Quick Answer:
If a sink that was just repaired now stops draining intermittently, something under the sink was likely disturbed — a misaligned trap, a loose connection, or debris pushed into the line. Start by reproducing the stoppage, then inspect under-sink connections and confirm trap alignment after repair. Clean and flush the trap and tailpiece before assuming a larger problem.
Why This Happens
During a repair, parts can be moved, gaskets can shift, and small bits of debris (pipe dope, plumber’s tape, putty, or mineral buildup) can be pushed into the drain. A trap that is slightly rotated or not seated straight can hold water or collect debris, causing intermittent slow draining. Venting problems or partial blockages further down the line can make the issue appear only after some use.
If the stoppage looks like it comes and goes suddenly, think of two common patterns: the drain may clear and then clog again — **Sink drains then backs up** — or the sink may handle short flows but fail after extended use — **Sink only stops when water runs long**. Each pattern points to a different likely cause (loose debris vs. progressive blockage or venting).
Step-by-Step What to Do
1. Reproduce the problem safely
- Run the tap and note exactly when the sink slows or stops (right away, after a few seconds, only when you run hot water, etc.).
- Place a bucket under the trap before you start working to catch any water or debris.
2. Inspect under-sink connections and confirm trap alignment after repair
- Look for loose slip nuts, cross-threaded fittings, or a trap that is rotated so the outlet doesn’t line up with the drain tailpiece.
- Ensure the trap is not sagging and that the inlet and outlet are in a straight line. A misaligned trap can hold water and catch debris.
3. Clear and clean the trap and tailpiece
- Loosen the trap slip nuts and remove the P-trap. Empty it into the bucket and inspect for hair, putty, or other debris.
- Use a wire brush or a small bottle brush to clean the trap and the visible tailpiece. Rinse them with water and reassemble, hand-tightening the slip nuts and then giving a small additional turn with pliers if needed (don’t over-tighten).
4. Flush and test
- With the trap reinstalled, run hot water for several minutes to flush small debris further into the line or out of the house. If possible, run a combined flush by filling the sink and releasing the stopper to force a stronger flow.
- Check for leaks at every connection and watch whether the intermittent stoppage returns. If the sink still clogs only after long runs, the issue may be farther down the line or related to venting.
5. Try a simple mechanical clear for deeper clogs
- If cleaning the trap didn’t fix it, use a hand auger (plumber’s snake) through the tailpiece to 6–8 feet to dislodge a clog. Avoid chemical solvents at this stage (see safety notes).
- Re-test by running water and observing. If the problem returns only after long runs, consider checking venting or calling a professional.
What Not to Do
- Do not accept intermittent stoppage after repairs without a proper flush.
- Do not pour caustic drain cleaners into the sink if you plan to work on the trap — they can burn skin and damage pipes.
- Do not overtighten plastic slip nuts or try to force fittings that are cross-threaded; you can crack fittings or strip threads.
When to Call a Professional
- If the sink still stops intermittently after you’ve inspected and cleaned the trap and tailpiece.
- If water backs up into other fixtures (showers, tubs, or other sinks) — that suggests a main drain or sewer line problem.
- If you find persistent leaks, damaged pipes, or you’re uncomfortable disassembling the trap safely.
Safety Notes
- Wear gloves and eye protection when removing the trap — the water may contain grit or sewage.
- Turn off water only if you need to disconnect supply fittings; for trap work the shutoff is usually not needed, but keep a bucket handy.
- Avoid mixing chemical drain cleaners with mechanical cleaning: residue can splash and cause injury. If drains have been treated chemically, ventilate the area and consider calling a pro.
Common Homeowner Questions
- Q: How long should a post-repair flush take to prove the problem is fixed?
A: Run hot water for several minutes and test normal use for a day; intermittent issues often show up within normal use cycles. - Q: Can a slightly crooked trap cause a stoppage?
A: Yes — a misaligned or sagging trap can catch debris and cause intermittent slow drainage. - Q: Is it OK to use a plunger after reassembling the trap?
A: Yes, a plunger can help with surface clogs, but avoid plunging aggressively if you suspect broken fittings or a leak.