Quick Answer:
If toilets overflow only when more than one is flushed at the same time, the house drain or sewer line is likely near its capacity or partially blocked. Flush toilets one at a time and watch how each behaves to help narrow the cause. If backups continue or you smell sewage, contact a plumber.
Why This Happens
Homes share drainpipe capacity among fixtures. When two or more toilets, or a toilet and another drain, release a lot of water at once, the main line may not move the combined flow fast enough. That can push water back up into lower fixtures or overflow a toilet.
- Partial clogs, tree roots in the lateral line, or a narrowing in the main sewer reduce capacity.
- Older or undersized plumbing may handle single fixtures fine but struggle with simultaneous use.
- Recent changes to plumbing or water pressure can change flow behavior—see Overflow after pressure change and Overflow after remodel for related situations.
Step-by-Step What to Do
1. Stop using other fixtures
Immediately avoid running washers, showers, or other toilets. Limiting flow gives the system a chance to recover and reduces the amount of sewage that can back up.
2. Flush toilets one at a time and observe behavior
Flush a single toilet and watch:
- If it drains normally, wait a few minutes and flush a second toilet alone.
- If each toilet works alone but overflows when flushed together, that points to a capacity limit in the main line or a partial blockage.
3. Look for other warning signs
- Gurgling sounds from drains, slow draining sinks, or water rising in other fixtures during a flush suggest a shared blockage.
- Smell of sewage, repeated backups, or water outside fixtures are more serious and need professional help.
4. Try safe, basic clearing methods (if applicable)
- Use a plunger on the affected toilet(s). Do not use excessive force or inappropriate tools.
- A toilet auger can help a local clog in the bowl trap, but it won’t clear a deep main-line blockage.
5. Record what you found
Note which toilets overflowed, whether single flushes were fine, and any noises or slow drains. This information speeds diagnosis for a professional.
What Not to Do
- Don’t assume simultaneous overflow is normal—main drain capacity matters and can indicate a serious issue.
- Don’t keep flushing multiple toilets to “see what happens.” That can make backups worse and spread sewage.
- Don’t pour large amounts of chemical drain cleaner into toilets as a first fix; chemicals can damage pipes and create hazards.
- Don’t attempt major main-line repairs yourself; a plumber is appropriate for deep blockages or sewer-line work.
When to Call a Professional
- Multiple fixtures back up at once, or toilets overflow even when flushed individually.
- Persistent slow drains, gurgling noises, or foul sewage smells.
- Evidence of sewage outside a fixture, or you cannot clear the issue with a plunger or simple auger.
- If you suspect a main sewer line problem (tree roots, collapsed pipe, or large blockage).
Safety Notes
- Avoid contact with wastewater; wear gloves and clean hands thoroughly afterward.
- If flooding occurs, turn off water to the toilet at the shutoff valve to limit further overflow.
- Avoid mixing chemical cleaners and do not enter standing sewage water. Call a professional for major backups.
Common Homeowner Questions
- Q: Why does it only overflow when more than one toilet is flushed?
A: Because the combined flow can exceed the main drain’s capacity or reveal a partial blockage. - Q: Can I clear this myself?
A: Try a plunger or toilet auger for a local clog, but main-line issues require a plumber. - Q: Is this an emergency?
A: If sewage is backing up, smells are strong, or water is spreading outside fixtures, call a plumber right away.
For more related articles, see the nan hub.