Rigid pipe, clay and cast iron, cracks when it is overloaded. Flexible pipe, PVC, ABS and HDPE, does something else: it deflects. The crown moves toward the invert, the sides bow outward, and a round pipe becomes an oval. A little deflection is normal and designed for; a lot is a problem, because it reduces capacity, catches debris, stops a camera head and rules out lining. On camera, ovality is easy to miss on a fixed head that rolls and obvious on a self-leveling head that keeps the pipe’s shape in a consistent orientation, and learning to see it is part of inspecting any plastic lateral.
Quick answer: ovality shows on camera as a pipe whose opening is wider than it is tall, with the crown flattened and sometimes a visible crease along it, in a plastic pipe that is otherwise sound. Mild deflection is a condition note. Deflection that visibly reduces the bore, that the camera head slows at, or that shows a crease or buckle along the crown is structural and needs a spot repair, because a liner follows the host shape and the pipe will continue to deflect. The cause is nearly always poor bedding, a surface load, or both.
Why plastic deflects
Flexible pipe gets its strength from the soil around it. Properly bedded in compacted material, the soil at the sides resists the pipe’s tendency to bow outward and the pipe carries its load as a ring. In loose or poorly compacted backfill, with voids at the sides, the sides have nothing to push against and the crown drops. Add a surface load, a vehicle, a driveway or a footing, and the drop increases. Deflection is therefore a bedding failure first and a load failure second, and it is most common in laterals laid in a hurry and backfilled with whatever came out of the trench. Settlement after construction covers the load side.
What the camera shows
With a self-leveling head the pipe’s opening ahead is a circle in a sound section and a horizontal oval in a deflected one, with the crown closer to the lens than the invert. In mild cases the difference is subtle and best seen by comparing against a sound section nearby. In severe cases the crown is visibly flat, there may be a crease or a white stress line along it where the plastic has yielded, and the head bumps the crown as it passes. A deflected section often has a belly in it as well, because the same bad bedding let it settle. On a fixed head the picture rolls and the oval can be mistaken for a trick of the angle, which is one of several reasons self-leveling heads are recommended for plastic laterals.
How much is too much
Engineering standards for buried flexible pipe set deflection limits as a percentage of diameter, typically around five percent for new installation, and inspection standards grade deflection by severity. A camera cannot measure percentage, but it can show whether the deflection is slight, moderate or severe by the shape of the opening and by whether the head passes freely. A practical rule: if the oval is noticeable but the head passes without slowing and there is no crease, note it and re-inspect in a year. If the head slows, the crown is creased, or the bore is clearly reduced, it is a repair. PACP defect codes gives the grading vocabulary.
Why lining does not fix it
A cured-in-place liner takes the shape of the host pipe. In an oval host it cures oval, with the same reduced capacity, and the host continues to deflect under the same bedding and load, taking the liner with it. Most lining contractors will decline a visibly deflected section or require a spot repair first. The repair is to excavate the section, re-bed it in compacted material, replace the deflected pipe and backfill properly, which is the same repair as a belly and is often the same dig. CIPP lining explained covers the host-pipe requirements.
Where to look
Under driveways and paths. Near the foundation, where backfill is deepest and least compacted. Where the lateral crosses a trench dug later for another utility. Anywhere a camera’s sonde locate lands on a parking spot. And in any plastic lateral laid by a builder in a hurry, which is many of them. Deflection is also common in HDPE storm laterals and corrugated drainage pipe, which deflect more readily than PVC, and inspecting a French drain covers the corrugated case.
What to record
The distance where the oval begins and ends, how the opening compares with a sound section nearby, whether a crease or stress line is visible along the crown, and whether the head slowed. A sonde locate at the centre of the deflected section, because it usually lands on the thing that caused it. A still from the deflected section beside one from a sound section makes the report clear to a customer who has never seen the inside of a pipe.
Common questions
Is a slightly oval PVC pipe a problem?
Mild deflection is common and tolerated. Note it, locate it, and re-inspect. Severe deflection with a crease or a reduced bore is a repair.
Can an oval pipe be lined?
Not usefully. The liner takes the oval shape and the host keeps deflecting. Re-bed and replace the section first.
Why did the camera stop in a plastic pipe with no blockage?
Possibly a deflected section where the crown has dropped enough to catch the head. Back off and look at the shape of the opening.
Related guides
See ABS pipe defects on camera, the best sewer cameras for PVC pipes, and the belly guide.