Stop running water anywhere in the house until the line is cleared. Every flush, shower or load of laundry adds to what has nowhere to go.
What it usually means
Each fixture has its own small drain, and those branches join into one main line that carries everything out to the city sewer or a septic system. A clog in one branch affects one fixture. A clog in the main line affects everything upstream of it.
The lowest fixtures show it first. A basement floor drain, a lower-level shower or a first-floor toilet will gurgle or fill when water runs somewhere higher in the house.
Likely causes
- Tree roots growing into the sewer line through joints or cracks
- Grease, wipes or paper that has built up where the line narrows or sags
- A collapsed, offset or bellied section of older pipe
- A blockage at the city connection or a full septic tank
What you can safely check
Note which fixtures are affected and which are not. If only one bathroom is slow, the problem is probably in that branch. If a toilet gurgles when the washing machine drains, the main line is the likely cause.
Chemical drain openers do not reach a main-line clog and can make the work harder and more hazardous for whoever clears it.
When professional help makes sense
A main-line blockage needs a drain machine or water jetting through a cleanout, and often a camera afterward to see why it happened. If sewage is coming up into the house, treat it as urgent.