How Can a Sag in a Gravity Sewer Line Be Fixed?

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Sewage pipes can become clogged if there is a sag in the line.

Many homeowners do not realize the intricate piping that moves sewage waste from their structure to a municipal sewage pipeline. Most sewer lines use gravity as a method of moving waste water and solids down a pipe. However, sewer lines can develop problems, such as sagging. Sagging is repairable, although it can be costly based on the repair method chosen.

Sewer Line Design

Sewer lines use positive slopes to take advantage of gravity's force pulling waste water and solids away from a building. Typically, a sewer pipe is installed at an angle that ensures a 1/4-inch fall for each foot of piping length. In fact, the slope can be as small as 1/8 inch, if the installation is limited in space, although this is not a preferable spacing.

Sewer Line Sag Identification

A sewer line sag is also known as a belly. This sag generates a negative slope along the pipe's length, creating a pooling area for water and waste. Typically, sags are caused by improper soil compaction, as well as soil shifting. The pipe becomes unstable, slowly sagging with the soil's movement.

Dig and Replace

The old-fashioned method of repairing a sag is digging and replacing. A contractor digs down into the soil until he reaches the sag. The contractor replaces the section and fills the trench. On average, a dig-and-replace process can cost about $50 to $60 for each foot. However, many factors can increase the price, including the piping's depth and moving above ground structures.

Trenchless Technology

Newer piping replacement methods have been devised over the years, including trenchless techniques, costing about $40 to $80 per piping foot. In-line expansion is a trenchless method that uses the older pipe to guide a new pipe into place. An expansion head pushes the old pipe forward while pulling the new pipe in from behind, negating the need to dig. Soil is compacted as the expansion head moves forward, creating a solid area for the new pipe to remain at a positive slope. Another method is sliplining. This technique slips a smaller pipe within the old pipe. After the new pipe is positioned, the old pipe slides away and is discarded.


Contractors typically use cameras for verifying the extent of sagging for a particular piping length. Some sags may not be severe enough to warrant immediate repair. However, homeowners must consider that an ignored sagging pipeline can worsen, possibly clogging with solids over time.


Amy Rodriguez

Writing professionally since 2010, Amy Rodriguez cultivates successful cacti, succulents, bulbs, carnivorous plants and orchids at home. With an electronics degree and more than 10 years of experience, she applies her love of gadgets to the gardening world as she continues her education through college classes and gardening activities.