The mainline is the central artery of a property wastewater system. All branch drains and soil stacks feed into this single large-diameter pipe. It is typically buried in the yard and relies on a steady downward slope to carry waste away using gravity. Blockages here will cause sewage to back up into the lowest fixtures in the building.
Heard on the job
The kitchen sink is draining fine, but the toilets are bubbling, so we definitely have a clog in the mainline.
Insider tip
Tree roots are the number one enemy of older mainlines, especially those made of clay or cast iron.
Also heard as
- main sewer line
- building sewer