A slip-joint washer is the ring-shaped seal inside a removable tubular drain connection, commonly found under a sink. A slip nut holds the assembly together and presses the washer against the matching surfaces. The washer seals around the tube; the nut's threads supply the force. Washer shape, material and size must match that particular connection.
A slip-joint washer is the ring-shaped seal inside a removable tubular drain connection, commonly found under a sink. A slip nut holds the assembly together and presses the washer against the matching surfaces. The washer seals around the tube; the nut's threads supply the force. Washer shape, material and size must match that particular connection.
The washer is a separate part of the joint
In a typical tubular connection, a smooth tube slides into a fitting. The nut and washer fit around the tube, and tightening the nut brings the washer into its seat. This arrangement lets compatible drain pieces be assembled without gluing every connection.
That description does not make every under-sink joint a slip joint. A threaded tailpiece, a gasketed strainer and a solvent-welded drain fitting can use different sealing methods. Identify the actual connection before applying a washer rule to it. Our kitchen sink installation guide shows where slip joints appear in an assembled sink drain.
Tapered and square-cut washers are not identical
Many plastic tubular connections use tapered washers, but taper is not a universal feature. Oatey's tubular product guidance describes its True Blue tapered TPE washers, rigid polyethylene washers supplied with specific products, and square-cut rubber washers supplied with its brass tubular products. Those are manufacturer-specific examples of why appearance and material matter. If an identified rubber-like washer has a hardness specification, elastomer durometer explains that separate property.
A reducing washer also has a particular job: it adapts a specified smaller tube to a larger compatible connection. It does not make any two pipe sizes interchangeable. Charlotte Pipe's DWV catalog lists trap adapters with specified washer combinations, including different washers for different tubular sizes.
Choose from the fitting's parts list or instructions rather than assuming an old washer's color identifies a universal replacement. A flattened, damaged or incorrectly sized ring cannot be corrected simply by forcing the nut tighter. The correct orientation also depends on the washer and its seat; a tapered-washer instruction should not be transferred to a square-cut washer. The compression set entry explains a laboratory property of an identified elastomer; the appearance of an old washer does not identify that test result.
What to check before choosing a replacement
Record the tubular size, whether the drain assembly is plastic or brass, and any maker or model marking. Photograph the assembled connection and keep its original parts available for identification. Ask whether the replacement is the specified washer for that nut, fitting and tube.
Thread tape does not replace the sealing ring in a slip joint. Other threaded connections nearby may have different instructions, as our thread-sealant guide explains. Oatey also states that its polypropylene tubular products are assembled with slip nuts and washers rather than PVC or ABS solvent cement.
If the fitting is cracked, the tube is corroded or the drain must be forced sideways to meet, have the assembly assessed instead of treating the washer as the only possible problem. Stop using a leaking fixture until the repair can be checked.