Belly in Sewer Line: How to Spot and Measure a Sag on Camera
A belly is a section of sewer line that has settled below grade, so water and solids collect in it instead of draining. On camera, the lens rolls down into standing water, may go completely under, then climbs back out onto a dry invert. The pipe wall itself can be intact. The problem is the grade.
Why sewer lines develop a belly
The pipe follows the ground it sits on. When the ground under one section drops, the pipe drops with it. The usual causes:
- Poor bedding at installation. Pipe laid on loose backfill, rocks or uneven trench bottom settles unevenly.
- Washed-out bedding. A leaking joint or crack upstream can wash fines out of the soil under the pipe.
- Disturbed ground. A later excavation next to or across the lateral (another utility, a footing, a pool) can undermine the trench.
- Soil movement. Expansive clays that swell and shrink, and freeze-thaw cycles, move the pipe over the years.
Once a low spot holds water, the wet soil around it can soften further, so a belly can deepen over time.
What a belly looks like on camera
Pushing downstream, the signs show up in a predictable order:
- Water level rises. The water line creeps up the frame over a few feet. With a self-leveling head it stays flat at the bottom of the image, which makes the rise easy to read.
- Lens goes under (deep sags). The picture turns murky or goes dark as the head submerges. Paper, grease and fine sediment often sit on the invert here.
- Water level drops back. The head climbs out and the invert is visible and draining again.
The third point matters. NASSCO training material on coding sags makes the same case: if you don’t record where water returns to normal, a rise in water could just be flow from a lateral or infiltration, not a sag.
Rule out the look-alikes
- Debris under the water. If the water level reads high but the lens stays dry and the head rides high in the pipe, the camera is probably driving over a deposit or obstruction, not through a sag.
- Downstream blockage. A partial stoppage further down backs water up over a long run. The level won’t drop back until you pass the blockage.
- Flow from upstream. Someone running a tub during the scope raises the water everywhere. Ask for fixtures off.
How to measure it
You can’t put a level on a buried pipe, but the video gives you enough for a useful record.
| What to record | How | Why it matters |
|---|---|---|
| Start footage | Counter reading where water begins to rise | Tells the dig crew where to open up |
| Deepest point | Footage plus water depth as % of pipe diameter | Main severity indicator |
| End footage | Where the invert is dry and draining again | Gives the length of the sag |
| Depth of line at the low point | Sonde reading at the deepest footage | Sets excavation depth and cost |
| Solids in the sag | Note paper, grease, sediment | Shows whether it is already trapping waste |
For water depth, estimate the percentage of the diameter that is under water. It helps to read it at a joint, which gives a fixed reference in the frame. In PACP, a sag shorter than 3 feet is coded at its deepest point with the end footage in the remarks; a longer one is coded as a continuous defect with a start and finish.
A sonde in the camera head plus a receiver lets you mark the low point on the surface and read its depth. If you do a lot of belly calls, a camera with a built-in transmitter is worth it; see sewer cameras with a locator.
How serious is it?
Severity is a judgment call, and there is no single industry cutoff for “too deep.” Weigh these together:
- Water depth at the low point. A few percent of diameter that drains slowly is a different problem than a pipe that is half full or submerges the head.
- Length. A 3-foot dip at a single joint behaves differently than 15 feet of sag.
- Solids. A sag that is already holding paper and grease will keep causing stoppages.
- Other defects inside it. Offset or separated joints and cracks in the low spot let water out and soil in, which feeds the settlement.
- History. A line with repeat backups at the same footage is telling you the answer.
Fixes
Monitor. A short, shallow sag with no solids and no backup history can reasonably be recorded and re-scoped later. Put the footages in the report so the next inspection can compare like for like.
Maintenance. Periodic jetting keeps a moderate sag from building up solids. It does not change the grade.
Excavate and re-grade. The permanent fix. The crew opens the section, re-beds the pipe on proper compacted bedding at the right slope, and replaces damaged pipe. The start and end footages from your video, plus the sonde locate, define the dig.
Pipe bursting and lining. Lining does not remove a belly, because the liner follows the host pipe. Pipe bursting generally follows the existing path too. Neither is a grade correction, even though either may be the right call for other defects in the same line.
Tell the customer plainly: the camera shows where water sits and for how far. What it cannot tell you is exactly how many inches the pipe dropped. That is why the report gives footages and water depth rather than a slope number.
Related defects
A belly often sits next to offset joints, because the settlement that dropped the pipe also pulled joints out of line. In clay and Orangeburg, standing water at leaking joints also invites root intrusion.
FAQ
Can a belly be fixed with a liner?
No. A cured-in-place liner follows the host pipe, so it keeps the same low spot. Lining can seal cracked or separated joints inside a sag, but only re-grading the pipe removes the belly itself.
How much standing water is too much?
There is no universal cutoff. Record the start, the deepest water level as a percent of diameter, and the end of the sag, then weigh that against the line's history of backups. A short, shallow sag on a line that has never backed up is often a monitor item; a long, deep one that submerges the camera and collects solids usually is not.
Why did the camera show a sag the first time and not the second?
Water level depends on flow at the moment you scope. If someone ran a fixture upstream, water can back up from a partial blockage or a lateral and look like a sag. Run the line with fixtures off, and confirm that the water level returns to normal past the low spot before you call it a belly.
Does a belly get worse over time?
It can. Water and waste that sit in the low spot keep the bedding wet, and settlement that caused the sag can continue. Re-scoping after a set period, with footage and water depth compared against the first video, is the way to find out.