Most Likely Causes (Ranked)
Homes built in the 1950s-1980s commonly have copper supply lines embedded in or under the concrete slab foundation. The copper corrodes from contact with aggressive soil chemistry, electrolysis from grounding currents, and the abrasion of the pipe against the concrete as it expands/contracts with temperature changes. Pinhole leaks develop on the underside of the pipe (in contact with soil) and may not be detectable for months until enough water saturates through the slab.
Concrete slabs shift, settle, and expand/contract with temperature and moisture changes. Pipes embedded in or below the slab are squeezed against the concrete. Over decades of micro-movement, the pipe wall thins at contact points. Earthquakes, nearby construction, and drought/flood cycles accelerate this movement. The leak develops at the abrasion point.
Copper pipes can act as electrical conductors if the home's grounding system uses the plumbing as a ground path. Stray electrical current accelerates corrosion at specific points on the pipe. This is more common in homes where the electrical system was improperly grounded to the plumbing or where nearby utilities create ground currents.
Kinked pipes, excessive flux on solder joints (flux is acidic and corrodes copper from the outside), pipes laid on sharp rocks, or insufficient sleeving through the slab all create premature failure points. These defects may take 10-20 years to manifest as leaks.
Homes with hot water recirculating systems have additional pipes under the slab that run hot water constantly. The constant heat accelerates corrosion and abrasion. These lines fail 20-40% sooner than standard supply lines because they never cool down.
How to Fix It
Spot Repair (Jackhammer Access)
$500-2,000- 1A leak detection service pinpoints the exact location using acoustic equipment and thermal imaging ($150-400).
- 2The concrete is jackhammered at that location (3x3 foot opening minimum).
- 3The damaged pipe section is cut out and replaced with new copper or PEX.
- 4The concrete is re-poured and the flooring repaired.
- 5Best for: a single, isolated leak. Not recommended if multiple leaks are likely (system-wide corrosion).
Reroute Above Slab (Best Long-Term)
$2,000-6,000- 1Instead of digging up the slab, the leaking line is abandoned in place and a new PEX line is run through the walls or attic to the fixture.
- 2PEX is flexible, corrosion-proof, and has a 50+ year expected lifespan.
- 3This eliminates the risk of future slab leaks on that line.
- 4Multiple lines can be rerouted during the same project, addressing all at-risk pipes.
- 5Best for: homes with copper under the slab that's showing signs of system-wide corrosion.
Epoxy Pipe Lining (Trenchless)
$3,000-8,000 for whole house- 1The interior of existing pipes is coated with an epoxy lining that seals pinhole leaks and prevents future corrosion.
- 2The pipes are cleaned with abrasive, then the epoxy is blown through to coat the interior.
- 3Works on both supply and drain lines.
- 4Controversial: some plumbers question long-term durability. Manufacturer warranties are typically 10-25 years.
- 5Best for: multiple small leaks throughout the system where rerouting isn't practical.
When to Call a Professional
A slab leak can undermine your foundation. If you see new cracks in walls, doors that won't close, or floors that feel uneven, the leak may have been active for months.
Turn off the water supply to the house immediately if the leak is significant (running meter, wet floor). This prevents further foundation damage.
Hot water slab leaks are particularly damaging because the warm, moist environment under the slab promotes mold and erosion faster than cold water leaks.
If you have copper pipes under the slab and one has leaked, others are likely approaching failure. Consider a whole-house reroute rather than chasing individual leaks.
How to Prevent It
Monitor your water bill monthly — a gradual increase is often the first sign of a slab leak.
Read your water meter before and after a 2-hour period of no water use. Any movement = a leak somewhere.
If your home has copper under the slab and is 30+ years old, have a plumber evaluate the system proactively.
Address stray electrical current on plumbing — have an electrician verify proper grounding.
Frequently Asked Questions
The top 5 signs: unexplained water bill increase, running water meter when nothing is on, warm spot on the floor, damp carpet or flooring, and foundation cracks or doors that stick.
Leak detection: $150-400. Spot repair: $500-2,000. Reroute: $2,000-6,000. Epoxy lining: $3,000-8,000. Foundation repair (if needed): $2,000-10,000+.
Most policies cover the resulting water damage (floor replacement, mold remediation) but not the pipe repair itself. Some policies have specific slab leak endorsements. Read your policy carefully and document everything with photos.
Spot repair: cheapest, best for a single isolated leak. Reroute: best long-term solution for copper systems with corrosion issues. Reline: best for systems with many small leaks where rerouting is impractical. Most plumbers recommend rerouting.