How to Fix a Leaky Pipe Joint

Dry the suspect area completely with a paper towel, then watch for 60 seconds with a flashlight to confirm exactly which joint is weeping — water travels along pipes and the visible drip is often inches from the source. There are three common joint types: threaded (metal male threads screwed into a female fitting, sealed with tape or dope), compression (a brass nut squeezes a ferrule onto the pipe — common on shutoff valves and supply lines), and slip-joint (a plastic nut and washer on a P-trap). Each is fixed differently, so confirm the type before grabbing tools.

For a sink or toilet supply line, turn the angle stop valve (the small oval handle on the wall behind the fixture) fully clockwise until it stops. For drain or trap leaks no shutoff is needed, but stop running water at the fixture. Open the faucet to release residual pressure and let the line drain into a bucket. Place a shallow pan or folded towels directly under the joint to catch the water still trapped in the trap or the section you are about to open.

For threaded metal joints, hold the fitting body steady with one wrench and turn the nut or pipe counterclockwise with a second wrench — using two wrenches prevents twisting the rest of the pipe run. For compression fittings, hold the valve body with one wrench and back off the compression nut with the second. For slip-joint plastic nuts on a P-trap, you can usually loosen them by hand or with slip-joint pliers — no wrench needed and never with metal-on-plastic force. Catch any standing water in your bucket as you break the seal.

Look closely at the male threads with a flashlight. You are looking for galling (silvery torn metal), cross-threading (threads going at the wrong angle), or a hairline crack in the female fitting. On compression fittings, inspect the brass ferrule — if it is deformed, cracked, or has cut into the pipe, replace it with a matching new one. On slip-joint connections, check the beveled nylon or rubber washer for flattening, cracks, or stiffness. Replace any worn part now; sealant cannot save a damaged ferrule or torn washer.

Use a wire brush or stiff old toothbrush to scrub off the remnants of the old tape, dope, mineral scale, and any debris from both the male and female threads. For stubborn mineral deposits, soak a rag in white vinegar and wrap it around the threads for 10 minutes, then scrub again. Wipe everything dry with a clean rag — new sealant adheres far better to clean, dry threads than to a fitting with old sealant or moisture on it.

Point the open end of the male thread toward you and wrap PTFE tape clockwise around the threads three to five times, stretching the tape lightly so the thread profile shows through. Start one thread back from the very end of the pipe — wrapping over the lip can shed bits of tape into the water line. For pipe dope, brush a thin even coat over all the male threads with the applicator brush, just enough to fill the thread valleys. Use only one or the other on a typical residential joint, not both.

Bring the two parts together by hand only and start the threads with a slow counterclockwise turn first — when you feel a slight click, the threads have aligned. Now turn clockwise and thread the fitting in by hand as far as it will go (typically three to four full turns on a 3/8-inch supply line). If the threads bind or feel gritty before they are fully engaged, back it off and re-check for cross-threading. The pipe must enter the fitting perfectly straight, with no side load.

Hold the fitting body steady with one wrench and turn the nut or pipe clockwise with the second wrench. Tighten an additional one half to one full turn past hand-tight — that is all. The single biggest mistake DIYers make is overtightening, which cracks brass fittings, crushes ferrules, or splits plastic nuts. If the joint still drips after a half turn, stop, disassemble, and re-tape rather than cranking harder.