The joints in a sink's P-trap seal with slip joint washers — beveled (cone-shaped) rings that a slip nut compresses into the joint as it tightens. A drain that drips at a nut after you've tightened it needs a new washer, not more muscle. Here's how slip joint washers work, standard sizes, and flat vs beveled substitution.
How a slip joint seals
Slide the slip nut onto the pipe, then the washer with its beveled face pointing away from the nut (toward the fitting it enters). As the nut threads on, it drives the cone into the tapered mouth of the mating fitting, wedging rubber against both pipe and fitting. Hand-tight plus a quarter turn is enough — over-tightening splits plastic nuts and extrudes the washer.
Standard slip joint washer sizes
| Trap size | Where used |
|---|---|
| 1-1/4" | Bathroom sink drains |
| 1-1/2" | Kitchen sinks, laundry tubs, tub drains |
The size refers to the pipe OD the washer slides over (its ID). Measure the old washer's ID, OD and thickness with our measuring guide if it's non-standard — older and imported traps vary.
Beveled vs flat washers in drain joints
Beveled washers are correct where a nut drives into a tapered fitting mouth — the geometry does the sealing. A flat rubber washer is correct at square-shouldered joints, like where a basket strainer or tub shoe clamps against a flat face. If a flat washer replaced a missing cone washer in a tapered joint, it will seep no matter the torque — the profiles must match the joint.
Best material for drain washers
Drain service is intermittent warm, soapy, sometimes greasy water. EPDM and neoprene both serve well; neoprene's moderate grease tolerance gives it a slight edge under kitchen sinks, while EPDM lasts longest against hot water and cleaning chemicals. Softer natural rubber conforms best to worn chrome traps. More context in the plumbing washer guide.
Troubleshooting a dripping trap
Drips after washer replacement usually mean: washer installed backwards (bevel toward the nut), a cracked slip nut, a scored or out-of-round pipe end, or mismatched 1-1/4" → 1-1/2" parts without a reducing washer. Reducing slip joint washers step a 1-1/4" tailpiece into a 1-1/2" trap — check both diameters before buying.
FAQ
Which way does a slip joint washer go?
Beveled/tapered side away from the nut, pointing into the fitting. The nut's job is to push the cone into the taper.
Do slip joint washers need plumber's putty or tape?
No — the compression of the cone washer is the seal. Thread tape on slip nut threads does nothing (the threads aren't the sealing surface) and putty belongs under strainer flanges, not in slip joints.
Can I use a flat rubber washer in a slip joint?
Only at square-shouldered connections. Tapered slip joints need the beveled profile; a flat washer will weep there.