Everything You Need to Know About P-Traps

With a history that goes back to 1775, the P-trap is an essential fixture in modern plumbing, especially after improvements by the legendary Thomas Crapper in the late 1800s. You’ll find this curvy pipe—which after Crapper’s modification, is shaped like an inverted “P” instead of an “S”—connected to every sink, tub and shower drain in your house, as well as other drains.

So why is it there? A plumbing trap holds just enough water in its P-shaped bend to create an airtight seal that prevents sewer gas (and vermin) from entering your home. Besides being noxious, sewer gas is flammable, so a P-trap serves both sanitary and safety purposes. Read on to learn about the traps in your home.

P-Trap Sizes and Materials

P-trap pipes are available in different materials, but the assemblies are similar. Your P-trap can be polypropylene (white or black), ABS (black), PVC (white) or 18-gauge brass (chrome-plated or natural).

Plastic traps come in inside-diameter sizes of 1-1/4 inches for a standard bathroom sink; 1-1/2 inches for a laundry sink, bathtub or standard kitchen sink; and two inches for a shower or floor drain.

Handling Drain Clogs in the P-Trap

P-traps can clog, but not all sink clogs occur in the trap. If your bathroom sink drains slowly, it may be because of hair around the stopper or below it, and you can usually remove this with a long wire hook. However, if the clog is deeper than that, it’s probably inside the P-trap. The same design that traps water can also trap hair, food and diamond earrings.

Identifying P-trap clogs

Justin Cornforth, who runs a full-service plumbing and drain company, has this advice: “If you notice musty odors or drain flies (fruit flies, drain flies, even cockroaches) congregating around a drain, the clog is likely just past the trap, its organic buildup attracts pests.

A clogged P‑trap often causes gurgling or suction sensations in nearby fixtures (like the toilet or shower) from air movement through a sluggish drain, pointing to trap resistance.”

To clear a clog, try a plunger after first blocking the sink overflow and other drain openings. Avoid caustic drain cleaners that can damage fixtures and pipes. A plumbing snake can clear the trap, but won’t retrieve that diamond earring.

If you’re lucky, your sink’s trap has a cleanout nut located on the lowest portion of the bend. You can remove that and clear many clogs. Grab an empty bucket, place it under the nut, then remove the nut and let the contents drain out. If your trap lacks a cleanout nut, you can easily disassemble the trap and find that earring. You might not even need tools.

How to Remove and Reassemble a P-Trap

The P-trap connects the tailpiece at the sink end and the drain line (also called the waste line) at the opposite end. Those connections use slip nuts and beveled cone washers. An older home with metal P-traps might use squared and thick rubber washers with accompanying flat metal washers. These fittings can stick, and if that happens, Cornforth recommends using a little cooking oil or WD-40 to loosen them. Either way, if you’re taking apart your P-trap to remove a clog, pay close attention to the placement of the washers and reassemble them the same way.

• Pro tip: If you’re tackling this job because the P-trap is leaking, it’s best to buy a new PVC P-trap. It will last longer than metal and costs only about $10. If your plumbing is visible — underneath a pedestal sink, for example — you can purchase decorative P-traps.

Here are the steps:

1. Grab some rags and place an empty bucket below the trap.
2. Slowly loosen the nut connecting the P-trap on the outflow side. Pay attention to the placement of the washers inside and be ready for water to come out. If your P-trap was installed properly, you should be able to remove it by hand. If not, a set of tongue-and-groove pliers will give you a good grip on slip nuts.
3. You might find the clog in the trap or the union joint. Clean those areas with water and a brush and replace the slip nut washers if the connection shows any sign of leakage — and I hope you found the diamond earring.
4. Start reassembling the trap by replacing the drain line end, then reconnect the tailpiece to the sink. Keep any slip nut connections loose for now.
5. Connect the sink side of the P-trap to the tailpiece and position the drain line end to align with the drainpipe. That drainpipe slides into the wall with some adjustability for length, easing the final connection to the P-trap.
6. Place each beveled washer so the large end contacts the slip nut.
7. When tightened, a slip nut eases the washer into a leak-free connection.

Pro tip: Do not use plumbing tape or putty on any of these connections. Some plumbers use plumber’s grease on threaded connections, but these are compression fittings, so that’s unnecessary. Make sure to tighten all the connections, but don’t overtighten them, or you risk distorting the washer or cracking the pipe.

Check for leaks

Once all the fittings are secured, run two tests. First, turn on the tap and run water through the sink while inspecting the P-trap connections for leaks. Second, fill the sink basin with a couple of inches of water, then remove the drain plug and let the water flow through the trap.

If you have a leak, loosen the slip nut where the leak appears, inspect the placement of the washers, then reinstall. Most leaks happen because a washer is installed incorrectly or a slip nut is threaded improperly or overtightened.

Simple P-Trap Option

Some modern P-trap systems use push-connect fittings, designed to make installation easier. These fittings connect the P-trap to the sink and the drain without the need to align a cone washer or properly thread and tighten a nut.

Inside each fitting, an O-ring and a locking ring create the seal. A release collar at the top of each fitting secures the connection. To undo a fitting, simply compress the release collar and disconnect the pipes — no tools required.

Push-connect P-trap assemblies in 1-1/4-in. and 1-1/2-in. sizes. They are especially useful in small cabinets with hard-to-reach pipes, making installation and service work easier. These systems are approved in most plumbing codes, but always check local requirements. They are intended for use with drainpipes, not pressurized or potable water pipes.

FAQ

What is a dry P-trap?

A dry P-trap is one with no water. The most common cause is suction created in the drain pipes by a blocked vent. You can remedy this situation temporarily by pouring water into the drain to refill the trap, but eventually you will have to unblock the vent.

Can I replace a P-trap myself?

It’s totally doable, says Cornforth. Just be sure to match sizes (either 1-1/4 in. or 1-1/2 in.). “When reconnecting slip nuts, finger-tight plus a 1/4 turn with pliers is enough; over-tightening distorts washers and causes leaks. Testing after installation is a must. Additionally, if you doubt your abilities, you can always choose push-fit options that make replacement tool-free and nearly foolproof. They are ideal for tight cabinet spaces or beginner DIYers.”

About the Expert

• Justin Cornforth is the owner and CEO at Ace Home Co., which provides electrical, plumbing and HVAC services.