Short answer: The basement floor drain is the lowest drain in the house, so it is where a main sewer problem shows up first. This Old House lists the usual causes: clogs from wipes and grease, tree roots, sagging or broken pipes, and heavy rain overloading the city sewer. If water rises in the floor drain when you run the washer, flush a toilet or shower upstairs, stop using water and call a plumber to clear the main line.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Water comes up when the washer drains | Partial main line clog | Stop using water; check other drains | Plumber |
| Several drains slow and gurgling | Main sewer line blockage | Run one fixture and watch the floor drain | Plumber |
| Backs up only during heavy rain | City sewer overloaded or storm water in the line | Note if neighbors have the same problem | Plumber and city |
| Clear water, no smell, after rain | Groundwater entering the drain | Check grading and sump pump | Often |
| Sewage smell but no water | Dry floor drain trap | Pour water into the drain | No |
Do this now
- Stop using all fixtures: toilets, sinks, showers, the washer and the dishwasher.
- Keep people and pets away from the water; sewage can make you sick.
- If water is near outlets or appliances, turn off that circuit only if you can reach the panel safely.
- If you have an outdoor sewer cleanout and can reach it safely, opening the cap can relieve the pressure so sewage spills outside instead of in the basement (This Old House).
- Call a licensed plumber.
What the plumber will do
- Snake (auger) the main line from the cleanout.
- Hydro jet tough clogs like grease and roots.
- Run a camera to find roots, breaks or a sagging section.
- Recommend a backflow (backwater) device if your area is prone to city sewer surcharges.
Prevent the next backup
- Do not flush wipes (even “flushable”), diapers or paper towels.
- Keep grease, oil and coffee grounds out of drains.
- Have the line inspected if you have big trees near the sewer or an older clay or cast-iron pipe.
- Ask your insurer about a sewer backup endorsement; standard policies often exclude it.
When to call a professional
- Sewage or water comes up through the floor drain.
- More than one drain is slow or gurgling.
- It happens again after the line was cleared (roots or a broken pipe).
- Sewage soaked carpet, drywall or stored items (call a restoration company).
What it usually costs
HomeGuide (2026) lists main line snaking at about $200 to $500, hydro jetting at $600 to $1,400, and a sewer camera inspection at $125 to $500. Sewage cleanup runs about $7 to $15 per square foot, and minor backup repair and cleanup about $500 to $2,000.
Frequently asked questions
Why is my basement floor drain backing up when I do laundry?
The washer sends a lot of water quickly. If the main line is partly blocked, that water backs up at the lowest opening: the floor drain.
Should I pour drain cleaner in a backed-up floor drain?
No. A main line clog is too far away, and the chemicals make the water more dangerous for you and the plumber.
Is sewage in the basement dangerous?
Yes. This Old House notes sewage exposure can cause stomach illness and infections. Wear gloves, boots and eye protection, and leave major cleanup to professionals.
Related problems
Sources
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.