What Problems Can Bellies in Sewer Lines Cause?

Many homeowners give little thought to the intricately installed sewer lines extending from a home to the main municipal sewer collection lines. The homeowner's sewer lines move waste, such as dishwater and toilet refuse, down pipes for eventual treatment and disposal. However, anomalies along the sewer line, such as bellies, can cause sewage problems for the homeowner.

Sewer Line Design

Sewer lines rely on gravity and the conducive effect of sloped piping for moving waste water. A sewer line is laid on a positive slope to transport waste water away from the home. In fact, the pipe's slope must equal a drop of 0.25 inches or more for every one foot in distance that the pipe extends. This slope value allows gravity to exert its pull on the waste water and bear it away to the municipal sewer lines. The pipe must have a positive slope for the waste water to travel efficiently. Demanding installations may allow a smaller slope value, such as 1/8th of an inch, if local ordinances allow, but even if permitted, slope this minimal may hinder effective movement.


A belly in a sewer line is also referred to as sag. Essentially, the pipe never had, or no longer has, a positive slope over a specific portion of its run, and in that portion the pipe actually bows downward in a negative slope. Problems arise when water enters the belly. Part of the waste water flow will move past the belly, but some will remain trapped as stagnant water. The next time waste water flows through the piping, it will strike the stagnant water, hindering the waste's movement. Over time, solids will begin accumulating in the belly, forming a solid clog. The entire pipe may become clogged if solids continue to stack within the belly (See Reference 1).

Belly Causes

Many external factors can contribute to a belly formation. Loose and shifting soil surrounding the sewer line allows the pipe to move within its immediate area, causing it to slump into a sagging position. The original plumbing contractor for the home may not have adhered to local regulations. The line may have broken at the joints. As the line breaks, the pipe settles into a different position, possibly creating a belly.

Determining The Damage

Excavating and repairing a sewer line can be a costly experience. Currently, many contractors use camera technology that is snaked into a sewer line to scope out the extent of the damage. Many homeowners, when advised of the presence of a belly or shown photographs of it, allow the belly to remain if the sag is not too extreme. This is especially true if the homeowner is not having any immediate problems with sewage movement. However, bellies that could potentially worsen warrant a repair. Ignoring an extreme belly piping anomaly can back waste water into the home, causing interior damage.