A sewer belly is a dip, or belly, in a sewer line is a common problem in underground sewers. In addition to “sewer belly,” you might also hear it called a “dip” or “low spot” in the line.
To understand how a sewer belly can be a problem for you, you have to first understand the basic design of your sewer system.
All sewer lines have to be set at a grade, which means that the pipes are always sloping downhill. Similar to an amusement park slide, the pipe has a top high point (your house) and a bottom (the street or the public sewer underneath the street). This setup allows gravity to ensure that the sewers constantly flow in the right direction.
Like the amusement park slide, proper assembly is important. A sewer pipe with a belly is the same as a slide assembled with a pool halfway down: the ride down will be smooth until you hit the pool. To continue to the bottom, you must swim across the pool, and then continue the slide to the bottom. This seems like no problem if you can swim, right? Wrong.
With waste water, a pool located in the middle of the pipe simply creates an area for solid waste to settle and build up, eventually creating a clog. As a result, bellies must be repaired as soon as the issue is discovered.