Key Takeaways
- A slab leak is a pressurized water line leaking under your concrete foundation — it wastes water 24/7 and undermines the slab.
- Top signs: hot spots on the floor, an unexplained water-bill spike, the sound of running water, low pressure, mold, and new floor/wall cracks.
- The meter test confirms it: if your water meter still spins with the main valve off and no water running, water is escaping somewhere.
- Detection typically costs $150–$400 and takes 1–3 hours — acoustic listening, thermal imaging, or tracer gas pinpoints the leak.
- Insurance often covers the resulting water damage, but not the pipe repair itself. Check your policy before work starts.
What Is a Slab Leak?
A slab leak is a leak in a pressurized water line running beneath (or within) your home’s concrete slab foundation. Unlike a visible pipe leak, it hides — eroding soil under the slab, wicking moisture upward, and running your meter around the clock until it’s found.
Denver’s expansive bentonite clay soil makes this a local specialty: the soil swells when wet and shrinks when dry, stressing buried copper lines until pinholes form. Add freeze-thaw ground movement, and slab leaks are one of the most common serious calls we run.
What Are the 6 Signs of a Slab Leak?
No single sign proves a slab leak — but two or more together mean it’s time for professional detection.
1. Hot Spots on the Floor
A warm patch on a tile or wood floor — especially with no heating vent nearby — is the classic slab-leak tell. Hot-water lines leak most often (heat accelerates copper corrosion), so the floor above literally warms up. Walk barefoot; you’ll feel it.
2. Unexplained Spike in Your Water Bill
A bill that jumps with no change in usage means pressurized water is escaping. (Note: this applies to supply lines, which are metered — unlike sewer lines.) Denver Water’s online portal lets you track gallons per day — a sudden sustained rise is your early warning. Turn on usage alerts if you haven’t; they catch leaks weeks before you’d notice.
3. The Sound of Running Water When Nothing’s On
Hearing water moving inside walls or under floors with every fixture off is never normal. Stand quietly near bathrooms and the water heater’s wall — a faint hiss or rush points at the leak’s zone.
4. Low Water Pressure
A leak bleeds off pressure before water reaches your fixtures. Diagnose it: if pressure dropped at one fixture, it’s local (aerator, valve). If it’s everywhere, suspect the main — or a slab leak. Ask a neighbor: if their pressure is fine, the problem is inside your property line.
5. Mold, Mildew, or Musty Smells
Slab moisture wicks upward and feeds mold under flooring and inside wall bases. Priority checklist: musty smell with no visible source → check baseboards and flooring edges near plumbing walls; warping laminate or buckling wood → moisture is coming from below; don’t just bleach it — mold returns until the moisture source is fixed. If growth is extensive, an IICRC-certified remediation firm handles it after the plumbing repair.
6. New Cracks in Floors or Walls
Water eroding soil under the slab lets sections settle — and settling cracks concrete and drywall. Monitor cracks: mark the ends with pencil and date them. A crack that grows over weeks alongside any sign above is urgent.
How Do I Confirm a Slab Leak? The Meter Test
Turn off every fixture, then watch your water meter: if it still spins, water is escaping.
- Shut off all faucets, appliances, and irrigation.
- Find your water meter (usually in a ground box near the curb).
- Watch the low-flow indicator (small triangle or star) for 10–15 minutes.
- Still moving? You have a leak. Next: shut off your home’s main valve — if the meter stops, the leak is inside your property; if it keeps spinning, it’s in the service line to the street.
Escalation rule: meter spinning with the main off and no water running = call for professional leak detection now, not next week. Every day is hundreds of wasted gallons plus slab erosion.
How Are Slab Leaks Detected?
Pros pinpoint slab leaks with acoustic listening, thermal imaging, or tracer gas — detection typically runs $150–$400 and takes 1–3 hours.
- Acoustic equipment amplifies the hiss of pressurized water escaping — effective through concrete.
- Thermal imaging spots the temperature anomaly of a hot-line leak.
- Tracer gas (a safe hydrogen-nitrogen mix) is pumped into the line; the gas escapes at the leak and is detected at the surface — the most precise method for tricky cases.
You get a marked location, not a demolished floor. Detection first, concrete second — always.
What Are the Repair Options?
| Option | What it involves | Best when |
|---|---|---|
| Spot repair | Open the slab at the leak, fix the section, repour | Single accessible leak, newer plumbing |
| Reroute | Abandon the under-slab line; run new pipe through walls/attic | Recurring leaks, hard-to-access location |
| Whole-home repipe | Replace all supply lines (often with PEX) | Aging system, multiple leaks, polybutylene or failing copper |
The honest guidance: a single pinhole in an otherwise young system = spot repair. The third pinhole in five years = stop patching and repipe. Each slab opening costs money and disruption; at some point the repipe is cheaper than the next three spot repairs.
Does Homeowners Insurance Cover Slab Leaks?
Often partially: the resulting water damage (flooring, drywall, mold remediation) is commonly covered, but the pipe repair itself and slab access usually are not.
Ask your agent specifically: Is “sudden pipe rupture” covered vs. “long-term seepage” (often excluded)? Are access costs (breaking concrete to reach the pipe) covered? What’s your deductible relative to the repair cost?
Document everything — photos of damage, the detection report, all invoices. Insurers pay documented claims faster.
Frequently Asked Questions
Can a slab leak fix itself?
No. Pressurized leaks only grow. Mineral buildup can slow a pinhole temporarily, but pressure reopens it — usually at 2 AM.
How long can a slab leak go undetected?
Months. Slow leaks show up first as a creeping water bill or faint mustiness. That’s why usage alerts and the meter test matter — they catch what your eyes can’t.
Will a slab leak make my foundation collapse?
Collapse is rare; settlement and cracking are common. The real costs are water waste, mold, and ruined flooring — all of which grow with time.
Should I turn off my water if I suspect a slab leak?
If the meter test confirms a leak, yes — shut the main off when water isn’t needed (overnight, while at work) until it’s repaired. It stops the waste and limits damage.
Copper vs. PEX for slab-leak repipes?
PEX, routed overhead. It resists the corrosion that causes copper pinholes, expands slightly under freeze pressure, and — routed through walls/attic instead of the slab — never puts a pressurized joint under concrete again.
Suspect a slab leak? Professional Plumbers Denver pinpoints them with acoustic and tracer-gas detection across the metro — marked location, straight repair options, upfront pricing. Call (720) 390-2652, 24/7.
