Slab Leak Detection and Access Decision

Purpose

This procedure ends a slab leak visit with three things written down: a marked point measured to two fixed references, the segment it belongs to, and a signed access decision naming open-and-repair, reroute or repipe with the criterion that decided it. The expensive mistake is not missing the leak, it is opening a slab at a point confirmed by only one method. Concrete comes up once, and if the pipe is not under the hole the customer has a hole, a dust event, and a tech guessing at the next swing.

Scope

Covers locating a pressurized potable supply leak under a concrete slab in a house, small retail space, office suite or multifamily unit, and producing the access recommendation.

Does not cover the demolition, repair or repipe execution, which the repipe of an occupied home phasing SOP owns; drain, waste and vent leaks under slab, which belong to the sewer camera inspection SOP; or the active flooding call, which is the burst pipe emergency response SOP and starts at the shutoff, not a gauge. Where the only complaint is a high bill with no warm spot, no sound and no damage, start at the high water bill investigation SOP.

Roles and the handoff

Procedure

1. Answer the electrical question first. If water stands near a receptacle, panel, furnace or appliance cord, nobody enters and the customer calls an electrician or the utility. Acceptance: a yes or no on standing water near electrical equipment, logged with a time, before meter work. Wrong: a tech already kneeling on a wet floor. Stop rule: work resumes only after a qualified person under 29 CFR 1910.332 and 1910.399 isolates the circuits per 29 CFR 1910.333(b)(2), and if nobody on site is qualified the work goes to an electrician. Hazard: this step is the hazard.

2. Quantify the loss at the meter before hunting it. All fixtures closed, ice maker and irrigation off, read the register, wait a timed interval of at least 15 minutes, read again. Acceptance: a gallons-per-minute rate from the two readings and the elapsed time, arithmetic printed. Wrong: "the leak dial is spinning", with no rate for the recheck. Stop rule: a still register proves flow below what that meter resolves, not zero, so switch to a pressure hold. Hazard: never enter a meter pit or vault, a permit-required confined space under 29 CFR 1910.146; read from grade.

3. Separate hot from cold at the heater's cold inlet. Close it and watch the meter for a full timed interval. Acceptance: the rate falls to zero, putting the leak on the hot side, or holds at the step 2 rate, putting it on the cold. Wrong: reading for 30 seconds, inside the noise of a slowly filling toilet. Stop rule: a rate that only partly falls means two paths are open, usually a recirculation return tied to the cold, so close the return and repeat. Hazard: piping here sits at tank temperature, commonly 120 to 140 F, and 140 F water scalds in a couple of seconds, so glove up and open no fitting here.

4. Prove the loss is inside the building envelope. Close the house-side valve downstream of the meter and read the register over a timed interval. Acceptance: the register stops, placing the loss on the building side. Wrong: assuming a warm floor means a slab leak while an irrigation valve weeps on the same service. Stop rule: a rate continuing near the step 2 figure with the house isolated leaves this procedure and becomes a service line locate, trenched or bored, not opened through a floor. Hazard: buried valve boxes hold wasps, spiders and broken lids, so look before reaching and turn with a key.

5. Scan for a thermal plume on the suspect side. Segment pressurized, heater running on a hot-side leak, camera emissivity set per its manufacturer's documentation, floor swept in a slow overlapping pattern. Acceptance: a surface delta recorded with both the plume center and surrounding slab temperatures, and the outline photographed against a fixed feature. Wrong: chasing a sun patch or a duct run under the slab. Stop rule: a delta under about 2 F does not clear the slab, since carpet, pad, thick tile and depth all flatten it, so go to step 6 without a thermal mark rather than declaring no leak. Hazard: this reads the floor FACE, not the pipe, so never quote it as pipe temperature; concrete cover and the internal film sit in between. It is also prolonged kneeling on a damp floor beside energized appliances and cords, so re-confirm the step 1 standing-water answer before you kneel, keep extension cords out of the wet area, and wear knee protection.

6. Confirm with a second, independent method. Acoustic on a pressurized line, or tracer gas where the line is quiet, run per the acoustic and tracer gas leak location SOP. Acceptance: the second method's peak sits within 18 in of the thermal mark, measured tape-to-tape. Wrong: calling the plume's warm center the hole, when water travels downstream through the sub-base and carries the warmth. Stop rule: more than 18 in apart, do not average and do not pick a favourite; run a third method or isolate a shorter segment. Hazard: you are kneeling over a pressurized line, so keep the gauge in sight and never raise pressure above normal static to make a leak louder, which opens the next weak joint.

7. Measure the mark to two fixed references and photograph it. Acceptance: two distances in inches with the reference named for each, plus a photo showing the mark and one reference in the same frame. Wrong: a chalk X that gets mopped, or a measurement to an island being replaced next month. Stop rule: where the room offers no two permanent references, mark the wall base and record the offset, because the repair crew works from your numbers and nothing else. Hazard: none; record the step as checked rather than leaving the field blank.

8. Scan for tendons, rebar and conduit before the access decision, not before the saw. Sweep at least 24 in around the mark, within the locator's stated depth range for reinforced concrete, and chalk every hit. Acceptance: a scanned outline drawn on the floor and photographed, with the nearest obstruction clearance in inches. Wrong: assuming conventional reinforcement because the house looks old; post-tensioned residential slabs have been common in several regions for decades. Stop rule: a tendon within 12 in takes opening off the table at that point and the recommendation moves to reroute. Hazard: a post-tension tendon holds stored energy and a cut one whips hard enough to kill, so a slab whose tendon layout is unknown is released in writing by the engineer of record, working to the Post-Tensioning Institute's repair and penetration guidance in the edition that engineer names, which binds you through the engineer's written release and through the permit rather than directly - never by this shop.

9. Write the access decision against stated criteria. Open-and-repair where two methods agree, every scanned obstruction clears by at least 12 in, the surface can be restored, and material and age do not argue for replacement. Reroute where the point sits under cabinetry, under tile the customer cannot match, or inside that clearance. Repipe where this is a repeat failure or the condition report says end of life. Acceptance: one of the three in writing with its deciding criterion, signed by the lead. Wrong: promising "we can just cut here" before the scan is read. Stop rule: no demolition is scheduled off a field opinion. Hazard: none; a desk decision, deliberately kept off the floor.

10. Put the water back and prove the building is as safe as you found it. Reopen the house valve slowly with a high fixture running, then walk every fixture. Acceptance: static within about 5 psi of the arrival value at the same gauge point, every fixture running solid water, no new drip at any valve you touched. Wrong: snapping the main open, which slams the system and drives scale into fill valves. Stop rule: a valve you closed that will not reseal is replaced today, not noted, since you introduced that failure. Hazard: repressurizing puts a tech's face at fittings, so crack it a quarter turn and stand off the plane of any joint; and if any metallic water line was cut, the bonding path through it is broken, so a bonding jumper of the size NEC Article 250 requires, in the edition your AHJ has adopted, goes on before the section is separated and stays until the permanent bond is restored; continuity across the repair is then measured and the reading recorded, not assumed, and where restoring the permanent bond is beyond your license the jumper stays and an electrician closes it out the same day.

The record this produces

Ten fields before the tech leaves: the standing-water answer with its time; the loss rate with both register readings and the elapsed time; the hot-or-cold verdict; the building-side verdict; both step 5 temperatures; the second method and the distance between marks; the two reference distances; the nearest obstruction clearance; the access decision with its criterion; and the post-restore static.

They land on the address, not the invoice line, and the office uses the loss rate for the utility adjustment request most utilities will not process without a documented rate.

One worked pass, including a failure

Single-story slab-on-grade, built 1996, copper under slab. Complaint: a warm patch in the hall and a bill roughly double last month's.

Step 1: no standing water, floor damp underfoot in the hall only, logged 08:12. Step 2: register 2214.0 cu ft, then 2217.0 cu ft after a timed 20 minutes. At 7.48 gal per cu ft, 3.0 cu ft is 22.4 gal, so 22.4 divided by 20 is 1.12 gpm, about 1,610 gal per day.

Step 3: the heater cold inlet closes and the register stops inside the 15 minute interval, so the leak is on the hot side. Step 4 still runs and confirms the loss is building-side.

Step 5: plume center 78 F against 69 F on surrounding slab, a 9 F delta, roughly 4 ft long running toward the bathroom, photographed against the hall door jamb.

Step 6: acoustic peaks 14 in from the plume center, back toward the heater. That is inside the 18 in gate, so the acoustic mark is taken, and the direction is the expected one.

Step 7: 31 in from the hall door jamb, 22 in from the linen closet trim, both photographed.

Step 8 FAILS. The locator finds a post-tension tendon 6 in from the mark, inside the 12 in clearance the step demands. Stop rule taken: opening there is off the table and nobody cuts. The tendon line is chalked and photographed.

Step 9: with opening excluded, the decision is reroute. The hot run to that bathroom group is abandoned in place and re-run overhead, and the lead signs it that afternoon with the 6 in tendon clearance as the criterion.

Step 10: the house valve reopens with the tub spout running. Static reads 62 psi at the laundry bib against 64 psi on arrival, a 2 psi difference, inside the 5 psi acceptance. Nothing metallic was cut, so no bonding jumper was required, recorded as checked. The reroute is a separate visit closing under the pressure test on a repair before close SOP.

When the site does not match this procedure

Shared meter in a multifamily building. Step 2 cannot produce a rate. Hold pressure on the isolated unit supply, record the drop over a stated interval, and say in the record that no gallons-per-minute figure exists for this address.

No working shutoff anywhere. That is its own visit under the shutoff location and labeling SOP. Do not run this procedure on a building you cannot isolate.

References

• 29 CFR 1910.333(b)(2) electrical work practices, with qualified-person scope at 29 CFR 1910.332 and 1910.399; 29 CFR 1910.146 for permit-required confined spaces such as meter pits.
• NEC Article 250, bonding of metal water piping and continuity when a metallic line is cut, in the edition your authority having jurisdiction has adopted.
• Post-Tensioning Institute guidance on penetrating post-tensioned slabs, cited in step 8, and the engineer of record.
• Infrared camera and cable locator manufacturers' documentation, for the emissivity setting in step 5 and the locator depth range in step 8.
• See related: the Plumbing acoustic and tracer gas leak location SOP, high water bill investigation SOP, repipe of an occupied home phasing SOP, and pressure test on a repair before close SOP.