Which Joint to Check First on a Leaking Supply Line: Fitting vs Washer vs Valve Decision Tree

Why this matters

A wet spot under a sink or behind a toilet sends customers into "the whole thing is leaking" mode, but a supply line has only three or four places it can actually weep, and they fail for different reasons. Chasing the wrong joint means you tighten a fitting that was fine, crack a compression nut that was holding, or replace a hose when the real leak is the stop valve packing nut behind it. Water tracks along surfaces and drips from the lowest point, so the puddle almost never sits under the actual source. Working the joints in the right order, top of the run to bottom, lets you find the real weep in one dry-then-wet pass instead of guessing and re-wetting your own work.

Symptom presentation

What the puddle and the customer's words usually mean:

• "Water under the sink, can't tell from where": the leak is upstream of the puddle. Dry everything, lay paper towel, and let it reappear.
• "Drips when I run the faucet, stops when off": the leak is downstream of the stop valve, on the supply riser or at the faucet shank nut.
• "Wet even with the faucet off": the leak is at or upstream of the stop valve, or at the valve stem/packing, which holds pressure constantly.
• "Started after I shut the water off and back on for another repair": the stop valve packing nut loosened or the valve seat moved. Classic.
• "Toilet tank line sweats": condensation, not a leak, on a cold supply in a humid room. Confirm before you chase a fitting.

Quick checks

1. Dry the entire supply path with a rag: stop valve body, packing nut, valve outlet, supply line nuts at both ends, and the fixture shank.
2. Lay a single layer of paper towel or a dry tissue at each joint. Water shows up as a defined wet ring exactly at the source.
3. Run the fixture, then shut it. Watch which towels wet, and how fast.
4. Touch each joint with a dry fingertip and slide upward. The highest joint that comes away wet is the source; everything below is just runoff.

Isolation tree

Work from the wall/floor outward to the fixture.

1. Is the joint wet with the fixture off and the supply on?

   • Yes -> the leak holds under static pressure. Suspect the stop valve packing nut, the valve-to-stub fitting, or the valve outlet nut. Snug the packing nut gently (a quarter turn) and recheck. Go to step 2.
   • No, only wet when running -> the leak is downstream of the valve. Go to step 3.

2. Packing-nut snug did not stop it?

   • Wet at the threaded connection where the valve meets the copper/PEX stub: compression ferrule or sweat joint. A compression nut may take a careful eighth-turn snug; if it weeps after that, the ferrule is deformed and the connection must be remade.
   • Wet at the valve body casting itself: the valve is failed and must be replaced.

3. Downstream joint wet only when running:

   • Top supply nut (at the faucet/toilet shank): re-seat the line, check the cone washer or gasket. A braided line has a built-in gasket; a flexible or rigid riser uses a cone or rubber washer that can be cracked or missing.
   • Bottom supply nut (at the valve outlet): same check, plus confirm the line is not cross-threaded.
   • The line itself bulging or weeping mid-span: braided line liner has failed; replace the whole line.

Confirming diagnosis

• Static leak at the stop valve: with the line disconnected and the valve closed, the valve outlet should not drip. If it drips closed, the valve seat is bad. If the packing nut weeps only when open, the packing is the issue.
• Washer/gasket leak: remove the supply nut, inspect the cone or rubber washer for cracks, flat spots, or a missing piece. A clean, intact washer that reseats and stays dry confirms it was the original (re-seat fixed it); a damaged one confirms replacement.
• Fitting/ferrule leak: a compression joint that weeps after one careful snug has a deformed ferrule; remaking it dry confirms the diagnosis.
• Condensation false alarm: wipe dry, leave off for an hour with no water use. If it re-wets evenly along the cold pipe with no defined source ring, it is sweat, not a leak.

Remediation

• Loose packing nut: snug a quarter turn, no more. Over-tightening splits the bonnet.
• Failed compression ferrule: cut the line back if possible, install a new ferrule and nut, or replace the riser. Do not pile thread tape on a compression joint; it does not seal there.
• Cracked or missing washer/gasket: replace with the correct cone or flat washer for the riser type. Hand-tight plus a quarter turn with a wrench on a plastic nut.
• Failed stop valve: replace with a quarter-turn ball stop. If it is a soldered or crimped connection to the stub, that work may exceed a basic handyman scope in some jurisdictions.

References

• International Plumbing Code (IPC), Chapter 6, Water Supply and Distribution, for supply fittings and shutoff requirements.
• International Residential Code (IRC), Chapter 29, Water Supply and Distribution.
• ASME A112.18.1 / CSA B125.1, Plumbing Supply Fittings, for supply-line and stop-valve standards.
• ASSE 1061, Performance Requirements for Push-Fit Fittings, for fitting connection types.