Quick Answer:
If the basement floor drain starts filling while the laundry runs, stop using any water-using appliances immediately. Remove the drain strainer and clear visible debris, then slowly pour a 5‑gallon bucket of water into the drain to test flow. If the drain still won’t accept water, find and carefully open the nearest exterior sewer cleanout (wear gloves and eye protection). If sewage backs out of that cleanout, cap it, stop draining more water, and document the water level and any odors for your plumber.
Why This Happens
Basement drains back up for a few common reasons: a blockage in the house drain or main sewer line, solids or lint from the washer piling in the trap, or municipal sewer problems downstream. Sudden backups during laundry often point to a clog near where the washer line meets the main drain, but a full main blockage will affect multiple fixtures.
Other homeowners sometimes see similar situations after time away—compare the symptoms to situations like Backup after long vacation or when the washer itself overwhelms the floor drain, as in Basement drain floods when washer drains. For a broader look at causes and patterns, see the cluster hub topic Basement Floor Drain Backups.
Step-by-Step What to Do
1. Stop all water use
- Turn off the washer, and don’t run sinks, toilets, or other appliances. Stopping water flow prevents the basin from rising further.
2. Remove the strainer and clear visible debris
- Use gloves and a flashlight. Lift out the strainer or grate and pull away hair, lint, or gunk you can reach by hand or with a small tool.
- Do not stick your head directly over the drain — avoid inhaling any fumes.
3. Test with a 5-gallon bucket
- Slowly pour one 5‑gallon bucket of water into the drain. Pour over 30–60 seconds, not all at once.
- If the water goes down, the partial obstruction may have cleared enough to run the washer slowly. If it stands or rises, there’s a deeper blockage or sewer issue.
4. Locate and open the nearest exterior sewer cleanout
- Find the cleanout outside along the foundation, near the street, or at a low point on the property. It usually has a round or square cap.
- Wear gloves and eye protection. Loosen the cap slowly—gas and sewage can spray or vent under pressure.
- If nothing comes out, the clog is further downstream or the line is blocked. Replace the cap securely when done.
5. If water comes out of the cleanout
- If sewage or grey water backs out of the cleanout, cap it and stop draining any more water into the system.
- Note the water level, take photos if safe, and record any strong sewage smell. This information helps the plumber diagnose where the blockage is.
6. Prepare for the plumber
- Keep the area clear, note which fixtures were in use, and be ready to explain what you tried (bucket test, cleanout behavior, odors).
- While waiting, avoid running more water or attempting risky DIY repairs if sewage is present.
What Not to Do
- Don’t pour chemical drain cleaners into a pooled basement drain — they can damage pipes and the environment and won’t fix a main sewer blockage.
- Don’t keep running the washer or other fixtures after the drain starts backing up.
- Don’t open the cleanout without gloves, eye protection, and confidence in what you’re doing; if you can’t access it safely, call a pro.
- Don’t enter a flooded basement alone or touch sewage with bare skin.
- Call a professional when sewage or strong odors are present, when water continues to rise after stopping fixtures, or if you can’t access the cleanout safely.
When to Call a Professional
- Sewage is backing into the basement, or you smell strong sewage odors.
- Water continues to rise after you stop using water and after testing the cleanout.
- You cannot locate or safely open the cleanout, or the cleanout shows signs of pressure or forceful discharge.
- Repeated backups occur, suggesting a failing sewer line or tree root intrusion.
Safety Notes
- Wear gloves, eye protection, and old clothing when working near sewage.
- Avoid breathing fumes; ventilate the area if possible without spreading contaminated water.
- Wash hands and disinfect tools and surfaces that contacted the water. Dispose of contaminated rags or absorbents per local rules.
- If anyone is exposed to raw sewage and develops symptoms, seek medical advice.
Common Homeowner Questions
- Why did this start when I ran the laundry?
Washer lint and soap scum can collect near the junction of the washer line and main drain, causing backups when volume increases. - Can I clear a main sewer clog myself?
No — main sewer clogs often require a plumber with a camera and rooter equipment; DIY attempts can make things worse. - Will my homeowner’s insurance cover this?
Coverage varies; document damage and contact your insurer, but many policies exclude sewer backups unless you have specific endorsements.