Sewer problems
Signs of a main sewer line clog
Updated · Editorial team; not yet technically reviewed
Short answer
A main line clog affects more than one fixture: drains on the lowest level back up first, the tub or shower fills when a toilet flushes, and toilets gurgle when the washer drains. One slow sink is a branch clog; several fixtures acting up together point to the main line, and you should stop using water until it's cleared.
Main line or branch line?
| What you notice | Branch clog | Main line clog |
|---|---|---|
| Fixtures affected | One | Several, often all |
| Lowest drain | Normal | Backs up first |
| Flush a toilet | No effect elsewhere | Tub or floor drain bubbles or rises |
| Washer drains | Normal | Toilet gurgles or a drain overflows |
| Cleanout cap | Dry | Water standing in it |
What to do right away
- Stop running water: no flushing, laundry or dishwasher.
- Don't pour chemical drain cleaner into a main clog; it won't reach and makes the line hazardous to open.
- If you have an outdoor cleanout, a plumber can clear the main from there.
- If sewage reached the floor, keep people and pets away and see sewer backup cleanup.
What causes main line clogs
Tree roots at pipe joints, grease, wipes and paper towels, sags that collect waste, and broken or collapsed pipe. If the main clogs more than once, a camera inspection shows which; in many cities the line to the street is the owner's responsibility (see who owns the sewer line).
Damage, cleanup and insurance
Sewage backups carry bacteria, so anything soaked with sewer water, such as carpet, pad, drywall near the floor and porous items, usually needs to be removed rather than cleaned. Wear gloves and boots, keep children and pets away, and ventilate the area. Professional cleanup crews disinfect and dry the space to prevent mold; document the damage with photos before anything is thrown away.
Standard homeowner's policies often exclude sewer backups unless you have a water backup endorsement, which is inexpensive to add. Check your policy now rather than after a backup. If the blockage was in the city's main, contact the utility promptly, as some cities have claim procedures and deadlines. Installing a backwater valve can prevent future backups from the main, and some cities offer rebates for it.
Common questions
Why does my tub fill up when I flush the toilet?
Water from the toilet can't get past a main line clog, so it rises at the next lowest opening, often the tub or shower.
Which drain backs up first in a main line clog?
The lowest one in the house, usually a basement floor drain or first-floor tub or shower.
Can I clear a main sewer clog myself?
Small augers rarely reach; a plumber's machine through the cleanout is the usual fix.
Is a main line clog an emergency?
If sewage is backing into the house, yes; stop using water and call for help.
Why is there water standing in my sewer cleanout?
Water in an open cleanout means the main is blocked downstream of it, toward the street.
Do wipes cause main line clogs?
Yes. Even wipes labeled flushable don't break down like toilet paper and snag in the main.
General information; codes vary by location. See our editorial standards.