap to slow evaporation.

How to Clear a Clogged Drain Past the Trap

Clogs love P traps. That curve is where hair, soap scum, grease, and debris settle over time.

Here’s a safe homeowner method to clear a clogged drain past the trap:

1. Place a bucket under the trap to catch water.
2. Unscrew the slip nuts carefully and remove the P trap.
3. Clean out any visible debris inside the trap.
4. If the clog is deeper than the trap, use a drain snake or auger to reach further down the line.
5. Reinstall the trap, making sure the seals are tight.
6. Run water to confirm it’s flowing smoothly.

Avoid dumping harsh chemicals down your drain. While they might clear a blockage temporarily, they can also corrode pipes and harm the trap over time.

If the clog is stubborn or recurring, that’s a sign of a larger issue like a misaligned trap or a partially blocked main line. That’s when calling a professional plumber makes sense.

How Often Should You Replace a P Trap?

P traps are durable, but they don’t last forever. Here’s a general guideline:

• PVC traps: 10–15 years or more, depending on use and water quality.
• Metal traps: often 5–10 years due to corrosion and wear.

You should consider replacement if:

• There’s visible rust or corrosion.
• It’s leaking around the seals.
• It clogs frequently despite proper cleaning.
• You still have an outdated S-trap.

Replacing a P trap is a relatively straightforward job, but proper installation and venting are key to long-term performance.

Why S-Traps Are No Longer Used in Plumbing

S-traps were once standard, especially in older homes built before modern venting codes. The main problem is siphonage: when water flows down too quickly, it creates a vacuum and pulls the remaining water out of the trap.

Once the water seal is gone, there’s nothing stopping sewer gas from entering your home. This is both unhealthy and unsafe.

P traps, on the other hand, work with proper venting to maintain that water barrier. They’re code-compliant and far more reliable.

If you discover S-traps in your home, it’s a smart investment to have them replaced with P traps. It’s not just about convenience—it’s about health and safety.

Why Contractors Often Get P Traps Wrong

This might surprise you, but Trap (plumbing) issues often come from rushed or outdated installations. Some contractors use shortcuts or install traps without proper venting because “that’s how it’s always been done.”

Common errors we see:

• Installing traps without ensuring they’re vented properly.
• Using fittings or dimensions that don’t meet code.
• Forgetting to check the slope of the trap arm.
• Reusing old, rusted S-traps instead of replacing them.

These mistakes can lead to recurring clogs, odors, and health concerns—and homeowners are the ones who end up dealing with the mess.

Trap Plumbing and Home Maintenance

Here’s the truth: traps aren’t just a “contractor thing.” Every homeowner should know where their traps are and how to maintain them. It’s a simple way to prevent bigger problems down the road.

Here are a few easy habits to keep your trap plumbing in top shape:

• Run water regularly through all drains, even rarely used ones.
• Use strainers in sinks to catch hair, food, and debris.
• Avoid pouring grease or harsh chemicals down drains.
• Visually inspect traps a few times a year for leaks or rust.
• Learn how to remove and clean a P trap safely.

A little knowledge can save you hundreds of dollars in service calls.

Bonus Tip: Evaporation Prevention for Seasonal Homes

If you own a vacation home, cottage, or rental property that sits empty for long periods, your traps can dry out completely. That can lead to sewer smells when you return.

Here’s what you can do:

• Pour a small amount of mineral oil into each trap before leaving.
• The oil floats on top of the water, slowing evaporation dramatically.
• When you return, just run the water for a few seconds to flush it out.

This simple trick can make a huge difference in seasonal maintenance.

When to Call a Professional

Not every trap issue needs a plumber—but some do. Call a professional like DrainRooter Plumbing if:

• The trap leaks even after tightening.
• You can’t clear a clog past the trap.
• There’s a persistent sewer odor.
• You suspect you have an illegal S-trap.
• The trap is corroded, cracked, or poorly installed.

A professional inspection can prevent structural water damage, mold growth, or ongoing plumbing headaches.

Final Thoughts

The Trap (plumbing) or P trap may be small, but it’s one of the most important pieces of your home’s drainage system. It protects you from odors, pests, harmful gases, and messy backups.

Understanding how it works, why S-traps are outdated, and how to maintain your trap plumbing gives you real control over your home’s health and comfort.

You don’t need to be a plumber to know the basics. Just knowing where your traps are, how they work, and what red flags to watch for can save you from expensive repairs down the line.

If your traps are giving you trouble—or if you suspect your plumbing isn’t up to modern code—our licensed technicians at DrainRooter Plumbing are ready to help.

📞 Call us today to book an inspection or service and keep your home’s plumbing working the way it should.

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