Most P-trap leaks come down to one small part: the slip joint washer. These thin rings get squeezed between the slip nut and the pipe, and when one is the wrong shape, faces the wrong way or has hardened with age, you get a slow drip that never seems to stop. There are only a handful of washer types, and once you know which goes where, fixing a leaky trap gets a lot easier.
What a trap washer does
A slip joint lets one pipe slide inside another so you can adjust the length and angle of the trap. The joint is sealed by a washer that the slip nut pushes into the gap between the two pipes. The washer does not need sealant or tape. Its shape does all the work, which is why using the right one in the right direction matters so much.
Common washer types compared
| Washer type | Shape | Where it goes | Typical cost |
|---|---|---|---|
| Beveled (tapered) slip joint washer | One flat side, one tapered side | Most trap joints, tapered side toward the pipe it seals into | About $1 to $3 each, $5 to $10 for a kit |
| Flat washer | Even thickness both sides | Some tailpiece to strainer joints and threaded connections | About $1 to $3 |
| Reducing washer | Beveled, sized to step down | Joining a 1 1/4 in tailpiece to a 1 1/2 in trap | About $1 to $3 |
| Polyethylene (plastic) washer | Firm white or clear plastic | Plastic traps, resists drain chemicals | Usually included with kits |
| Rubber washer | Soft black rubber | Metal traps, older fittings | About $1 to $3 |
| Flanged tailpiece washer | Flat with a lip | Top of the tailpiece at the strainer | Usually included with strainers |
Washer sizes match the pipe: 1 1/4 inch for most bathroom sinks and 1 1/2 inch for kitchen sinks and tubs. If you are not sure which size your sink uses, our guide on P-trap size for a bathroom sink shows how to measure.
Which way does a beveled washer face?
This is where most leaks start. The slip nut goes on first, then the washer, with the flat side facing the nut and the tapered side pointing toward the fitting the pipe slides into. When you tighten the nut, it presses the taper into the mouth of the fitting and the washer seals the gap.
Put the washer on backward and the flat face meets the fitting. It may seal for a day, then start to weep as the joint warms and cools.
How to replace a trap washer step by step
- Put a bucket under the trap and loosen the slip nut on the leaking joint by hand or with channel lock pliers.
- Slide the nut back and pull out the old washer. Note which way it faced.
- Clean the pipe end and the fitting mouth with a rag so no grit is trapped in the seal.
- Slip the nut onto the pipe, then the new washer, flat side toward the nut and taper toward the fitting.
- Push the pipe into the fitting so it sits straight, with no sideways strain.
- Thread the nut on by hand. If it binds, back off and restart, since cross threading is a common cause of leaks.
- Tighten by hand until snug, then add about a quarter turn with pliers for plastic nuts, a little more for metal.
- Run water hard for a minute and wipe the joint with a dry tissue to check.
If the joint still drips, our full guide on a P-trap leaking at the slip nut covers other causes like a cracked nut or a misaligned pipe.
Plastic or rubber?
Use whatever the trap came with. Plastic traps are designed for firm polyethylene washers, which hold their shape and stand up to drain cleaners. Rubber washers suit metal traps, where the softer material grips the thin brass. Mixing them is usually fine for a quick fix, but rubber washers in a plastic trap can squeeze out of shape if overtightened. If you are refitting a trap after cleaning it, our guide on how to clean a P-trap without removing it may save you the trouble next time.
Safety and code notes
Slip joints must stay accessible under most codes, which is why they are used under sinks and never inside walls or under floors. Do not add pipe dope, putty or tape to a slip joint washer, since it can stop the washer seating and some compounds attack plastic. Overtightening plastic nuts cracks them, so go by feel. Wear gloves, as trap water carries bacteria, and swap any washer that is hard, flattened or split rather than trying to reuse it.
Frequently asked questions
Which way does a P-trap washer go?
The flat side faces the slip nut and the tapered side points toward the fitting the pipe slides into.
Do P-trap washers need plumber's tape?
No. Slip joint washers seal on their own. Tape or dope can actually stop them seating properly.
What is a reducing washer for?
It joins a smaller pipe to a larger fitting, most often a 1 1/4 inch tailpiece into a 1 1/2 inch trap.
Can I reuse an old trap washer?
It is better not to. Old washers harden and flatten, and a new one costs only a dollar or two.
Rubber or plastic washers, which is better?
Use plastic washers on plastic traps and rubber on metal traps, matching what the trap was designed for.