Slab Leak Detection - Troubleshooting

When this comes up

Slab leaks are pinhole or pipe failures in the copper, galvanized, or PEX water supply lines buried under the concrete slab of homes on slab foundations. Common in Sun Belt + Southwest where slab construction dominates (post-1970 in TX, AZ, NV, FL, CA suburbs). A single slab leak carries a real repair bill on its own, + can cause many times that amount in secondary damage if not caught. This is the systematic diagnostic.

Symptoms

Customer reports:

• Hot or wet spot on the floor
• Carpet damp without visible water source
• Hissing sound under floor when fixtures off
• Spike in water bill
• Tile or hardwood floor warping
• Mold smell from one room
• Water heater running constantly (hot-side slab leak)
• Cold-water pressure low at one fixture but not others

Severity: small pinhole leaking ounces per hour → 0.5-2 gpm major break. A water bill spiking to several times its normal amount is classic.

Diagnostic procedure

Step 1: Verify there IS a leak

1. Shut off all fixtures + appliances using water
2. Read water meter
3. Wait 20-30 minutes (verify no automatic ice maker, irrigation, water softener regenerating)
4. Re-read meter
5. Any movement = leak in the system

Where to verify: indoor only (shut main, see if leak stops) vs outdoor (irrigation, hose bibs). Most slab leaks are indoor.

Step 2: Isolate hot vs cold

Shut hot-water heater inlet:

• If meter slows or stops: leak is on hot side
• If meter continues: leak is on cold side

Hot-side slab leaks more common (heat + corrosive water accelerate copper corrosion).

Step 3: Locate by symptom

• Hot spot on floor: leak directly under it (heat from hot water radiates)
• Damp spot: leak nearby (water travels along gravel base)
• Hissing sound: stethoscope or amplifier traces direction

Step 4: Acoustic leak detection

Listen via electronic equipment:

• Ground microphone (Fluke 9090, SubSurface): heard at surface
• Pressurized listening (close valves, listen for water flow)
• A meaningful equipment investment

Acoustic finds leak typically within 12-18" of actual location.

Step 5: Tracer gas detection

When acoustic doesn't work (deep slab, multiple leaks, very small leak):

• Inject hydrogen + nitrogen mix into system
• Gas escapes at leak; sensor finds it at surface
• More expensive equipment, several times the cost of acoustic gear
• Highly accurate

Step 6: Thermal imaging

For hot-side leaks:

• Infrared camera shows hot spot on floor
• Tile + hardwood radiate heat differently; interpret carefully
• A modest camera investment, far less than tracer-gas equipment

Step 7: Pressure testing

For confirmation OR when symptoms unclear:

• Cap individual branches
• Pressurize each separately
• Identify branch with leak

Repair options

Once located, three repair strategies:

Option 1: Spot repair (most common)

1. Jack-hammer concrete at leak location (typical 2x2 ft hole)
2. Excavate to pipe
3. Repair: solder coupling, push-fit, or PEX patch
4. Pressure test
5. Repour concrete
6. Restore floor covering

Cost: the cheapest of the three options, typical. Quick. Doesn't address other potential failures.

Option 2: Reroute (around slab)

Run new pipe through attic, walls, ceiling instead of under slab.

1. New PEX or copper run overhead
2. Disconnect failing slab pipe at both ends
3. Cap or abandon
4. Test new run
5. Drywall repair where needed

Cost: somewhat more than a spot repair. More invasive but more future-resistant. Works when slab pipe is suspected to have multiple weak points.

Option 3: Whole-house repipe

For homes with multiple slab leaks or aged copper (40+ years):

1. New PEX system through walls + ceilings
2. Abandon all slab piping
3. New manifold often included
4. 1-3 weeks scope

Cost: several times a spot repair. Most invasive but permanent. Pre-1990 copper homes in active failure: this is the conversation.

Decision math

Insurance considerations

Homeowner's insurance:

• Coverage for SLAB LEAK detection: usually covered (good news)
• Coverage for slab pipe repair: varies (some include, some exclude)
• Coverage for floor + finish damage: usually covered (water damage)
• Coverage for repipe: typically NOT covered (preventive)

Customer should:

• File claim before repair starts
• Get insurance adjuster on phone before repair quote
• Provide invoice + photos to insurance
• Some insurance assigns benefits direct to plumber

Most slab-leak repair customers get partial reimbursement. Education matters.

Common pitfalls

• Misdiagnosing irrigation as slab leak: shut irrigation valve to verify
• Missing the leak by 6 ft: acoustic vs visual confidence matters
• Damaging post-tension cables: many slab homes have steel tensioning cables; locate BEFORE cutting
• No customer briefing on noise/dust: jack-hammering is loud + dusty
• No floor repair coordination: cut concrete + leave bare = customer mad
• Failing to pressure-test after repair: missed leak in adjacent fitting
• Quoting spot repair when reroute makes sense: customer disappointed in 18 months when second leak found

Customer talking points

Initial visit:

1. "We confirmed a leak: meter shows water flowing with everything off."
2. "Hot side / cold side; we'll locate now."
3. "Locating fee $X; if leak repair follows, often credited to repair cost."

After locating:

1. "Leak is at [location], [depth] below slab."
2. "Three options: spot repair $A, reroute $B, full repipe $C."
3. "Pros + cons of each."
4. "Insurance covers [X portion]; here's the documentation you'll need."

Honest recommendation. Customers respect "you should consider repipe" more than "we can spot-repair another year of leaks."

Post-repair customer education

For copper-pipe slab homes (still common):

• Water softener pre-conditions water + extends pipe life
• Acid neutralizer if pH < 6.5
• Annual water test to monitor
• Watch water bill for sudden increases

For PEX repipe customers:

• PEX outlasts copper in most water conditions
• Annual flush of manifold valves
• Filter at point of entry recommended

Locator certifications

Professional leak locators sometimes hold:

• Caspian Locator certification (manufacturer)
• ASA (American Leak Detection) franchise certification
• ASSE 7000 + 7010 (some states)

For most plumbers: acoustic detection learnable in 2-3 days hands-on. Equipment investment is meaningful for entry-level. Tracer gas is a premium tier, several times the acoustic-gear cost, for difficult cases.

References

• ASSE 7000 Standards (leak detection equipment)
• AWWA water-loss methodology
• Manufacturer locator manuals (Fluke, SubSurface, Pollard)
• Insurance industry water damage protocols