Quick Answer:
An intermittent sink backup usually means a partial blockage, a venting issue, or a problem further down the shared drain. Track when it happens over several days to spot a pattern, then try simple inspections and safe clearing steps. If multiple fixtures are affected or the problem gets worse, call a professional.
Why This Happens
- Partial clog in the P-trap or drain line that lets water pass slowly until use patterns overload it.
- A shared drain or blocked vent can let water back up when another fixture runs or when air pressure changes.
- Main sewer line issues, tree roots, or collapsing pipes can cause unpredictable backups that come and go.
- Appliance discharge (dishwasher, washing machine) may send extra flow into the sink’s line at certain times.
Step-by-Step What to Do
1. Track when the backups occur
Keep a simple log for several days. Note date, time, what fixtures were used, whether the toilet was flushed, and what you were running (dishwasher, washing machine). This helps identify patterns like backups during heavy use or after a neighbor’s water use.
2. Note obvious patterns
- If backups reliably happen when the toilet flushes, that points toward a shared drain or partial main-line blockage. Sink backs up when toilet flushes
- If backups occur only when two fixtures run at once, you may have a capacity or slope problem. Sink backs up when two fixtures used
3. Clear the sink strainer and trap
Remove the strainer and clean out hair and gunk. Place a bucket under the P-trap, loosen the slip nuts, and inspect and clean the trap. Reinstall carefully and test the sink.
4. Use a plunger safely
Seal the sink overflow, fill the basin partly with water, and plunge firmly. If the sink has two basins, block the other side when plunging. Repeat a few times and test.
5. Try a hand auger for a short clog
If plunging fails, a small hand auger (drain snake) can reach clogs beyond the trap. Feed slowly, rotate, and pull back any debris. Stop if you feel resistance that won’t move — that may be farther down the line and require a pro.
6. Check other fixtures and vents
Run water in nearby sinks, showers, or the toilet to see if backups follow a specific use. Avoid climbing on the roof to clear vents yourself unless you are experienced; vent problems are best handled by a pro.
7. Continue tracking results
Keep updating your log for a few more days after each step. If the backups persist, become more frequent, or start affecting multiple fixtures, stop and call a plumber.
What Not to Do
- Do not keep using fixtures if backups worsen.
- Do not pour strong chemical drain cleaners into a sink with sewage backup — they can be dangerous and often won’t fix deeper clogs.
- Do not force tools or use powered snakes you are not trained to operate; you can damage pipes.
When to Call a Professional
- Backups affect more than one fixture or happen throughout the house.
- Foul sewage smell, slow drains in multiple drains, or water backing into toilets or tubs.
- You cannot clear the clog with basic tools or the clog is beyond the trap.
- Repeated or worsening incidents after you’ve done basic checks and tracking.
Safety Notes
- Wear gloves and eye protection when working under the sink. Keep a bucket and rags handy.
- Turn off and unplug nearby electrical devices if water is present.
- Wash hands thoroughly after handling drain debris. Treat any contact with sewage as a contamination risk.
- If you’re unsure about working on roof vents, sewer lines, or using powered equipment, hire a licensed plumber.
Common Homeowner Questions
- Why does the sink sometimes clear itself? Small partial clogs can shift or break up, letting water pass until new debris builds up again.
- Can a plunger always fix this? A plunger can clear many sink clogs but won’t fix blockages farther down the line or problems with vents and main drains.
- How long should I track before calling a pro? Track for a few days to identify patterns; call sooner if backups spread, smell like sewage, or get worse quickly.