Sewer repairs under concrete are the expensive ones, because every foot of error is another foot of saw cut, excavation and patch. A 512 Hz sonde passes through concrete itself without difficulty; the problem is the reinforcing steel most slabs contain, which distorts the field, and the fact that the pipe under a slab is usually the building drain, shallow and close to other services. This guide covers how to get a locate on a slab that is good enough to cut once, and when to stop trusting the sonde and use another method.
Quick answer: locate the sonde on the slab from four directions and average; locate it again at two or three more points along the pipe, including any points where the pipe is under soil or an unreinforced section, and draw the route through all of them; check the camera’s distance counter against the route; treat the receiver’s depth as unreliable under reinforcement and estimate depth from the pipe’s known invert at a cleanout instead. Mark with paint or tape on the concrete, cut a small inspection hole first, and confirm the pipe before the full cut. Where the slab is heavily reinforced or post-tensioned, or where the locate will not settle, hire ground-penetrating radar or core a pilot hole.
What is under a slab
In a slab-on-grade house the building drain runs under the floor with branches to each fixture; in a basement the main drain and floor drain branches run under the basement slab; under a driveway or patio the lateral passes on its way to the street. Depth is usually shallow, a foot or two below the slab for a building drain, and other services, water, gas, electrical conduit, may share the space. Slab homes covers why this inspection is so often needed.
Why reinforcement matters
Welded wire mesh or rebar in the slab re-radiates the sonde’s field and smears the peak at the surface, shifting it, broadening it and making the depth read too shallow. Unreinforced residential slabs and lightly reinforced driveways are usually locatable with care. Commercial floors with heavy rebar, post-tensioned slabs with steel cables, and slabs over steel decking can defeat a 512 Hz locate entirely. Rebar and metal interference covers the physics and the signs.
Technique on the slab
Push the head to the finding and hold it there. Approach the peak from four directions with the receiver vertical, marking each reading with a small chalk mark, and take the centre. Reduce the receiver’s gain until the peak is as sharp as it will go. Note whether the peak is clean or smeared and record it. Then move the head a few feet along the pipe in each direction and repeat, so there are three or more marks along the route; a straight or gently curving line through them is the pipe. Marking a dig covers the basic locate.
Locate where there is no steel
Where the pipe leaves the slab, at the foundation wall, a cleanout, a floor drain or a section under soil, the sonde can be located cleanly, and those points anchor the route. Two clean locates either side of a slab section and the counter reading between them often place a finding under the slab more reliably than a locate through the steel. Tracing a lateral covers building the route.
Use the counter
The camera’s distance counter gives the finding’s position along the pipe from the entry, and if the route is known from the locates, the counter places the finding on it independently of the sonde’s behaviour under steel. Where the receiver’s peak and the counter’s implied position disagree, the counter is usually right about distance and the receiver wrong about position. Counter accuracy matters here.
Depth under a slab
Do not trust the receiver’s depth under reinforcement. Instead, measure the pipe’s depth at a cleanout or where it exits the foundation, assume the normal fall, and estimate depth at the finding from that. A building drain under a slab is typically a foot or two below the concrete, and a saw cut and careful excavation will reach it. Typical sewer depths and depth limits set expectations.
Marking concrete
Paint marks on concrete are permanent for practical purposes; use chalk or tape for provisional marks and paint only the final position. Mark the centreline and the two offsets the excavator should cut to, and write the distance from the cleanout beside the mark so it can be checked.
The inspection hole
Before a full cut, core or saw a small hole at the marked centreline and probe for the pipe. If it is found, the full cut follows with confidence; if not, the hole shows which way to adjust. One small hole costs little; a long cut in the wrong place costs the whole job. Repair under a slab covers what comes next.
When to use other tools
Ground-penetrating radar sees the pipe and the rebar and is unaffected by the steel in the way a sonde is; it is hired in for commercial slabs and for any locate that will not settle. A pilot core at the best estimate, followed by a camera or a probe through the core, is the other option. Both cost more than a locate and less than a wrong cut.
Other services
Water, gas and electrical conduit under a slab share the space with the drain. Sweep the cut area with a passive power mode if the receiver has one, consult the house’s plans, and cut with the care that a possible conduit demands. Passive locating covers the sweep.
Common questions
Can a sewer camera locator work through concrete?
Through the concrete, yes. Through the steel reinforcement in it, with difficulty: the peak smears and the depth reads wrong. Locate from several points and use the counter.
How accurate is a sonde locate under a slab?
Under light reinforcement, within a foot or so with careful technique. Under heavy steel, possibly not usable; cut a small inspection hole before the full cut.
What is the alternative when the sonde cannot be found under concrete?
Ground-penetrating radar, or a pilot core at the best estimate from the counter and the route.
Related guides
See sewer line locators, the locator finder, and multi-frequency locators.