# Signs of a Slab Leak Under Your Foundation | Comfort Pros

> Water bill spikes, warm floor spots, running water sounds, foundation cracks and mildew odor - what each means, and why waiting multiplies the repair cost.

URL: https://www.comfortprosaz.com/guide/signs-of-a-slab-leak/
Last-Modified: 2026-08-25

![Hand resting on a tile floor with a visible damp patch near a baseboard](/images/featured/hand-resting-on-a-tile-floor-with-a-visible-damp-p.webp)

Explainer 5 min read

# Signs of a Slab Leak Under Your Foundation

Water bill spikes, warm floor spots, running water sounds, foundation cracks and mildew odor - what each means, and why waiting multiplies the repair cost.

## What a Slab Leak Actually Is

In slab-on-grade construction, the water supply lines run through or beneath the concrete foundation. When one of those lines develops a leak, the water has nowhere obvious to go. It saturates the soil under the slab, wicks up through the concrete, and works its way into flooring and wall bases.

There is no dramatic moment. It is a slow failure that announces itself indirectly, which is exactly why so many of them run for months before anyone acts.

If you already know you have one, 

emergency plumbing service

[/emergency-plumbing/ →](/emergency-plumbing/)

 covers what happens next, including how we locate it before opening any concrete.

## The Warning Signs, and What Each One Means

**A water bill that jumped without an explanation.** The most reliable early indicator. A leak running continuously under a slab moves a surprising volume of water, and it shows up on the bill before it shows up anywhere else.

**A warm spot on the floor.** This one is specific to hot water line leaks and it is close to conclusive. If an area of tile or flooring is noticeably warmer than the surrounding floor and there is no heating source under it, hot water is escaping beneath the slab there.

**The sound of running water with everything off.** Stand in a quiet house with every fixture closed. A faint continuous hiss or rush, particularly audible near a floor or a wall base, is water escaping under pressure.

**Damp patches, discoloured flooring or lifting tile.** Water wicking up through concrete. By this stage it has been running for a while.

**A mildew or musty smell** without a visible source, especially near floor level. Moisture trapped under flooring.

**New foundation cracks, or doors and windows that have stopped closing properly.** Saturated soil moving under the foundation. This is the expensive stage.

**Reduced water pressure** across the house, as supply is diverted through the leak.

![Technician using acoustic leak detection equipment on a residential floor](/images/content/technician-using-acoustic-leak-detection-equipment.webp)

> **The meter test settles it**
> 
> Shut off every water-using fixture and appliance, note the low-flow indicator on your meter, wait half an hour without using water, and look again. Movement means water is escaping somewhere.

## Why This Housing Fails This Way

Slab leaks are not randomly distributed. They cluster in specific housing generations for specific reasons.

**Mesa and the East Valley’s 1970s and 80s stock.** Slab-on-grade construction plumbed with galvanized supply lines and early soft copper run under the concrete. Both develop pinhole failures with age, and Mesa produces more slab leak calls than anywhere else we serve.

**Hard water accelerates it.** Maricopa County water at 15 to 25 grains per gallon works on pipe interiors continuously.

**Soil movement.** Expansive soils in parts of the valley move seasonally, and pipe under a slab has nowhere to flex.

**Thermal cycling on hot lines.** The hot water line expands and contracts with every draw, and decades of that against unyielding concrete produces wear at contact points.

**Pre-1980 Santa Monica and Westside housing** shows a comparable pattern for the same reasons.

## How We Find It Without Demolishing Your Floor

This is where the horror stories are avoided. We locate first, then open concrete, and only at the leak.

**Acoustic detection.** Sensitive ground microphones listen for the sound of pressurised water escaping through concrete. Surprisingly precise once ambient noise is controlled for.

**Thermal imaging.** Picks up the temperature signature a hot water leak creates across a slab. Fast and effective on hot lines.

**Line isolation and pressure testing.** Confirms which circuit is leaking before anyone starts looking for where.

Only then does concrete open, typically a small area rather than a trench.

| Approach | Concrete opened | Typical outcome |
| --- | --- | --- |
| Locate acoustically, then open | Small, targeted | Repair completed, minimal restoration |
| Open and search | Large, sometimes repeatedly | Substantial restoration cost |

## Repair or Reroute

Once we find it, there are two honest options.

A **spot repair** opens a small area, replaces the failed section and patches. Right for a single leak in otherwise sound pipe.

An **overhead reroute** abandons the under-slab run entirely and routes new pipe through the attic or walls. This costs more up front on a single leak, and frequently costs less over five years when the pipe is early soft copper or polybutylene, or when the same line has leaked before. No further concrete work, ever.

We show you both numbers rather than defaulting to whichever is a bigger job. 

What repairs cost

[/guide/emergency-plumbing-cost-arizona-california/ →](/guide/emergency-plumbing-cost-arizona-california/)

 has the ranges.

## What to Do Right Now

1.  Run the meter test to confirm water is escaping
2.  Photograph any damp areas, flooring damage or cracking before touching anything
3.  Notify your insurer early rather than after the repair
4.  Call for detection, not for demolition
5.  If water is actively pooling, close the main shutoff. 
    
    Burst pipe first steps
    
    [/guide/burst-and-pinhole-pipe-leaks-first-ten-minutes/ →](/guide/burst-and-pinhole-pipe-leaks-first-ten-minutes/)
    
     covers that sequence.

The one thing not to do is wait and watch. This is a failure whose cost compounds weekly.

FAQ

## Common Questions

### How do I confirm I have a slab leak?

Do the meter test. Shut off every fixture and appliance that uses water, including ice makers and irrigation, then look at your water meter and note the position of the low-flow indicator, usually a small triangle or dial. Wait fifteen to thirty minutes without using any water and check again. If it has moved, water is escaping somewhere in the system. That does not prove it is under the slab, but combined with a warm floor spot or the sound of running water with everything off, it is strong enough to act on.

### Does insurance cover slab leak repair?

It varies by policy, but the common pattern is that the resulting water damage and the cost of accessing the pipe are covered while the pipe repair itself is not. Documentation matters a great deal to that outcome. Photograph everything before any cleanup, notify your insurer early rather than after the work, and keep the leak detection report. We provide findings and photographs as standard for exactly this reason.

### How urgent is a slab leak?

Very. Water escaping under a foundation saturates the soil beneath it, and saturated soil moves. That movement produces foundation cracking, door and window misalignment and tile separation, all of which cost far more to remedy than the plumbing repair does. Meanwhile the water is wicking into flooring and drywall, and in a warm house that becomes a mould problem within days. The cost of a slab leak climbs every week it runs.

## Need help with emergency plumbing?

No pressure either way. If you would rather just talk it through with someone who works on these systems every day, the phone lines go straight to our teams.

Emergency Plumbing

[/emergency-plumbing/ →](/emergency-plumbing/)

 

Contact us

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