A drip under the sink that only appears when the tap is running is almost always a slip joint. Slip joints are the large plastic or metal nuts that hold the trap together, and they seal with a simple tapered washer. When that washer is worn, missing, flipped or misaligned, water finds its way out. The good news is that this is one of the cheapest and fastest leaks to fix in a house.
Why slip nuts leak
The nut does not seal by itself. It squeezes a tapered washer into the gap between two pipes. Leaks happen when:
- The washer has hardened or cracked with age
- It was installed backwards
- The pipes are out of line, so the washer only seals on one side
- A plastic nut has been overtightened and cracked
- Someone added thread tape, which stops the nut seating
Find the exact joint first
Dry every joint with a paper towel, put a dry towel under the trap and run water. Water runs downhill along the pipe, so the lowest wet spot is often not the source. Feel above it with dry fingers or tissue.
| Where it drips | Likely cause | Fix |
|---|---|---|
| Tailpiece to trap | Washer flipped or worn | New washer, bevel toward the joint |
| Trap to trap arm | Pipes misaligned | Loosen, realign, retighten |
| Down the nut itself | Cracked plastic nut | Replace the nut or trap kit |
| At the wall | Worn wall washer or adapter | Replace washer or adapter |
| Basin strainer | Putty or gasket failure | Reset strainer |
Step by step fix
- Put a bucket under the trap and undo both nuts by hand. Plastic nuts should not need tools.
- Pull the washers and look at them. The flat face goes toward the nut and the bevel faces into the joint.
- Wipe the pipe ends clean and fit new washers. An assorted pack costs a few dollars.
- Line the pipes up so they slide together without bending anything.
- Hand tighten, then add about a quarter turn with pliers on metal nuts only.
- Fill the basin, pull the plug and watch every joint as the full volume runs through.
If the trap meets the wall at an awkward angle, see connecting a P-trap to a wall drain.
Things that make it worse
Do not use thread tape or pipe dope on slip joints. They seal on the washer, not the threads. Do not crank plastic nuts with pliers. And do not force a misaligned trap together; adjust the tailpiece or trap arm length instead. If the trap sits lower than the wall outlet, read P-trap lower than drain pipe.
When to replace the whole trap
If nuts are cracked, threads are stripped or a chrome trap is corroded inside, replace the kit. A PVC trap kit usually costs under $15 and takes ten minutes. Chrome versions cost more but look better under a pedestal sink.
Understanding the parts of a sink trap
It helps to know what you are looking at before you take it apart. Starting at the basin, the tailpiece drops straight down from the strainer or pop-up drain. It joins the J-bend, the curved section that holds water. The J-bend then joins the trap arm, which runs to the wall or floor and connects to the drain through a wall adapter or trap adapter. Each of those joints is a slip joint with its own nut and washer, so a typical sink trap has three or four places where a leak can start.
Bathroom traps are usually 1 1/4 inch and kitchen traps 1 1/2 inch. Mixing sizes is common, and it is done with a reducing washer, which is thicker and stepped. Fitting a standard washer where a reducing one belongs is a frequent cause of drips that never quite stop no matter how hard you tighten. Our guide to P-trap washer types shows the differences.
Before you buy parts, take a photo of the assembly and measure the outside diameter of the tailpiece and trap arm. Bringing the old washer to the store also saves a second trip.
Safety note
Kitchen traps may contain hot water or drain cleaner residue. Wear gloves and eye protection if anyone has poured chemicals down recently, and never mix cleaners.
Frequently asked questions
Should I use plumber's tape on a P-trap?
No. Slip joints seal with the washer, and tape can stop the nut seating properly.
How tight should a slip nut be?
Hand tight plus about a quarter turn. Plastic nuts usually need no tools.
Why does my trap only leak with a full sink?
A full basin pushes more water through at once, which exposes weak washers.
Can I reuse old washers?
If they are soft and undamaged, yes, but new ones are cheap.
My trap still drips after new washers. What now?
Check alignment first. If the trap arm enters the wall at a slight angle, the washer is squeezed unevenly and will weep no matter how new it is. Loosen the wall nut, adjust the trap arm length or swivel the J-bend until the pipes meet squarely, then retighten. If the pipe ends are scratched or out of round, a new trap kit is the quickest cure.