Why is my sink P-trap leaking?
Updated August 2026
The drip in the cabinet under your sink is usually loose slip nuts at the trap joints, which back off over time and let water weep from the connections. Hand-snug them first, since that fixes most P-trap leaks for free. A worn slip washer is next, then a cracked plastic trap or a corroded metal one that weeps mid-span. Dry the trap, run water, and watch for the first bead so you tighten the right joint instead of guessing your way around the trap.
Safety first: a slow trap leak soaks the cabinet base and grows mold behind the panel, so dry the cabinet fully, set a paper towel under the trap to catch a recurring drip, and clear anything stored below before the particleboard swells and the smell sets in.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Loose slip nuts at the joints | Most common | Hand-tighten the plastic nuts a quarter to half turn, snug but not cranked. | Free |
| Worn slip washer or gasket | Common | Replace the beveled nylon or rubber washer inside the joint, taper toward the connection. | $3–8 |
| Cracked plastic trap | Occasional | A hairline split leaks under flow; swap the trap for a new PVC one. | $8–20 |
| Corroded metal trap | Occasional | Old chrome-over-brass traps rot through at the bend; replace with PVC. | $10–25 |
| Misaligned or cross-threaded joint | Occasional | Re-seat the washer square and re-thread the nut by hand so it does not cross-thread. | Free |
| Clog raising back-pressure | Rare | Debris in the trap backs water up and forces it past a tired washer; clean the trap. | $0–20 |
Check these first
- Dry the trap and the cabinet floor, then run the tap and watch each joint for the first bead of water.
- Feel the slip nuts by hand; if one turns easily, it has backed off and is likely your leak.
- Snug loose slip nuts a quarter to half turn by hand, then test again before reaching for tools.
- Check for a cracked trap bend or a corroded metal trap that weeps mid-span, not at a joint.
- Fill and drain the basin fully; a leak that shows only under a full bowl points to a marginal joint.
Prevent it next time
A P-trap leak is usually a slow-loosening joint, so the prevention is a periodic check and clean. When you clean the trap to clear the gunk that slows the drain, re-seat the washers square and hand-snug the slip nuts, following our how to clean a sink P-trap steps. Knowing what a P-trap is makes reassembly obvious, and staying ahead of clogs avoids the back-pressure that pushes water past a tired washer, a case our cost of ignoring slow drains makes plainly.
When to call a pro
Call a plumber when the leak is above the trap, at the tailpiece from the sink or the drain arm that disappears into the wall, or when a metal trap is so corroded it crumbles as you loosen it. Those runs are harder to seal and sometimes need re-piping, roughly $100–250 for a visit. Tightening slip nuts or swapping a $5 washer or a $15 PVC trap is squarely DIY and takes about 15 minutes with a bucket underneath.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
Why does my P-trap only leak when I run the sink?
Because the trap only fills with moving water when you run the tap or drain a full basin. A joint that looks dry and tight when idle can weep the moment water flows past a loose slip nut or a worn washer. Put a dry paper towel under each joint, run water, and the first wet spot names the leak. It is almost always a connection you can hand-tighten, not a crack, so start there before replacing anything.
Can I hand-tighten P-trap slip nuts, or do I need tools?
Hand-tight is usually the target for plastic slip nuts, plus maybe a quarter turn with slip-joint pliers if it still weeps. Cranking them with tools cracks the plastic or distorts the beveled washer, which causes the very leak you are trying to fix. Loosen the nut, make sure the nylon or rubber washer sits square with its taper toward the joint, then snug it by hand. Test with running water before adding any tool pressure at all.
Should I put plumber's tape on P-trap slip nuts?
No. Slip-joint connections seal with a beveled washer compressed by the nut, not with threads, so Teflon tape does nothing useful there and can actually keep the washer from seating. Reserve thread tape for tapered threaded pipe joints. For a leaking trap, replace the slip washer, seat it square, and hand-tighten. If a joint still leaks after a fresh washer and a proper snug, the trap or tailpiece is likely cracked and should be swapped out.
Why does my metal P-trap keep leaking after I tighten it?
Old chrome-over-brass traps corrode from the inside and thin out, so the metal itself weeps or the threads no longer hold a seal no matter how hard you tighten. Over-tightening only splits it further. The lasting fix is to replace it with a PVC trap kit for $10–25, which will not corrode and seals with a simple washer. Match the pipe diameter, usually 1-1/4 or 1-1/2 inch, and hand-snug the new slip nuts rather than wrenching them.