Quick answer
A slab leak is a leak in a water supply or drain pipe that runs under or inside a home’s concrete slab foundation, where the escaping water is hidden by the concrete and often shows up first as a high water bill, a warm floor spot or the sound of running water.
Because the pipe cannot be seen, finding the leak takes testing. Repair means opening the floor, tunneling, or rerouting the line through walls or the attic.
Quick facts
The key facts about Slab Leak, with sources:
- Where it happens
- Supply or drain pipe under or in a slab-on-grade floor 2
- First home check
- Read the water meter, use no water for two hours, read again 1
- PEX under slab rule
- Continuous lengths only, no fittings under the slab 2
- Soil link
- Leaks wet expansive clay and can move the slab 4
- Repair routes
- Spot repair, reroute, or repipe 2
Key takeaways
- A slab leak is any leak in piping buried under or cast into a concrete floor.
- Pressurized supply leaks run around the clock and push up the water bill; drain leaks only leak when water goes down the drain.
- EPA’s two-hour meter test is the simplest first check for a hidden supply leak.
- Water under a slab can move expansive clay soil and with it the foundation.
- Rerouting a line overhead often avoids breaking concrete; the right choice depends on pipe age and how many leaks there have been.
What counts as a slab leak: supply lines vs. drain lines
A slab leak is any leak in a pipe buried under or cast into a concrete floor.
Many homes in warm regions sit on a slab-on-grade foundation: a concrete floor poured straight onto compacted soil.
The water service, hot and cold lines, and drains are laid in the soil first and the slab is poured over them.
The PPFA PEX design guide describes this sequence: tubing is buried in sand or pea gravel, placed under the rebar, and sleeved where it rises through the slab 2.
When one of those buried pipes leaks, you have a slab leak. Two kinds matter. A supply leak is on a pressurized line, so it leaks all the time. A drain leak is on the gravity drain system and leaks only when fixtures drain. They are found and fixed in different ways.
A leak on the hot side is the easiest to notice, because it wastes heated water and can warm a patch of tile or bare concrete above it.
Slab leak vs. foundation crack
A cracked slab is not proof of a leak, and a leak does not always crack the slab. Roanoke County’s building office notes that expansive soils move houses on their own, and that a plumbing leak under the house is one of the ways water gets there 4. Test the plumbing before blaming either.
How a slab leak starts and spreads under the floor
- Pipe stress: high pressure, fast water, abrasion or a poor joint
- Small breach in the buried pipe
- Water soaks the sand and soil under the slab
- Clay soil swells or washes out 4
- Signs above: wet floor, warm spot, high bill
The order varies. A pressurized hot line can show a warm floor in days, while a slow drain leak may go unnoticed for months until odor or settling appears.
Types of slab leaks and the sign each one gives
| Type | What leaks | Typical sign |
|---|---|---|
| Hot supply leak | Pressurized hot water line | Warm spot on the floor, water heater running often, meter moving |
| Cold supply leak | Pressurized cold line or water service | Meter moves with all water off 1; damp floor or wall base |
| Drain leak | Gravity drain or sewer line under the slab | Odor, slow drains, damp spot after use; meter does not move |
| Fitting or joint leak | A joint left under the slab | Any of the above; the PEX guide bars fittings under slabs for this reason 2 |
Example: reading the two-hour meter test
EPA’s WaterSense program gives a simple test: read the meter, use no water for two hours, and read it again. If it changed at all, you probably have a leak 1. The readings below are an example of how to interpret results.
| Reading | Start | After 2 hours, no use | What it suggests |
|---|---|---|---|
| House A | 0412.30 | 0412.30 | No supply leak found; if signs persist, test the drains |
| House B | 0412.30 | 0412.48 | Something is using water; check toilets first, then call for leak detection |
| House C | 0412.30 | 0413.90 | Large flow with nothing on; shut the main and call a plumber |
- 9,300+ gallonsWasted a year by the average household’s leaksEPA WaterSense
- 9%Of homes have leaks wasting 50 gallons or more a dayEPA WaterSense
- ~1 trillion gallonsWasted a year by household leaks nationwideEPA WaterSense
Source: [1]
Units on the dial vary by utility (cubic feet or gallons), so compare like with like. Silent toilet leaks are a common cause of a moving meter, so rule them out before assuming a slab leak.
Where slab leak clues show up: the bill, the floor and the report
The first clue is often on paper. A water bill that jumps with no change in habits is a reason to run the meter test. EPA notes that household leaks waste nearly 1 trillion gallons a year nationwide, and that the average home’s leaks waste more than 9,300 gallons a year 1.
Inside the house, look along the floor: a warm patch on tile, damp carpet with no spill, cracked or lifting flooring, mildew at the base of walls, or the sound of running water when every tap is off. Outside, a wet area along the foundation can point to a leak near where pipes enter.
On a plumber’s report or quote, slab leak work is often listed as electronic leak detection, line isolation, spot repair, reroute or repipe. Our leak detection and repair service covers each.
Why fixing a slab leak early pays off
What early repair protects
What it cannot undo
- Does not fix soil movement that has already happened 4.
- A spot repair leaves the rest of an aging pipe in place.
- Concrete and flooring work may still be needed to reach the pipe.
- Insurance terms vary; check your policy rather than assume coverage.
What the meter test and leak detection cannot tell you
The meter test only finds leaks on pressurized lines. A drain leak under the slab will not move the meter, so it needs a camera, a dye test or a drain pressure test instead. Our page on sewer camera inspection explains how that works.
Detection also gives a location, not a cause. If one copper line has failed from erosion or water chemistry, nearby sections may follow.
The Copper Development Association says velocities above 8 feet per second in cold lines and 5 in hot lines risk erosion-corrosion 3. A plumber should look for the cause before choosing a repair.
What drives the cost of a slab leak
Detection, the repair route and the flooring on top set the cost of a slab leak. We do not publish prices, because fair numbers need dated local quotes. These factors move the total:
- Detection time and method, and whether supply or drain is involved.
- Repair route: breaking concrete, tunneling, or rerouting through walls or the attic.
- Flooring over the leak, such as tile or hardwood that must be lifted.
- Pipe material and condition; a second leak may favor a reroute or whole-house repiping.
- Water damage cleanup and any soil or foundation work.
- Permits for rerouting or repiping where your city requires them.
The leak cost calculator estimates what an ongoing leak wastes, and the repipe cost estimator helps compare a full repipe.
How a plumber finds and repairs a slab leak
- Confirm a leak: Meter test with all water off 1; isolate hot and cold sides with valves.
- Locate it: Pressure testing by line, then electronic listening or thermal imaging to pinpoint the spot.
- Choose a route: Spot repair through the slab, a tunnel from outside, or a reroute above the floor.
- Repair to code: PEX run under a slab must be one continuous length with no fittings, and sleeved where it passes through the slab 2.
- Test: Pressure test the repaired line before closing anything up.
- Restore: Patch concrete and flooring, and dry the area.
Spot repair
- Break concrete at the leak
- Fix the one joint or section
- Best for a single, isolated failure
Reroute
- Abandon the buried line
- Run new pipe through walls or attic
- No floor break in most rooms
Repipe
- Replace the whole supply system
- Often chosen after repeat leaks
- Replaces buried runs with accessible ones
Water, electricity and gas lines
If water is pooling near outlets, appliances or the electrical panel, stay out of the water and shut power at the panel only if you can reach it from a dry spot.
Never cut into a slab yourself: gas, electrical and post-tension cables can run through it. Shut the main water valve and call a licensed plumber.
How to lower the risk of a slab leak
- Keep house pressure in a normal range; PNNL notes lower pressure reduces leaks and burst hoses 5.
- Run the two-hour meter test a few times a year 1.
- Keep soil moisture steady around the foundation with good drainage and even watering, the advice building offices give for expansive soils 4.
- Address water chemistry if you have had copper pinhole leaks.
- Note where lines enter the slab so you can watch those spots.
Know your shutoff
A slab leak can turn urgent fast. Learn where your main water shutoff valve is now, and make sure it turns.
Slab leak warning signs, and when to call
- Water meter moves when all water is off 1.
- A warm or hot area on a tile or concrete floor.
- Sound of running water with every fixture off.
- Damp carpet, buckled wood floors or mildew at baseboards.
- New cracks in floors or walls along with any sign above 4.
- Water heater running far more than usual.
Water you can see spreading is an emergency: shut the main and see our plumbing emergencies guide.
PEX, copper and soil rules for pipes under slabs
Installation practice for PEX under slabs: the PPI/PPFA PEX Design Guide (2nd ed., 2013) says only continuous lengths without fittings go under a slab, all connections stay outside or above it, and tubing gets a nonmetallic sleeve where it passes through 2.
Energy code: the same guide lists piping under a floor slab among hot water pipes the IECC (Table R403.4.2, as cited there) requires to be insulated 2.
Copper: CDA’s Copper Tube Handbook (CDA A4015-26/24) suggests Type M hard or Type L soft for underground water service, notes plumbing codes govern which types are allowed, and gives velocity limits to avoid erosion-corrosion 3.
Soils: Roanoke County, VA applies the 2021 IBC, which lets the building official require soil testing where expansive soils are known 4.
Permits: reroutes and repipes usually need a plumbing permit. Rules depend on the code your jurisdiction adopted; check with your building department.
Slab leak vs. related problems
| Problem | Where | Main difference |
|---|---|---|
| Slab leak | Pipe under or in the concrete floor | Hidden by concrete; needs detection to locate |
| Pinhole leak | Any copper pipe | A cause; can happen under a slab or in a wall |
| Burst pipe | Any pipe, often from freezing | Sudden, high-volume failure rather than a slow leak |
| Main sewer line leak | Drain line outside the house | Gravity drain; does not move the meter |
| Foundation settlement | Soil under the slab | Soil movement that may or may not involve plumbing 4 |
Slab leak myths
- Myth A slab leak always means breaking up the floor.
- Reality Rerouting a line through walls or the attic often avoids it.
- Myth If the meter is still, there is no slab leak.
- Reality The meter test only checks pressurized lines; drain leaks need other tests 1.
- Myth Small leaks under concrete do no harm.
- Reality Water under a house can make expansive soil move the interior 4.
- Myth New PEX under a slab can have fittings like any other run.
- Reality The PEX design guide bars fittings under slabs 2.
- Myth Fix one leak and the problem is over.
- Reality If the cause is erosion or water chemistry, more of the same pipe may fail 3.
How expansive clay soil raises slab leak risk
Slab leaks are a bigger concern where slab-on-grade homes sit on expansive clay.
Roanoke County, VA, for example, maps potential expansive soil areas from the USDA soil survey and requires soil tests before building there 4.
In those areas, steady moisture around the foundation matters as much as the plumbing. Ask your building department whether your area is mapped for expansive soils.
When a slab leak matters, and what to do next
It matters when the meter moves with everything off, when a floor feels warm, or when the bill climbs with no reason. It also matters when buying a slab home: ask about past leaks, reroutes and the pipe material.
Next step: run the meter test, then call for leak detection before choosing a repair. If the house has had more than one leak in the same pipe, compare a reroute with a full repipe. The guide below sorts four common findings.
- If Meter moving and a warm spot on the floor
- Then Likely a hot supply leak; call for leak detection
- If Meter still, but odor or damp after drains run
- Then Ask for a drain line test
- If First leak in newer pipe
- Then A spot repair may be enough
- If Repeat leaks in old copper
- Then Price a reroute or a repipe
Questions about Slab Leak
What are the signs of a slab leak?
The common signs are a water meter that moves when all water is off, a warm spot on the floor, the sound of running water with every tap closed, damp carpet or buckled wood flooring, mildew at baseboards and a water bill that jumps without reason.
Any one is worth a meter test; two or more is a reason to call for leak detection.
How do I test for a slab leak?
Start with EPA WaterSense’s meter test. Read the meter, use no water for two hours, and read it again. If it changed at all, you probably have a leak somewhere. Rule out toilets first, then have a plumber isolate the hot and cold lines and pinpoint the spot with listening or thermal tools.
What causes slab leaks?
Causes include joints left under the slab, abrasion against concrete or gravel, high water pressure, fast water that erodes copper from the inside, water chemistry, and soil movement.
The Copper Development Association warns that velocities above 8 feet per second cold or 5 hot risk erosion-corrosion. Expansive clay soils can also shift the slab and stress pipes.
Is a slab leak an emergency?
It can be. A large supply leak can flood floors and wet the soil under the foundation fast. If you see water spreading, shut the main water valve and call a plumber. A slow leak found by a meter test is urgent but not usually a same-hour emergency; book detection promptly.
Can a slab leak damage the foundation?
It can add to the risk. Roanoke County’s building office notes that the inside of a house on expansive soil can move when water gets under it, including through a plumbing leak.
Not every slab leak moves the foundation, but a long-running one on clay soil deserves both a plumber and, if cracks appear, an engineer.
How are slab leaks repaired?
There are three main routes. A spot repair breaks the concrete above the leak and fixes that section. A reroute abandons the buried line and runs new pipe through walls or the attic. A repipe replaces the supply system, often after repeat leaks. The right one depends on pipe age, leak history and flooring.
Does a hot water slab leak make the floor warm?
Often, yes. Hot water escaping under a slab can warm the concrete above it, so you may feel a warm patch on tile or bare concrete. The water heater may also run more than usual. A cold-line leak will not warm the floor, so the absence of heat does not rule out a leak.
Can PEX be installed under a slab?
Yes. The PPFA PEX design guide allows PEX under slabs if it is one continuous length with no fittings, buried in sand or pea gravel, and sleeved where it rises through the slab. All connections must be outside or above the slab. Your local code still decides what is allowed.
Should I reroute or repair a slab leak?
A single leak in newer pipe often suits a spot repair. Repeat leaks, old copper, or leaks under finished flooring often make a reroute the better buy, because it moves the line where you can reach it. Ask your plumber to price both, and compare with a whole-house repipe if the system is aging.
Sources
- U.S. EPA WaterSense, Fix a Leak Week, retrieved .
- PPI and PPFA (prepared by Home Innovation Research Labs), Design Guide: Residential PEX Water Supply Plumbing Systems, 2nd ed. (November 2013), retrieved .
- Copper Development Association, Copper Tube Handbook (CDA Publication A4015-26/24), retrieved .
- Roanoke County, VA, Office of Building Safety, Expansive Soils Policy & Procedures (revised 2/2025), retrieved .
- Pacific Northwest National Laboratory, Building America Solution Center: Service Water Pressure, retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Leak statistics and the meter test are from EPA WaterSense. Under-slab installation rules and the IECC insulation reference are from the 2013 PPI/PPFA PEX design guide (an industry publication). Copper types and velocity limits are from the CDA Copper Tube Handbook.
Soil behavior is from a county building policy (Roanoke County, VA). Pressure and leak context from PNNL. Detection methods and repair routes are described generally, without performance claims. The meter readings are an illustrative example. No prices.
Suggest a correction. We fix errors and say what changed.