A P-trap looks like a sideways “P”: a U-shaped bend joined to a straight outlet, with slip-nut joints and a cleanout that keeps a shallow water seal.
If you peek under a sink, that curved loop of pipe with a short horizontal run is the P-trap. The loop holds a small pool of water; the short run heads into the wall or floor. The silhouette resembles a sideways P, which is where the name comes from. You’ll see it under bathroom sinks, kitchen sinks, tubs, and showers.
Beyond the look, small cues separate a true P-trap from random elbows glued together. The fittings use compression or slip-joint nuts, there’s a union where the bend meets the outlet, and many kits include a cleanout plug on the bottom. Those details matter because the trap must keep its water seal while still coming apart for cleaning.
What A P-Trap Looks Like: At A Glance
Here’s a quick map of the pieces you’ll spot on most residential P-traps. Use the shapes to identify what’s what when you’re tracing a drain.
| Part | What It Looks Like | What It Does |
|---|---|---|
| J-bend (U-bend) | A smooth U-shaped curve with a flat bottom and two flared ends | Holds the water seal that blocks sewer gas and catches small debris |
| Trap arm | Straight section leaving the J-bend toward the wall or floor | Carries flow to the drain line or tee that ties into the vented system |
| Slip-joint nuts & washers | Hex nuts at each joint with visible beveled plastic or rubber washers inside | Let you hand-tighten, align, and remove the trap without glue or solder |
| Union (coupling) | Bulky ring where the J-bend meets the outlet | Makes the bend removable for cleaning or replacing |
| Cleanout plug (on some kits) | Small screw plug at the bottom of the U | Gives a fast way to drain the trap or fish out a dropped item |
| Wall tube | Short straight tube disappearing into an escutcheon at the wall | Connects the trap arm to the branch drain |
How The P-Trap Shape Works
The curve keeps a standing pool of water between the inlet and the outlet. That pool is a seal. It stops sewer gas and odors from drifting back through the drain while letting wastewater pass every time you run the tap. Codes call this the trap seal. Most codes specify a seal depth of about 2 to 4 inches; the International Plumbing Code section 1002.4…
Materials And Finishes You’ll See
For exposed sinks, traps are often chrome-plated brass. In cabinets, white PVC or black ABS is common because it’s light, easy to cut, and resists corrosion.
Where P-Traps Show Up
Any fixture that drains into the sanitary system needs a trap, either built in (toilets) or attached to the outlet (sinks, tubs, showers, laundry tubs). The trap sits close to the fixture outlet and ties into a vented branch so the seal doesn’t siphon. You’ll sometimes see a short vertical tailpiece from the sink, then the J-bend, then the trap arm heading into the wall tee.
P-Trap Vs. S-Trap: Spot The Difference
An S-trap dives straight down after the bend and loops again before turning upward, which can siphon the seal dry. A P-trap turns horizontal into a vented branch. That small shift in shape keeps the seal stable. Many jurisdictions ban S-traps outright; One example is the New York City plumbing code, which lists “S” traps on its prohibited list under Section 1002.3.
Fast Visual Cues
- If the outlet runs horizontally into the wall after the bend, you’re looking at a P-trap.
- If the outlet drops straight down toward the floor with no horizontal run and no nearby vent, that’s an S-trap pattern.
- Chrome kits with slim, polished nuts are usually P-traps for exposed lavatories; chunky white or black plastic kits are common under sinks and tubs.
P-Trap Sizes And Where Each Fits
Sink traps are sized to match the drain and the expected flow. Bathroom lavatories often use 1¼-inch kits, while kitchen sinks commonly use 1½-inch kits. Showers and some tubs step up to 2-inch. Local code and fixture listings decide the final call, but those ranges cover most homes.
Rules Of Thumb
- Lavatory sink: 1¼-inch trap and trap arm.
- Kitchen sink and laundry tub: 1½-inch trap; some regions allow a 1½-inch arm tied to a 2-inch branch.
- Shower or floor drain: 2-inch trap in many codes to handle hair and soap load.
How The Trap Arm Sits
The trap arm—the straight section after the bend—runs with a gentle fall so wastewater moves but the seal isn’t pulled out. A common requirement is a ¼-inch drop per foot for small diameters. Keep the arm short and nearly level to the eye; a steep down-angle hints at an S-trap layout.
Second Look: Shape Details That Confirm It’s A P-Trap
Once you know the basics, fine details clinch the ID:
- Symmetrical U-bend: Smooth inner walls and a full U, not a tight S-loop.
- Horizontal outlet: The arm leaves the bend at near-horizontal and heads to a tee, where vented air keeps the seal calm.
- Slip-joints, not glue: Most under-sink kits use compression nuts so you can remove the bend to clear clogs or rescue a ring.
- Optional cleanout: A small plug at the bottom of the U lets you drain the trap without touching the nuts.
Common Mistakes That Change The Look
- Elbows cobbled together into an S shape.
- A trap placed too far from the fixture outlet, leaving a long tailpiece that splashes.
- No vent near the trap arm, which can pull the seal dry and leave gurgling noises.
- Two traps in a row on the same fixture; codes call that double-trapping and don’t allow it.
Table Of Typical Sizes And Materials
Match what you see to this quick reference when you’re standing under a sink or opening a vanity panel.
| Fixture Area | Typical Diameter | Common Material/Notes |
|---|---|---|
| Bathroom lavatory | 1¼ in. | Chrome brass for exposed; PVC or ABS behind doors |
| Kitchen sink | 1½ in. | PVC or ABS; sometimes chrome on pedestal bar sinks |
| Shower or tub | 2 in. | PVC or ABS with solvent-welded trap body and union |
| Laundry sink | 1½ in. | PVC or ABS; deeper basket strainers help catch lint |
| Floor drain (residential) | 2 in. | Often with primer line to keep the seal filled |
Variations You Might Mistake For A P-Trap
Bottle Trap
This style hides the water seal inside a slim cylinder with a side outlet. Designers like it for modern, wall-hung basins. It can be a legal option when the product is listed for the job and the local code accepts the listing. To the eye, it looks more like a tube with a small can near the outlet rather than a classic U.
Drum Trap
Older tubs sometimes have a round canister with a large screw cap. That’s a drum trap. It’s bulky, catches hair, and can be tough to clean. Many regions have moved away from them because the seal can be unreliable and the body can corrode. When you see a round can with a big cap instead of a U, you’re not looking at a modern P-trap.
Low-Profile Tub Trap
Some tub and shower kits use a shallow U with a small union to fit tight joist bays. The shape still forms a P-trap: a U-bend plus a horizontal outlet. The giveaway is the sideways P outline and the single water seal in the bend.
Double Sinks: What The Setup Looks Like
With a double-bowl kitchen sink, you’ll often see a short “continuous waste” tube linking the two baskets and dropping into one P-trap. That’s normal when the two bowls are close together and the trap matches the listing. The one-trap layout keeps the water seal simple and leaves room for a disposer on one side if needed.
If the bowls sit far apart, you might see a longer cross tube with a center tee into one P-trap. Keep the tee near the basket outlets so air moves evenly across both sides and the trap pulls straight.
That layout keeps cleaning work simple.
Installation Clues That Back Up The ID
These layout signals pair with the visual shape so you can confirm what you’re seeing without taking anything apart.
- Close to the fixture: The trap sits just below the outlet so splashing is minimal.
- Short, level-looking arm: The arm has a gentle fall toward the wall, not a steep drop.
- Vented connection: The arm meets a tee that also carries a vent; that vent stabilizes the seal.
- Water seal depth: The U looks deep enough to hold water, usually in that 2–4 inch range set by codes.
Care And Quick Fixes
Most trap clogs happen right in the bend where hair and grit slow down. Keep a small bucket and a towel under the cabinet, loosen the two slip-nuts, and tip the J-bend into the bucket. Rinse it, reseat the washers, and snug the nuts by hand. If a trap has a bottom cleanout, back that plug out first to drain it with less mess.
If you spot an S-shape or a trap that gurgles and dries out, it’s time to call a licensed plumber to rework the outlet with a proper P-trap and vent. That swap protects indoor air and lines up with the code section linked earlier.
Recap: What You Should See
A sideways-“P” outline, a smooth U-bend, a horizontal arm, hand-tightened nuts, and often a cleanout. If your setup matches that picture and the sizes above, you’ve found the P-trap.