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

Many homeowners do not realize the intricate piping systems that move 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 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.

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. A belly in a sewer pipe could eventually lead to a clog that blocks the entire pipe.

Typically, sags are caused by improper soil compaction, as well as soil shifting. The pipe becomes unstable, slowly sagging with the soil's movement.

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.

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.

Sewer lines are sometimes accessible via a home's basement or crawlspace. If these heavy pipes are not properly secured to the home's structure with pipe straps, they can sag before ever entering the soil. These problems can occur as a normal part of the home's settling process.

Handy homeowners can bring these sagging pipes back in line. Cradle the pipe in a pipe strap attached to a floor joist, and adjust the strap until the sag straightens.

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.