A belly, or sag, is a low spot where the sewer pipe has lost its slope and holds water. It's usually a problem only if debris settles there and builds up into backups. Seattle's utility calls a sag under 25 percent of the pipe diameter minor, and 50 percent or more severe.
What is a belly in a sewer line?
Sewer lines move waste by gravity, so they are laid on a slope. Seattle Public Utilities, which publishes guidance for owners of private side sewers, says these pipes are built at a slight slope of a quarter inch per foot, and that slope "allows wastewater to use gravity to move debris through the pipe." A belly, also called a sag, is a section that doesn't have that slope. In inspection terms, it's often described as a sag holding standing water.
Is a belly always a problem?
No. The same utility says sags "are typically only a problem if they cause debris to settle and collect in the pipe." Settling debris may eventually result in backups. So the question is how deep the low spot is and what your line carries.
Seattle rates sag severity by depth compared with the pipe's diameter:
| Rating | Depth of sag | What the utility says |
|---|---|---|
| Minor | Less than 25 percent of the pipe diameter | Minor buildup of debris may occur, but typically not enough to cause negative effects. No repair. |
| Moderate | 25 to 50 percent | Would typically cause debris to settle and may eventually cause a backup. Inspect every two to five years. |
| Severe | 50 percent or more | Higher likelihood of debris buildup and a backup over time. Repair. |
Those are one utility's bands for side sewers, not a national code. Your city, your lender or your building's owner may use different cutoffs.
How a belly is graded on a PACP report
On a NASSCO PACP report, each finding gets a grade from 1 to 5. Geocadra says the grade comes from a combination of the defect code and its measured value, so ask what depth or percentage the inspector recorded for your sag. Check the same report for notes about debris deposits near the low spot. For how the code system works, see what PACP coding is.
What usually follows
- A shallow belly on a clean line: monitor it. Ask for a re-inspection date and a note on the water depth.
- A deeper belly or one with buildup: regular cleaning keeps the line moving, and the report should say where the low spot starts and ends by footage.
- A severe belly: Seattle's guidance is to repair it. Fixing one generally means correcting the pipe at that spot, which is a conversation about access, surface restoration and downtime as well as cost.
Roots, an offset joint or a crack can be the reason the pipe sagged, so look at the whole report and not the belly alone. For the basics, see what a plumbing camera inspection finds, and how to keep a sewer line from backing up.
HQ can scope your line, mark the location and walk through the report with you. Free estimates and upfront pricing on any repair.