In a slab-on-grade house built before the 1980s, the drain lines run in the soil under the concrete floor, and if the house is old enough they are cast iron. Nobody has seen them since the day the slab was poured. They have been wet on the inside and damp on the outside for fifty or sixty years, and cast iron corrodes in both directions. The only way to know their condition without breaking the floor is a camera, entered from a cleanout, the base of the stack, a pulled toilet or a floor drain, and the findings from that inspection decide whether the house needs a repipe, a liner, a spot repair or nothing at all.
Quick answer: under-slab cast iron is inspected from the nearest access toward the fixtures and toward the lateral. The camera shows scale, which is normal; channeling along the invert, which is the pipe wearing through; holes with soil behind them, which are leaks into the ground under the house; and offsets or separations at the lead joints. Scale alone is a maintenance note. Channeling, holes and separations are structural and mean the slab will be opened or the line lined. The sonde locate at each defect marks the floor above it.
Getting in
An interior cleanout on the building drain is the best entry, pushing toward the lateral and, from a two-way, back toward the fixtures. The test tee at the base of the stack covers the main building drain. A pulled toilet gives access to the closet bend and the branch below it, and a floor drain gives access to its own branch with a small head. The roof vent reaches the stack and the drain below. Most slab houses need two or three entries to cover the whole system, and entering from a floor drain and entering through a toilet cover the fixture routes.
What the camera shows
Scale. Rust nodules and ridges, heaviest on the invert and at joints. Normal for the age and not in itself a reason to open the floor. Channeling. A flat or missing invert, with the water running on soil. The pipe has worn through along the bottom and will not hold a liner; this is a replacement finding. Holes. Dark openings, usually on the invert, with soil or a void behind them. Water has been leaving into the ground under the slab, which is the cause of the soft floor, the damp carpet or the drain flies that prompted the inspection. Offsets and separations. At the lead joints, where the oakum has dried and the sections have moved. The channeling guide and the holes guide describe the two structural findings in detail.
Reading the whole system, not one line
Under-slab cast iron fails in patterns. The kitchen branch, carrying hot greasy water, is often the worst. The main building drain near the stack, where everything meets, is next. Bathroom branches that carry mostly clean water often look better. A house with channeling in the kitchen line and sound bathroom lines is a partial repair; a house with channeling everywhere is a repipe. The inspection should cover every reachable branch and the report should say which are sound and which are not, with distances, because the repair quote depends on the length of bad pipe, not on the fact that bad pipe exists.
Locating under the slab
With a sonde in the head, each defect can be located through the slab from above and marked on the floor. The depth estimate is usually small, a foot or two below the slab, and the locate is reliable away from rebar and footings. The marks turn a repipe into a plan: which rooms will be opened, where the trenches will run, what will have to be moved. For a spot repair, the mark is the hole. Locating a sewer line under concrete covers the technique.
The repair options
Lining, if the pipe is scaled but still round and continuous, after descaling; it seals the joints and the small holes without opening the floor. Spot repair through the slab, for a single channeled or holed section in otherwise sound pipe. Repipe through the slab, for channeling or holes in most of the system, which is the most disruptive residential plumbing job there is. And re-routing overhead, through the attic or along walls, which avoids the slab and is sometimes cheaper than trenching it. The camera findings, with distances and surface marks, are what let a contractor quote any of these without guessing. Repair under a slab compares them.
What to record
Every branch and run inspected, from which entry, with the condition of each: scaled, channeled, holed or offset, and the distances. Every branch that could not be entered, named as unreachable rather than assumed sound. The surface marks from the sonde locates. A repipe quote built on that record is a plan; one built on a single bad finding is a guess, and the difference can be the price of the house.
Common questions
How long does cast iron under a slab last?
Fifty to seventy years is typical. Many houses are past that, and the camera is the only way to know where a particular house stands.
Can the pipes be lined without breaking the floor?
If they are round and continuous after descaling, yes. Channeled or holed sections need a point repair first, which means opening the floor there.
Should I inspect before buying a slab house?
Yes, from every available entry. Under-slab cast iron is the single most expensive hidden defect in houses of that era.
Related guides
See why slab homes need sewer inspection, the best sewer cameras for cast iron pipes, and lead and oakum joints on camera.