Partial clog only in basement

Quick Answer:

If your basement floor drain runs slowly only while the washing machine is draining upstairs, first check the basement trap and cleanout. Clear any debris from the trap, make sure the cleanout cap is accessible, then run the washer while watching the cleanout to see whether the building main accepts the laundry outflow. If the trap is clean but the branch is blocked, remove the trap and snake from the basement cleanout toward the street. If you have a backwater valve, check it for sticking and consider adding or repairing sump/backflow prevention if backups come back.

Why This Happens

• Debris in the basement trap or near the floor drain will slow flow only at that fixture while larger flows from the washer overwhelm it.
• A partial blockage in the basement branch (lint, soap scum, grease, or small roots) can let slow drains pass but restrict heavier laundry outflow.
• A sticking backwater valve can hold back or restrict flow during high-volume discharge from the washer.
• If multiple fixtures are slow at the same time, the problem may be a larger line issue — **Slow drainage in entire house** or a main partial clog.

Step-by-Step What to Do

1. Make the cleanout and trap accessible

• Locate the basement floor drain and the nearby cleanout cap. Clear boxes or stored items away so you can work and observe.
• Keep a bucket and towels handy for any water when you open the trap or cleanout.

2. Inspect and remove debris from the basement trap

• Put on rubber gloves and a mask. Remove the trap cover or unscrew the trap if needed and scoop or pull out lint, paper, or other debris. Dispose of solids in a sealed bag.
• Rinse the trap with water to verify flow. Reinstall the cover snugly so it seals.

3. Run the washing machine while watching the cleanout

• With the cleanout cap accessible but closed, start a drain cycle (use a small test load if possible). Watch the cleanout area for surging, gurgling, or discharge when the washer pumps out.
• If the cleanout accepts the outflow with no backup or level rise, the main is likely handling the flow. If you see flow pushing back at the cleanout or surfacing, the branch or main may be partially blocked.

4. Remove and clean the trap, then snake from the basement cleanout toward the street

• If the trap was full again or you saw backups, remove the trap entirely. Place the bucket under it and remove debris by hand or with a small brush.
• Open the basement cleanout. Feed a drain snake (manual auger or powered where safe) into the cleanout and snake toward the street — that is, away from the house — to clear branch blockages. Advance the snake slowly and rotate to break up obstructions; retract and clean the cable frequently.
• After snaking, run water through the drain or run a short washer drain cycle to confirm improved flow. Reinstall the cleanout cap and trap cover securely.

5. Check a backwater valve and consider prevention if backups recur

• If your system has a backwater valve, open or inspect it for sticking debris or mechanical failure. Clean or replace the valve if it does not move freely.
• If backups happen repeatedly during heavy loads, consider a sump pump, improved backflow prevention, or rerouting laundry discharge to reduce risk. A plumber can advise on suitable equipment for your setup.

What Not to Do

• Avoid disconnecting sewer joints or pipes in the basement without proper seals, tools, and ventilation — doing so can release raw sewage and hazardous gases.
• Do not pour large amounts of chemical drain cleaners into the basement trap or cleanout. They can damage pipes and create hazardous fumes.
• Do not attempt major repairs if you encounter sewage at floor level, active backflow from the drain, or you cannot restore flow from the basement cleanout — in those situations, call a professional immediately.

When to Call a Professional

• You find sewage on the floor or active backflow from the basement fixture.
• You cannot clear the blockage using the basement cleanout or the snake meets solid resistance you cannot break safely.
• Multiple fixtures are backing up or you suspect the issue is a deeper line problem such as a root intrusion or a **Main sewer line partially clogged** situation.
• The backwater valve is damaged or you need a sump/backflow prevention system installed.

Safety Notes

• Wear gloves, eye protection, and a mask when handling traps and sewage-contaminated material.
• Ventilate the area; sewer gases can be harmful. Do not lean over an open cleanout or place your face near it.
• Keep electrical devices and outlets away from wet floors. Turn off power to nearby circuits if water is present near outlets or motors.
• When using a drain snake, advance the cable slowly and keep a firm grip. Sudden release or spring-back can injure you or damage the pipe.
• If you smell strong sewer gas inside the house or feel lightheaded, leave the area and call a professional immediately.

Common Homeowner Questions

• Q: Why is the basement drain only slow when the washer runs?
  A: The washer puts a large, concentrated flow into the line; a partially blocked branch or a dirty trap can handle small flows but slow during that surge.
• Q: Can I fix this myself with a snake?
  A: Yes, for small branch blockages you can snake from the basement cleanout toward the street, but stop and call a pro if you encounter solid resistance or sewage backing up.
• Q: Will a backwater valve stop this problem?
  A: A working backwater valve prevents sewer backflow into the basement but won’t fix a branch blockage; check the valve for sticking and consider it as part of a broader prevention plan.

More in this topic

For more related fixes and similar symptoms, see Main Line Partial Clogs.