Quick Answer:
Intermittent stoppages usually mean a partial blockage, a venting problem, or an appliance-driven flow issue. Start by running water and plunging, then systematically inspect the trap and drain line while you track triggers (time of day, duration, fixture use). If the problem returns often or spreads to other fixtures, call a professional.
Why This Happens
- Partial build-up of hair, grease, soap scum, or food in the trap or pipe that shifts and sometimes allows water through.
- P-trap or branch lines that fill and occasionally seal off flow, especially when another fixture runs.
- Vent stack blockage that makes drains slow or stop when pressure changes in the system.
- Garbage disposals or dishwashers that intermittently allow solids into the line and cause temporary blockages.
- Related patterns such as Sink drains then sludge appears or Sink drains fine then suddenly clogs can point to buildup or vent issues rather than a single item stuck in the pipe.
Step-by-Step What to Do
1. Observe and document
Keep a short log for a few days. Note the time of day, what you used the fixture for, how long the slow or stoppage lasted, and whether other fixtures were used at the same time. This tracking will help identify triggers like peak water use or dishwasher cycles.
2. Try a basic clear
- Run hot water for 30–60 seconds to see if moving water clears the line.
- Use a sink plunger (cover overflow if present) and plunge firmly for 15–30 seconds.
3. Check and clean the P-trap
- Place a bucket underneath, loosen the trap nuts, and remove the trap to inspect for debris.
- Clean the trap and reseal carefully. Tighten by hand and check for leaks.
4. Check downstream and the vent
- If the trap is clear but the problem returns, the clog may be further down. A manual auger (hand snake) up to 10 feet can reach the branch line.
- Observe roof vents if you can safely do so: leaves, bird nests, or ice can block venting and cause intermittent slow draining.
5. Track triggers for intermittents
Use the log you started earlier to match stoppages to events: dishwasher runs, laundry cycles, high-flow showers, or time of day. This will tell you if the issue is a flow/load problem, a one-way piece of debris, or a venting problem.
6. Re-test and reassess
After cleaning and tracking, test the sink under different conditions (cold/hot water, disposal on/off, other fixtures running). If the behaviour continues or gets worse, escalate to a professional inspection.
What Not to Do
- Do not treat random stoppages as harmless if frequency increases.
- Do not pour strong chemical drain cleaners into the line as a first fix—these can damage pipes and are dangerous if they splash during manual cleaning.
- Do not use an electric auger unless you are experienced; it can damage pipes or fixtures if used incorrectly.
- Do not ignore odors, sewer backups, or slow drains in multiple fixtures—these are warning signs of a larger issue.
When to Call a Professional
- If stoppages become more frequent or affect multiple drains at once.
- When you smell sewer gas or see sewage backing up into the sink or other fixtures.
- If you cannot reach or remove the obstruction with a hand auger and the issue recurs.
- When roof venting appears blocked and you are not comfortable inspecting it safely.
Safety Notes
- Turn off the water supply and wear gloves and eye protection when working under the sink.
- Use a bucket and rags to catch spills and protect cabinets from water damage.
- If you used chemical cleaners, ventilate the area and avoid mixing products; flush thoroughly before manual work.
- Don’t climb onto the roof or use ladders alone—hire a pro for vent inspections if needed.
Common Homeowner Questions
- Why does the sink only stop sometimes? Partial blockages or venting problems allow flow at times and block at others depending on pressure and what’s in the line.
- Will pouring boiling water help? It can dissolve grease and soap scum but won’t clear hair or solid debris and risks damaging some pipe materials if done repeatedly.
- How long should I track the problem before calling? Keep notes for several days to a week; call sooner if the frequency increases, odors appear, or other drains are affected.