Order summary
The mainline backwater valve is set into the sewer line under your basement floor, behind a removable cover. Inside is a flap that lets water leave the house but closes when sewage tries to come back. Debris, grease or a broken flap can hold it open or shut. Checking it is simple and needs no tools beyond gloves and a flashlight.
Numbered steps
- Stop using water in the house for an hour before checking.
- Open the access cover; most twist or lift off.
- Inspect the flap: you want it clean and free to move.
- Lift it gently with a gloved hand and let it close. It should seat evenly.
- Remove any debris, wipes or grease from around the flap.
- Check the cover's seal, then refit it firmly.
Plumber's tip: While the backwater valve is closed during a storm, limit water use. Don't run the laundry, dishwasher or showers until the storm passes.
Errors we see on site
- Leaving the cover loose; sewer gas and water can escape.
- Using water heavily in a storm while the flap is shut; that water has nowhere to go.
- Blocking access with storage or finished flooring.
- Forcing a stuck flap; it can break.
When the desk should come out
Call if the flap is cracked, does not seat, or the valve body is full of standing water. Those are signs of a damaged valve or a blockage downstream. We can clean or replace parts and inspect the line; see sewer line repair. Spring is a good time to check the valve and test the sump the same day using our sump test leaflet.
Quick checklist
- Open the access cap twice a year
- Check the flap moves and seals
- Clear grease or debris from the seat
Your questions on this one
How frequently do I check the valve?
Twice a year, plus after any backup.