Key facts: Sewer backup in the basement
- The basement floor drain is usually the lowest point on the house's drain system, so it overflows first when the main line is blocked.
- Sewage contains bacteria, viruses and other germs; keep people and pets away and wear gloves and boots if you must enter [1].
- Blockages from roots, grease and broken pipe, and stormwater entering sewers in heavy rain, are common causes [2].
- A backwater valve can help stop sewage from the public side entering a basement, but it does not fix a blocked private lateral [3].
Why the basement drain goes first
All the drains in a house join one building drain and one lateral to the public sewer. When that lateral blocks, wastewater from every sink, toilet, shower and appliance has nowhere to go. It backs up through the pipes toward the house and rises until it reaches the lowest opening, which in a house with a basement is almost always the floor drain, a basement shower or a laundry standpipe.
That is why a basement can flood with sewage when nobody has been downstairs all day. The water came from upstairs fixtures and, in some cases, from the public sewer itself.
Clues to the cause
| What happened | Likely cause |
|---|---|
| Backup after laundry or a lot of showers | A partial blockage in the lateral that cannot carry heavy flow |
| Backup only during heavy rain | Stormwater or groundwater entering the sewer system [2] |
| Neighbours backing up at the same time | A problem in the public sewer |
| Backups every few months | Roots at joints in an older lateral |
| Backup after a big meal or holiday | Grease and food waste in the line |
| Only the basement shower drains slowly | Early warning of a main-line restriction |
Getting the line cleared
- 1
Call for clearing
Say that sewage is in the basement and whether it is still rising. You will be connected with an independent contractor who can tell you on the call what they can do.
- 2
Relieve at the cleanout
Contractors usually open the exterior cleanout first so water drains outside rather than into the house.
- 3
Open the blockage
A cable machine or hydro jetter clears the lateral toward the street.
- 4
Film the cleared line
Once the line drains, a camera shows what caused the blockage and whether the pipe is damaged.
- 5
Fix the cause before restoring the room
There is no point refinishing a basement if the line will back up again.
Cleaning up after a basement backup
- Discard porous items soaked by sewage: carpet and padding, upholstered furniture, cardboard and paper [1].
- Clean and disinfect hard surfaces such as concrete, tile, metal and plastic.
- Cut out drywall and insulation that sewage reached, a little above the wet line.
- Dry the area thoroughly with fans and dehumidifiers before closing anything up.
- For large or finished basements, a professional cleanup and restoration company is usually the safer choice.
- Check your insurance policy; many standard policies exclude sewer backup unless a separate endorsement was added.
Preventing the next basement backup
Prevention starts with the cause the camera found. Roots call for clearing and then sealing the joints, often by lining. Grease calls for jetting and a change in kitchen habits. A sag or collapse calls for a repair at that spot or replacement.
For basements with fixtures below the level of the street, a backwater valve on the building sewer can close when sewage tries to flow back from the public side [3]. It needs to be installed correctly and checked periodically, and it does not protect against a blockage in your own lateral, because your own wastewater would then have nowhere to go.
Also check that downspouts and yard drains are not connected to the sewer, and keep the exterior cleanout accessible so the line can be cleared from outside next time.
Atlanta basements and older sewers
Basements are common in Atlanta's hilly neighborhoods, where houses step down sloping lots, and in many Buckhead and Brookhaven houses with finished lower levels or split-level layouts. Those lower floors are the first to feel a lateral blockage.
Atlanta's public sewer system was rebuilt in many places under federal consent decrees that ran for years [4], but private laterals were not part of that work. Inside the city, problems in the public sewer go to the Department of Watershed Management; in most of Brookhaven and other DeKalb County areas outside the city, to DeKalb County.
Ejector pumps and basement bathrooms
Some basement bathrooms and laundries sit below the level of the sewer lateral and drain into a sealed basin with an ejector pump that lifts wastewater up to the line. If only the fixtures connected to that basin back up while the rest of the house drains normally, the pump or its float switch may have failed rather than the lateral. Check whether the pump is running and whether a breaker has tripped, and mention the pump when you call.
FAQ
Basement backup questions
Why is sewage coming up through my basement floor drain?
The floor drain is the lowest opening on the system. When the main line is blocked, wastewater rises until it reaches it and spills out there first.
Is it safe to stay in the house during a basement backup?
Usually yes, if you keep out of the affected area, stop using water and avoid contact with the sewage. Leave if there is an electrical hazard you cannot safely isolate.
Can I clean up a basement sewer backup myself?
A small amount on a hard floor can be cleaned with gloves, boots and disinfectant. Larger backups or soaked finished rooms are best handled by professionals.
Will a backwater valve prevent all basement backups?
No. It helps stop sewage from the public side, but a blockage in your own lateral still needs to be cleared and fixed.
Why does my basement back up only when it rains hard?
Stormwater or groundwater may be entering the sewer through cracks or improper connections, on your side or the public side, overloading the pipes.
