# What are the signs of a slab leak?

> The signs of a slab leak, what else can cause each one, how to run a meter test, and what confirms a leak under the floor before any repair. Get an Estimate.

URL: https://levelrepipingcharlotte.com/guide/slab-leak-signs/

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A leak under a concrete floor rarely announces itself. Here are the signs that point to one, the ordinary things that cause the same signs, and the simple test that tells you whether water is leaving the house at all.

Last Updated: October 29, 2026

**Short answer:** Common signs of a slab leak are a warm spot on the floor, the sound of running water with every fixture off, a water meter that keeps moving, a jump in the water bill, damp flooring or a musty smell, and a water heater that runs more than usual. No single sign proves one; testing confirms it.

**Key takeaways:**

- Slab plumbing is common in newer Mecklenburg homes: 71% of single-family homes built in the 2000s and 82% built since 2020 sit on slabs.
- A moving meter with everything off is the most useful first test.
- A warm spot points to a hot water line; a cold line leak shows no warmth.
- Every sign has other possible causes, so testing comes before any concrete is cut.
- A reroute through the attic retires the under-slab lines instead of repairing one spot.

The signs of a slab leak are easy to miss because the pipe is under concrete. The water goes into the soil beneath the house, and what reaches you is indirect: a warm patch of floor, a sound, a meter that won’t stop, a bill that jumps. [Level Repiping Charlotte](https://levelrepipingcharlotte.com/) arranges [slab leak repair](https://levelrepipingcharlotte.com/slab-leak-repair/) and reroutes across Charlotte and the surrounding counties through licensed and insured contractors, starting with tests that confirm a leak before any floor is opened. This guide lists the signs, the other things that cause each one, and how to check whether water is leaving the house at all.

## Who has plumbing under a slab

Slab houses are common across Mecklenburg County, especially newer ones. County assessor records show 46.8% of single-family homes on slabs overall, rising to 71% of those built in the 2000s and 82% of those built since 2020. Pineville has the highest slab share of the county’s towns, at about 69%. Older Charlotte houses mostly sit on crawl spaces, where a leak drips onto the ground in plain view.

In a slab house the supply lines usually run under or through the concrete to each fixture group. They can be [copper](https://levelrepipingcharlotte.com/copper-repipe/), and in some newer houses [PEX](https://levelrepipingcharlotte.com/pex-repipe/). A leak can come from a pinhole in the pipe wall, a joint, or a spot where the pipe rubs against concrete or gravel as it expands and contracts.

## The signs, and what else causes them

Every sign below can come from a slab leak. Every one also has ordinary explanations, which is why no single sign proves anything.

| Sign | Why a slab leak can cause it | Other things that cause it |
| --- | --- | --- |
| A warm spot on the floor | Hot water leaking beneath the slab warms the concrete above it | Sunlight, a heating duct below the floor, a hot water line that is simply close to the surface |
| Sound of running water with everything off | Water escaping under pressure makes a hiss or rush | A running toilet, a refrigerator ice maker, irrigation, a water softener cycle |
| Water meter moving with everything off | Water is leaving the system somewhere | A running toilet, a dripping faucet, an irrigation leak, a service line leak in the yard |
| A sudden jump in the water bill | Water is escaping all day | Running toilets, irrigation, a leak on the service line, seasonal watering |
| Damp carpet, cupped wood or loose tile away from walls | Water wicks up through the slab | A wall leak, a dishwasher or refrigerator leak, high humidity, a roof leak |
| Musty smell or mildew near the floor | Constant moisture under flooring | Humidity, a drain leak, a past flood |
| Water heater running more than usual | It keeps reheating water that is leaking away | A failing heating element, a thermostat setting, more people in the house |
| Lower pressure or flow | Water is diverted before it reaches fixtures | Street pressure, a failing pressure valve, old pipe, clogged aerators ([low water pressure](https://levelrepipingcharlotte.com/low-water-pressure/)) |
| New cracks in flooring or walls | Water washes out soil beneath the slab | Ordinary settling, shrinkage, seasonal soil movement |

Charlotte Water says the leading cause of a high water bill is a private plumbing leak, and its list starts with running toilets and faucets. A toilet that runs silently can waste more water than a small slab leak, so rule it out first.

## The meter test

The meter test answers the first question: is water leaving the house when nothing is running?

1. Turn off every faucet, the ice maker, the washing machine and irrigation. Make sure no toilet is filling.
2. Find the water meter, usually in a box near the street, and lift the lid.
3. Watch the dial or the small leak indicator for several minutes. Note the reading.
4. If it moves, close the main shutoff at the house and check again.

If the meter stops when the house shutoff is closed, the leak is inside the house, and a slab leak is one possibility. If it keeps moving, the leak is likely on the service line between the meter and the house. The [water line ownership guide](https://levelrepipingcharlotte.com/guide/who-owns-the-water-line-charlotte/) explains why that part is the owner’s.

Toilet dye tablets or a few drops of food coloring in each toilet tank rule out the most common silent leak: if color appears in the bowl without flushing, the flapper is leaking.

## Hot side or cold side

A warm spot on the floor is strong evidence that the leak is on a hot water line, and so is a water heater that runs when no one is using hot water. A cold line leak gives no warmth. It shows up through the meter, the bill or damp flooring. Telling which side is leaking matters because it narrows where the leak is and shapes the repair.

## What confirms a slab leak

Signs suggest; tests confirm. Leak detection for a slab usually combines several methods: isolating the hot and cold systems and pressure testing each one, listening for the leak with acoustic equipment, and sometimes thermal imaging to follow a warm line under the floor. The aim is to confirm that a leak is under the slab, which line it is on and roughly where, before any concrete is cut. The [water leak detection](https://levelrepipingcharlotte.com/water-leak-detection/) page covers how that testing runs.

## What happens after a slab leak is found

There are two broad paths. A spot repair opens the floor at the leak, fixes that section and closes the floor again. It is the smaller job, and it leaves the rest of the under-slab pipe in service. A reroute abandons the under-slab lines and runs new pipe through the attic and down interior walls to each fixture, so nothing is left under the concrete to leak. Mecklenburg permit records include a slab house rerouted to move all waterlines into the attic to eliminate the lines under the slab.

Which path fits depends on the age and material of the pipe, whether this is the first leak or one of several, and what the flooring is. A first leak in otherwise sound pipe often points to a repair. Repeat leaks in old copper point toward a reroute or a [whole house repipe](https://levelrepipingcharlotte.com/whole-house-repipe/). The [repipe or repair guide](https://levelrepipingcharlotte.com/guide/repipe-or-repair/) and the [copper pinhole leaks guide](https://levelrepipingcharlotte.com/guide/copper-pinhole-leaks/) go further.

## What to do now

Shut off the main and call a plumber right away if water is surfacing through the floor, flooring is soaked, or the leak is near electrical outlets or appliances. If the only sign is a meter that creeps, there is time to rule out toilets and irrigation first and then arrange testing. Either way, write down when you noticed each sign and save recent water bills, because both help the person testing. The [repipe cost guide](https://levelrepipingcharlotte.com/guide/repipe-cost-charlotte/) covers how slab houses affect the price of a reroute or repipe. To talk through what you’re seeing, call [(704) 452-4386](tel:+17044524386).

**In this guide:**

- Who has plumbing under a slab
- The signs, and what else causes them
- The meter test
- Hot side or cold side
- What confirms a slab leak
- What happens after a slab leak is found
- What to do now

**Get an Estimate:**

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Questions

## More questions on this

### Can a slab leak be on the hot water line only?

Yes, and it is the easiest kind to notice, because hot water leaking under the floor warms the slab above it and keeps the water heater running. A cold line leak gives no warmth, so it usually shows up through the meter, the water bill or damp flooring instead. Testing the hot and cold sides separately tells which one is leaking.

### Why do slab leaks repeat in the same house?

Because the conditions that caused the first one are still there. The rest of the pipe under the slab is the same age and material, and it sits in the same soil and against the same concrete. A spot repair fixes one point, which is why some owners choose a reroute that moves all the lines out of the slab.

### How do I tell a slab leak from a leak inside a wall?

A wall leak tends to show at the wall: a stain, bubbling paint, or damp baseboard on one side. A slab leak shows in the floor: warmth, damp carpet or swelling flooring away from any wall, or nothing visible at all with the meter still moving. Some leaks sit where a line rises out of the slab into a wall, so testing settles it.

Next

## Where to go next

### The service behind this guide

- [Slab Leak Repair](https://levelrepipingcharlotte.com/slab-leak-repair/) Slab leak repair in Charlotte: the leak is found first, then fixed through the floor or bypassed with new lines run through attic and walls.

### Related guides

- Decision Guide [Should you repipe or repair leaks?](https://levelrepipingcharlotte.com/guide/repipe-or-repair/) One leak on sound pipe is a repair. A pattern of leaks, or a pipe with a known failure history, is a question about the whole system. The evidence that separates the two is usually in the house.
- Learn [Who is responsible for the water line from the meter to the house in Charlotte?](https://levelrepipingcharlotte.com/guide/who-owns-the-water-line-charlotte/) Charlotte Water draws a clear line at the meter. Here is what the utility owns, what the property owner owns, how to tell which side a leak is on, and when the service line is worth replacing with a repipe.
- Learn [What causes pinhole leaks in copper pipe?](https://levelrepipingcharlotte.com/guide/copper-pinhole-leaks/) Copper fails from the inside, usually at one spot first. The cause can be the water, the speed it moves, how the pipe was installed or the tube itself, and often more than one of those at once.
- Cost Guide [How much does it cost to repipe a house in Charlotte?](https://levelrepipingcharlotte.com/guide/repipe-cost-charlotte/) The typical Charlotte range for a whole house PEX repipe, why copper is quoted separately, how the Mecklenburg permit fee is set, and which parts of the job only a walk-through can price.
- Learn [How do you prevent frozen pipes in a crawl space?](https://levelrepipingcharlotte.com/guide/crawl-space-pipes-freezing/) Charlotte freezes most winters and sees a hard freeze every few years. Here is why crawl space and attic pipes are the ones that split, what the state code requires, and what helps before and after a cold night.