Slip Joint — Drain Fitting Sizes, Parts & Repair Tips

A slip joint is a compression fitting used in drain, waste, and vent plumbing that allows pipes to slide together and seal with a slip nut and neoprene or rubber washer instead of a permanent glued or threaded connection.

Slip Joint diagram — labeled parts and installation context

What It Is

A slip joint connects drain pipes at sink traps, tailpieces, and trap arms using a large hand-tightened nut and a soft washer that compresses around the pipe as the nut is tightened. The joint earns its name because the pipe can slide axially inside the fitting before the nut is locked down, making it easy to adjust length and alignment during assembly. Once snug, the washer creates a watertight seal without adhesive or thread sealant.

Slip joints are intentionally designed for accessibility. They appear throughout the drain assembly under kitchen and bathroom sinks precisely because they can be disassembled quickly for clearing clogs, adjusting height, or replacing worn washers. They are not rated for supply-side pressure and are restricted by code to accessible drain locations — they cannot be buried inside walls or enclosed cabinetry with no service access.

Types

The two standard slip-joint sizes are 1-1/4 inch for bathroom sink drain assemblies and 1-1/2 inch for kitchen sinks and most other fixtures. A third size, 2 inch, appears occasionally in laundry and floor drain connections. Slip-joint nuts and washers are sold in plastic (polypropylene), PVC, and chrome-plated brass. Brass fittings are more durable and hold torque better; plastic versions are lighter and resist corrosion but can crack if overtightened. Beveled washers seat against tapered socket ends, while flat washers are used with straight-cut slip nuts.

Where It Is Used

Slip joints connect the basket strainer tailpiece to the P-trap inlet, the P-trap outlet to the trap arm, and extension tubes when more length is needed. They are standard under every kitchen and bathroom sink drain assembly. Some dishwasher drain connections and laundry standpipe assemblies also use slip joints. They are never used on supply lines, gas lines, or any pressurized pipe.

How to Identify One

A slip joint looks like a large plastic or chrome nut threaded onto a smooth pipe with no pipe threads visible on the pipe itself. The nut can often be turned by hand or with light plier pressure. If you see a ribbed or knurled ring where two drain pipes meet under the sink, it is almost certainly a slip-joint nut. Leaking at the joint after years of service usually means the washer inside has hardened and compressed flat.

Replacement

Replacing a slip-joint washer costs almost nothing and takes a few minutes. Unscrew the slip-joint nut by hand or with slip-joint pliers, slide out the old washer, and press in a new one oriented with the taper facing the direction of the socket. If the plastic nut is cracked or the threads are stripped, replace the entire slip-joint nut and washer together. Match the pipe diameter (1-1/4 or 1-1/2 inch) when buying parts. No primer, glue, or plumber's tape is needed for slip-joint repairs.

Slip Joint — FAQ

Can a slip joint be used inside a wall?

No. Plumbing codes restrict slip joints to accessible locations only, meaning the joint must be reachable without cutting into a wall or ceiling. This is because the soft washer can harden and weep over time, and an inaccessible leak inside a wall causes serious damage before it is detected. If a drain connection must pass through a wall, a glued or permanent fitting is required.

Why does my slip joint keep leaking even after I tighten the nut?

Overtightening is the most common cause of persistent slip-joint leaks because it distorts or extrudes the washer out of position instead of sealing it. The washer may also be hardened, cracked, or the wrong profile for the fitting. Remove the nut, inspect the washer, and replace it if it feels stiff or shows any deformation. Hand-tight plus a quarter turn with pliers is usually sufficient.

What size slip-joint nut do I need for a kitchen sink?

Kitchen sinks almost always use 1-1/2 inch slip joints. Bathroom sinks typically use 1-1/4 inch at the tailpiece and may step up to 1-1/2 inch at the trap arm. Measure the outside diameter of the pipe where the nut sits, or bring the old nut to the hardware store for a direct match.

Is a slip joint the same as a compression fitting?

They work on a similar principle but serve different systems. A compression fitting on a supply line uses a brass ferrule that permanently deforms around copper or plastic tubing under high pressure. A slip-joint fitting is for low-pressure drain lines, uses a soft rubber or neoprene washer, and is designed to be disassembled and reassembled multiple times. They are not interchangeable.

How long do slip-joint washers last?

Most slip-joint washers last ten to twenty years under normal household drain use, but rubber washers exposed to harsh drain cleaners or very hot water can harden and fail sooner. If the drain under the sink has not been disturbed in many years and begins to drip, the washer is usually the first thing to replace before assuming a more serious problem.