Sewer problems
How to prevent basement sewer backups
Updated · Editorial team; not yet technically reviewed
Short answer
Basements back up when the sewer can't carry everything flowing into it, so water and sewage come up through the lowest drain. The most effective defenses are a backwater valve on the sewer line, a sump pump for foundation water, and keeping roof and foundation water out of the sanitary sewer. Several cities install these for free in flood-prone areas.
Why basements back up
- Overloaded public sewers: heavy rain overwhelms combined or leaky sewers, pushing sewage back up laterals.
- Clear water in the sanitary sewer: downspouts, foundation drains and cracked laterals add stormwater to pipes meant for sewage.
- Problems in your own lateral: roots, grease or a collapse block the line, and household water backs up.
- Overland flooding: surface water enters through windows, walls or floor cracks.
Defenses compared
| Option | Stops | Notes |
|---|---|---|
| Backwater valve | Sewage flowing back from the main | Install only on a sound lateral; needs a cleanout for maintenance |
| Sump pump | Groundwater around the foundation | Battery backup matters, since storms cut power |
| Downspout disconnection | Roof water entering the sewer | Send water at least several feet from the foundation |
| Lateral lining or replacement | Groundwater leaking into a cracked lateral | Camera inspection decides it |
City programs that help
| City | Program |
|---|---|
| Cincinnati | MSD Sewer Backup program: free prevention devices after repeat public-sewer backups |
| Columbus | Blueprint Columbus: free sump pumps, check valves and lateral lining in Blueprint neighborhoods |
| Detroit | Basement Backup Protection Program: free valves and sump pumps in 11 neighborhoods |
| Milwaukee | Downspout disconnection requirement and MMSD Pipe Check incentives |
During a backup
Stop using water
Every flush adds to the backup.
Stay safe
Avoid sewage and keep away from standing water near outlets.
Report it
Tell your sewer utility, which can check whether the public sewer is the cause.
Document
Photograph damage before cleanup. See sewer backup cleanup.
Checking your protection before storm season
If you have a backwater valve, open its access cover once or twice a year and make sure the flap moves freely and nothing is stuck in it. Clear debris, and confirm that the cover seals. A valve that stays shut won't protect the house and can block your own drains. If you're not sure one exists, a plumber can find it on a camera inspection.
Check that downspouts discharge away from the house and aren't connected to the sanitary sewer, which some cities prohibit because it overloads the system during storms. Test the sump pump and battery. Keep valuables and stored items off the basement floor. If your city offers a backflow prevention or downspout disconnection program, its rebates can cover much of the cost of installing protection.
Common questions
What's the difference between a backwater valve and a sump pump?
A backwater valve stops sewage flowing back from the sewer main; a sump pump removes groundwater from around the foundation. Many flood-prone homes need both.
Do backwater valves need maintenance?
Yes. Check and clean them periodically through the access cover so debris doesn't keep the flap from closing.
Why does my basement drain back up only when it rains?
Rain overloads the sewer or enters your lateral through cracks, so the lowest drain overflows.
Should downspouts be connected to the sewer?
Generally no. Many cities require or encourage disconnection so roof water doesn't overload sanitary sewers.
Can a backwater valve cause problems?
If installed on a broken lateral or without a working cleanout, it can trap sewage in your own line, which is why a camera check comes first.
Does insurance cover a sewer backup?
Usually only with a sewer backup endorsement, which isn't part of most standard policies.
General information; codes vary by location. See our editorial standards.