Cast iron does not fail evenly. The top of the pipe, which only sees sewer gas, often looks almost new after decades. The bottom, which sits in water, corrodes continuously, and once the scale on the invert flakes away the metal underneath thins until it is gone. What is left is a half pipe: an iron arch over a trough of soil, with the water running in the dirt. That condition is called channeling, and it is the most important thing a camera can find in an old cast iron drain, because it is the point at which lining stops being an option and the pipe has to come out.
Quick answer: channeling looks like a flat or missing invert, often with soil, gravel or roots visible along the bottom of the pipe, and a water line that no longer follows a curved surface. Early channeling shows as a lighter, flat stripe along the invert; advanced channeling shows bare earth. It is nearly always found under slabs and in crawlspaces where the pipe has been wet for decades, and it means replacement, not cleaning.
How channeling develops
Water sits on the bottom of a drain between uses. In cast iron that constant wetness drives corrosion on the invert while the crown stays dry. Scale builds, then flakes, and each cycle takes a little metal with it. Acidic or soft water speeds the process, as does a flat or back-graded line where water stands longer. After fifty or sixty years the invert can be paper thin, and after that it is gone. Because the crown is still intact, the pipe still looks like a pipe from the outside and still carries flow, which is why channeling is so often a surprise finding on a camera inspection ordered for a different reason.
What the camera shows
Early on, a lighter band along the bottom of the pipe where the scale has gone and bare, thinning iron is exposed. The band looks smoother than the scaled walls on either side. The pipe shape is subtly wrong, flatter at the bottom than the top. Later, the band becomes a flat floor with a rough edge where the iron ends, and the water runs across soil, sand or gravel. Roots may be growing in along the trough. In the worst cases the camera head drops into the void where the invert used to be. Dimmed LEDs show the flat floor far better than full power, and a self-leveling head keeps the invert at the bottom of the picture where it belongs. Our guide on scale and tuberculation covers the stage before this one.
Why it changes the repair decision
A cured-in-place liner needs a host pipe to hold its shape while it cures and afterwards. A channeled pipe has no bottom, so the liner has nothing to lie against and can sag, leak at the ends or fail to cure properly against wet soil. Most lining contractors will decline a channeled section or will insist on a point repair first, which under a slab means breaking concrete anyway. Pipe bursting needs a continuous host pipe to pull the bursting head through, and a channeled pipe may not provide one. That leaves open replacement, which under a slab is the most disruptive and expensive repair in residential plumbing. The camera is the only way to know which situation you are in before the quotes arrive, and the cast iron vs PVC guide explains what else to check on the same pass.
Where to look
Under the slab, from the first fitting inside the foundation to the point where the line exits the building, because that section has been wet and unseen for the whole life of the house. In the crawlspace, at any low point where hangers have sagged and water stands. At the base of the stack, where the vertical cast iron turns horizontal and water hits the invert hardest. And at the first few feet of the exterior lateral if it is also cast iron. A camera pass that starts at an exterior cleanout and heads to the street can miss all of it; the pass that matters heads back toward the house. The guide on entry points for a sewer camera covers how to get there.
Inspecting for channeling
Use a self-leveling head so the invert stays at the bottom of the picture. Dim the LEDs. Slow down and watch the floor of the pipe, not the walls. Note the distance where the flat band starts and ends, because the repair will be priced by the foot. Look for soil, roots or gravel on the floor. If the head drops or the picture suddenly shows earth, stop, record the distance, locate with the sonde if the system has one, and withdraw; a void in the invert is a place where a head can wedge. Then write it down plainly: a report that says “heavy scale” when the camera showed channeling costs the homeowner the chance to plan.
Common questions
Can a channeled pipe be lined?
Usually not without a point repair first. A liner needs a host pipe with a bottom. Ask the lining contractor to look at the footage before quoting.
Is channeling the same as a hole?
A hole is a local loss of wall. Channeling is loss of wall along the invert over a length. Both are structural, but channeling affects the whole section.
How urgent is it?
The pipe still carries flow, so it is not an emergency today. It is a repair that will be needed, and the sooner it is planned the less it tends to cost.
Related guides
See the best sewer cameras for cast iron pipes, collapsed pipe and the limits of a sewer camera, and the best sewer cameras for older homes.