A slab-on-grade house has its drain system in the soil under the floor, and the only ways to see it are the openings the plumbing itself provides: cleanouts, the base of the stack, toilet flanges, floor drains and the roof vent. A camera through those openings reaches most of the system, and a sonde in the head lets each finding be marked on the floor above it. That inspection is the difference between a repipe quoted from a guess and one quoted from a plan, and between a trench across the living room and a hole in the laundry. It also has limits, and an honest report states what was reached and what was not.
Quick answer: under-slab drains are inspected from every available entry, each covering a different part of the system: the main cleanout for the building drain and lateral, the stack base for the main run, each toilet for its bathroom group, each floor drain for its own branch, and the roof vent for the stack. Findings in cast iron are scale, channeling, holes and failed joints; in plastic, bellies, dry joints and crushed sections. Each finding is located through the slab with the sonde and marked on the floor. Branches that cannot be entered are reported as unreached, not assumed sound.
Planning the entries
Start with a sketch of the house’s fixtures and work out which pipe each one drains into. The main cleanout, if there is one, gives the building drain toward the street and, from a two-way, back toward the house. The test tee at the stack base gives the main building drain. Each toilet, pulled, gives its closet bend and its bathroom group. Each floor drain gives its branch with a small head. The kitchen sink, with the trap removed, gives the kitchen branch with a small head. The roof vent gives the stack. A typical slab house needs four or five entries to cover the system, and the cleanout guide describes each.
What the camera finds in cast iron
Scale on every wall, which is normal. Channeling along the invert, which is the pipe worn through and the finding that rules out lining. Holes with soil or voids behind them, which are leaks into the ground under the house and the cause of the damp floor or the drain flies. Offsets and separations at the lead joints. The kitchen branch and the main run near the stack are usually the worst; bathroom branches often the best. The under-slab cast iron guide covers it in depth.
What the camera finds in plastic
A belly where the trench settled under the slab, holding water and sediment. A dry joint that has pulled apart. A hub crushed during the pour. A deflected section under a bearing wall. Construction debris from the build. The pipe itself is sound; the installation is the question. The plastic-era guide covers the findings.
Locating through the slab
With the head at a finding, the sonde is activated and the receiver is walked across the floor above until it peaks. The depth under a slab is usually small and the locate is reliable away from rebar, footings and steel beams. Mark the floor with tape, note the distance reading beside it, and move to the next finding. The marks are the repair plan: a spot repair opens the slab at one mark; a repipe runs a trench between them; a re-route overhead avoids them. Locating a sewer line under concrete covers the technique and its interference problems.
What the inspection cannot establish
The condition of a branch the camera could not enter, typically a galvanized branch rusted shut or a small-diameter line behind a trap the head will not pass. The condition of the outside of the pipe. Whether a joint that looks sound is leaking outward. The exact depth of each pipe, which the sonde estimates. And anything in a section the camera could not reach because of a blockage. The report should list each of these as unreached rather than leaving them out, because a buyer or a contractor reading it will assume silence means soundness. Inspection limitations covers the point.
The repair conversation
With the findings located and marked, the options are lining the sound-but-leaking sections after descaling, spot repairs through the slab at the marks, a full repipe through the slab, or a re-route through the attic and walls that avoids the slab altogether. The camera findings, with distances and floor marks, let each be quoted without guesswork. Repair under a slab compares them.
Equipment for the job
A 23 mm reel with a sonde, a self-leveling head and an on-screen counter for the cleanout and stack-base entries, which cover the main runs. A 17 or 18 mm scope on a thin rod for the floor drains and the kitchen branch, which the full reel will not enter. A receiver for the floor marks. And a pulled-toilet entry for each bathroom group, which the 23 mm reel usually manages through the closet bend. The combination covers nearly every slab house; the equipment guides on this site cover each piece.
Common questions
Can you inspect pipes under a slab without breaking it?
Yes, from cleanouts, toilets, floor drains and the roof vent, with a sonde to mark findings on the floor.
How accurate is the floor mark?
Usually within a foot, away from rebar and steel. Take two locates from different approaches at each finding.
What if a branch cannot be entered?
It is reported as unreached. In an old house that is usually a galvanized branch that needs replacing anyway.
Related guides
See why slab homes need sewer inspection, the best sewer cameras for older homes, and the best sewer cameras for excavation planning.