Why Is There an 8-Inch Sewer Pipe in My Basement?

Basement Plumbing

• The basement or crawlspace in homes without basements is where municipal water supplies enter the building. The entry’s low point maximizes water pressure created for the entire community by water towers on hills. Wastewater also leaves the building using gravity falling through the house into the lateral connection to the city sewer that runs from 12 to more than 20 feet below the streets. Water supply pipes, normally 1- to 1 1/2-inch diameter pipes, may run through a water softener or water heater in the basement, but then rise, distributing water throughout each floor. Wastewater falls through the house to join into one main line and run toward the city sewer under the street.

Pipe Sizes

• City sewer lines may be 8- to 20 inches across. They run through cities angling downward, interrupted periodically by “lifts” that pump sewage up to more elevated lines that resume downward progress. Vertical household pipes with 3- to 4-inch diameters act both as collector drains and as vent stacks. Collectors connect 2-inch drains from sinks, tubs and other fixtures. They empty into larger pipes that run underneath basement floors and join into one pipe that may be as large as 8 inches in diameter. The main collector exits the house becoming the lateral connection with the city sewer.

Carrying Capacity

• The logical reason for using progressively larger pipes is avoiding clogs that form as solid matter compresses in smaller space. The less obvious reason is that carrying capacity improves as pipe diameter increases. An 8-inch pipe carries as much as twice the amount of water as a 6-inch pipe and five to six times the volume of a 4-inch pipe when moving water along declines as gentle as 1 inch in 100 feet. Those 8-inch pipes might be in the basement because drain and vent pipes are larger than average.

End of the Line

• Large, 8-inch pipes in basements may be exposed collectors. In older homes, they may represent the remnants of capped floor drains. One common reason for a visible 8-inch pipe is as the clean out, a stub of pipe with a cap, removed with a twist of a plumber’s wrench. One or more clean outs along the main line in the basement allow removal of obstructions in the main line. The clean out nearest the lateral that connects the house service to the city sewer might also provide access to tree roots that grow into cement pipes.