Sink backs up only when bathtub drains

Quick Answer:

If the sink only backs up when the bathtub drains, the tub and sink likely share a downstream pipe or vent and a partial blockage or poor venting is allowing bathtub flow to push water back into the sink. Start by stopping all water use, then drain the bathtub fully and watch the sink to confirm the connection. Small clogs can sometimes be cleared at the sink trap or at a nearby cleanout; persistent or multiple-fixture backups mean call a plumber.

Why This Happens

Most homes have branch drains that serve several fixtures. When the bathtub and sink share the same branch, water from the tub can force its way into the sink if:

• There is a partial clog downstream that reduces the available flow and raises pressure in the shared pipe.
• The vent serving that drain is blocked, so air can’t escape and one fixture displaces water into another.
• There are design issues (improper slope or a shared trap) that make the sink vulnerable when large volumes drain from the tub.

For related situations, homeowners sometimes see problems like Sink fills when floor drain used or Sink fills when irrigation runs when different parts of the system interact.

Step-by-Step What to Do

1. Stop using water immediately

Cease all water use to avoid making the backup worse and reduce the volume moving through the shared line.

2. Drain the bathtub fully and watch the sink

Run the tub and let it drain completely while someone watches the sink basin and drain. This confirms whether tub flow causes the sink backup and helps you assess timing and severity.

3. Check the sink trap and clean it

• Place a bucket under the P-trap, loosen the slip nuts, and remove debris. A partially blocked trap can contribute to backups.
• Only reassemble after removing debris and flushing the trap with water.

4. Try a sink plunger

Use a cup plunger on the sink (block any overflow holes first). Pump several times and then run a small amount of water to see if flow improves.

5. Check nearby cleanouts

Locate any accessible cleanout downstream of the fixtures. If you can open it safely, look for signs of backup at the cleanout before calling for more invasive work.

6. Test other fixtures

Run other drains one at a time to see whether the problem is isolated or affects multiple fixtures. Multiple fixtures backing up indicates a downstream issue.

7. If the problem is unresolved, call a professional

If clearing the trap and plunging don’t help, or if multiple fixtures back up, you’ll likely need a plumber with a drain camera or a proper snake to locate and remove the blockage.

What Not to Do

• Do not snake the tub drain without checking the sink line. Snaking the tub alone can push a downstream clog further or leave the actual shared blockage in place.
• Do not use concentrated chemical drain cleaners in large volumes; they often don’t reach the clog and can damage pipes or create hazardous conditions.
• Do not open sewer cleanouts or manholes without professional training—sewage exposure is dangerous.

When to Call a Professional

Call a licensed plumber if any of these occur:

• Multiple fixtures back up or backups keep returning after a DIY attempt.
• There is a strong sewage odor, slow drains across the house, or sewage appearing at floor drains.
• You cannot find or access the cleanout, or the blockage is beyond the sink trap and requires powered equipment or a camera inspection.

Safety Notes

• Wear gloves and eye protection when working on traps and drains.
• Avoid mixing chemical cleaners; residues can cause burns or toxic fumes.
• If you suspect sewage in the home, stop using water and call a professional—do not attempt major repairs yourself.

Common Homeowner Questions

• Why does the sink back up only when the tub drains?

  Because the tub and sink share a drain or the vent is blocked, so tub flow either pushes water into the sink or the sink can’t vent properly.

• Can I fix this by myself?

  Sometimes—cleaning the sink trap and using a plunger can help. If multiple fixtures back up or the problem returns, call a plumber.

• Will chemical drain cleaners solve it?

  No, they often don’t reach the blockage and can harm pipes; mechanical cleaning or professional service is safer and more effective.