Quick Answer:
If the sink drains fine during the day but backs up overnight, the most common causes are a partially clogged drain or a roof vent that becomes restricted when temperatures drop. Start by checking the trap and running a hot-water flush. If those don’t help, inspect the vent for frost or debris that can block airflow after dark.
Why This Happens
Plumbing vents allow air into the drain system so water flows freely. At night, colder temperatures or wind can cause roof vents to collect frost or debris, which reduces airflow and makes drains slow or back up. A partial clog in the P-trap or tailpiece can work during repeated daytime use but fail when flow is reduced.
Other signs that point to vent issues include gurgling noises from other fixtures. See Bathroom Sink Gurgles When Toilet Is Flushed for more about shared-vent symptoms.
Step-by-Step What to Do
1. Remove and check the stopper
- Take out the sink stopper and clean hair and soap buildup from the stopper and the drain opening.
- Run water to see if flow improves before moving to the trap.
2. Check the P-trap
- Put a bucket under the trap, loosen the slip nuts, and remove the trap.
- Clear any hair, gunk or sediment inside. Rinse the trap and reassemble, then test the drain.
3. Run a hot-water flush
- Boil or heat water safely, then pour it slowly down the drain in two or three batches. This helps dissolve grease and loosen soap scum.
- Follow the hot flush with cold water to push loosened material through the system.
4. Inspect the roof vent for frost or debris
- If you can safely access the roof, check the vent opening for frost, leaves, bird nests or other blockages. Clear debris carefully.
- Do not climb on the roof alone or in icy conditions. If the vent is frosted, warm air and sunlight usually clear it, but persistent frost may mean ventilation or insulation problems that need a pro.
5. Test overnight
- After the above steps, observe whether the sink holds up overnight. If it still backs up only at night, the vent is a likely cause and may require further inspection or repair.
What Not to Do
- Do not ignore seasonal vent issues. A vent that freezes or fills with debris at night can keep getting worse and lead to persistent backups or sewer odors.
- Avoid repeatedly pouring strong chemical drain cleaners. They can damage pipes and fittings and often won’t clear vent-related problems.
- Do not climb onto a slippery or unsteady roof to inspect vents without proper safety gear or assistance.
When to Call a Professional
- Call a plumber if clearing the trap and running hot water doesn’t stop the nightly backups.
- Ask a pro if the roof vent looks blocked but you can’t safely access it, or if vent freezing recurs — tests for venting and roof/attic insulation may be needed.
- If you smell sewer gas after the sink sits unused, get help promptly; that can indicate a dry trap or a vent problem. See Bathroom Sink Smells Like Sewer Gas After Sitting for related causes.
Safety Notes
- Wear gloves and eye protection when working under the sink. Keep a bucket handy to catch water from the trap.
- Do not use a ladder on icy, wet, or unsecured surfaces. Roof work can be dangerous—hire a professional if you’re unsure.
- Use caution with hot water to avoid scalding. Let heated water cool slightly before pouring it into the drain.
Common Homeowner Questions
- Why does it only happen at night? Cooler temperatures and reduced airflow at night can let frost or debris restrict roof vents, or reduced daytime use can mask a slow partial clog until the system rests.
- Will pouring hot water fix it permanently? Hot flushes help clear grease and soap buildup but won’t fix a blocked vent or a structural issue in the drain — consider them a first step.
- Is it safe to clear the roof vent myself? Only if you have stable footing, the right gear, and no icy conditions; otherwise hire a pro to avoid falls or injury.