A “low area” (a.k.a. Sag or Belly) is where the sewer pipe has lost positive slope going to its destination, i.e. the city sewer main or in some cases, a septic tank, With rare exception, sewers flow by gravity to where it ties onto the city’s main sewer and becomes their responsibility. If a line is properly installed, it will have a positive slope for the entire duration of its distance.
Under normal conditions, sewers are installed with a fall of 1/4″ per every foot of length. Under difficult circumstances (and with the city inspector’s OK) a bare minimum of 1/8″ fall per foot is acceptable, although not preferred.
A low area can be identified by seeing the sewer continue to hold water after the flow has stopped. Problems begin when incoming water is flushed down the sewer line, and it hits a “blockage” of standing water. Severe low areas cause the water (along with the debris it is transporting) to lose speed and settle in the low area, eventually causing a clog.
Low areas can be caused by a number of factors, such as: shifting soil, poor soil compaction, poor installation, or a broken pipe (usually at a joint) that is settling. Unfortunately, the only way to correct a low area is to excavate the affected area and re-install the pipe at the proper grade. Since this corrective action is so extreme and often costly, it should only be corrected if it is truly causing an actual problem, not just “lacking perfection” for a short distance. For instance, a low area holding 1/2″ of water for three or four feet is not “technically correct” per the plumbing code book. However, will this cause a problem? The answer is no.
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The problem now becomes … how much of a low area is too much?
The best way to find out is to ask the owner if they have had problems in the past. If so, they may have documentation from prior plumbing service calls on the distance out that the clog was cleared. This information, in conjunction with a camera inspection, may help determine if the low area is excessive.