A slab foundation is a strong, flat layer of concrete that serves as the base and floor of a building. Though concrete is extremely durable, it isn’t invincible. Poor construction, shifting soil, aging pipes, and other factors can cause foundation cracks and slab leaks.

In this guide, we’ll cover some of the most common signs of a slab leak. Early detection can help you avoid structural damage, expensive repairs, and reduced property value that often accompany foundation issues.

---

What Is a Slab Leak?

Often, a slab leak occurs when the plumbing pipes beneath a home’s concrete slab foundation—mainly copper water lines—corrode or otherwise deteriorate and leak water. This type of leak is specific to homes built on slab foundations.

If you suspect a slab leak, you need to investigate and address the issue right away. Unresolved leaks can cause significant structural damage and mold growth that can be expensive to fix.

Some piping materials are more prone to corrosion and leaks than others. To learn more about the different options available, check out the video below with plumbing expert Richard Trethewey:

---

What Are the Signs of a Slab Leak?

There are several signs that plumbing problems are lurking beneath a slab, including high water bills, wet spots on the floor, cracks in the walls, and unexplained mold or mildew growth. Watch for the following warning signs to avoid severe consequences of concrete slab leaks.

Cracks in Walls or Baseboards

As moisture from a slab leak penetrates the floor from below, it can cause visible cracks on the lower sections of interior walls or baseboards. These cracks can vary in size, though they often start as minor hairline fractures that widen over time.

A structural issue can also lead to wall cracks, so you should always hire a professional to evaluate them.

Hot Spot Areas on Your Floor

Imagine walking through your home barefoot and feeling warm or heated areas in random spots on the hardwood floor above your foundation. Known as hot spots, these localized sections typically align with the path of a leaking hot water pipe.

Not only do you need to be concerned about a leaking hot water line, but prolonged exposure to hot spots can be dangerous for young children and pets and may cause skin irritation or burns.

Low Water Pressure

If you can’t rinse the shampoo out of your hair in the shower or your dishwasher takes a long time to fill with water, you may have low water pressure.

Water escaping from a broken pipe beneath your slab can reduce water pressure throughout your home’s plumbing systems. The same can happen with clogged or damaged pipes, making it difficult to complete daily tasks.

Sounds of Running Water

If you hear the sound of running water, but the shower, sink faucets, dishwasher, and washing machine aren’t running, there could be a leak under your slab. The sound may be intermittent or continuous, but its persistence will likely give it away. 

Listen carefully to find the source, as a leak can be anywhere there are pipes, including behind drywall. Watch your water meter, too, to see if it continues to show water usage despite having all your water sources turned off.

Standing Water Around Your Home

If you have a slab leak, you may observe water pooling outside your home near the foundation or exterior walls. Standing water in basements or puddles in garages are other common signs.

While standing water is a concern, excessive rainfall can also create pooling. To differentiate between rainwater and a potential slab leak, look for standing water that remains for days after dry weather.

Sudden Increase in Your Water Bill

The average monthly water bill in the United States is $39.16, based on the latest data from World Population Review. Your bill may be higher or lower based on your location and water usage, but make note of your average monthly total.

While you can expect your bill to go up when enjoying summertime activities such as gardening or cooling off in the sprinkler, a significant or unexpected increase in your utility bills is worth investigating. Household leaks in your home, including beneath your slab, can waste as much as 90 gallons of water per day, according to the U.S. Environmental Protection Agency (EPA).

Unpleasant Mold or Mildew Smell

Smells related to cooking and pets can pop up in a home, though they fade with time. A musty or earthy smell can signal the presence of mold or mildew—a dangerous side effect of slab leaks.

Unlike temporary odors, the smell of mold or mildew is persistent and can become distinct and unpleasant. Since mold and mildew can pose health risks, reach out to a professional right away if you notice this smell and suspect a slab leak.

Wet or Damp Carpets

If areas of your carpet feel damp or wet to the touch, even when there hasn’t been a recent spill, you may have a slab leak. These moist patches are often found near walls or baseboards since the water seeps upward into the carpet from underneath.

One-time spills or pet accidents dry and don’t return. A persistent dampness that reappears despite drying can indicate an ongoing issue.

---

What Causes Slab Leaks?

Understanding the following common causes of slab leaks can help you take preventive measures and address potential issues before they escalate.

• Abrasion: Debris or sediment carried in the water can erode pipes, eventually causing leaks.
• General corrosion: As pipes age, chemical reactions can cause them to corrode, weaken, and eventually leak. Copper and galvanized steel pipes are especially susceptible to corrosion over time.
• High water pressure: Extremely high water pressure puts an added strain on the plumbing system, weakening the pipes.
• Improper installation: A poorly installed plumbing system may have loose connections beneath a slab. Low-quality pipes are also more susceptible to leaks.
• Shifting foundation: Soil movement, seismic activity, and even tree roots can put tremendous pressure on the slab and the pipes beneath. 
• Water acidity: Highly acidic or alkaline water can accelerate the corrosion process in metal pipes. This problem can be caused by rainwater, soil, pollution, or decaying organic matter.

Though modern PEX plumbing is more resistant to many of these problems, no pipes are completely immune to slab leaks. Improper installation or external factors can still lead to damage.

---

What To Do If You Suspect a Slab Leak

Slab leaks won’t fix themselves, and the longer they go on, the worse the damage becomes. If you suspect a slab leak, follow the steps below as quickly as possible:

1. Turn off the water. If you observe significant water intrusion or hear a strong rushing water sound, shut off your main water valve to prevent further water flow and damage. You may be able to skip this step for smaller or less obvious leaks.
2. Document the evidence. Take photos of any visible signs like damp spots, cracks, and water meter readings. This can be useful for both the plumber and potential warranty or insurance claims.
3. Contact your insurance. Many homeowner insurance policies cover slab leaks, but terms vary. Check your coverage and file a claim if applicable. Home warranties typically exclude slab leaks, but it’s worth reviewing your policy to find out.
4. Call a professional plumber. Slab leak repair is not a DIY job. Contact a reputable plumber with experience in slab leak detection and repair. If you aren’t dealing with a true emergency, gather quotes from multiple plumbers for comparison.

If the leak is small and easily accessible, plumbers may jackhammer through the concrete in that area, cut out the damaged pipe section, and replace it. As a less invasive option, they may clean the existing pipe and insert a resin-coated lining to seal the leak from within. Alternatively, they may keep the slab intact and reroute the affected pipe through the attic or wall.

The cost of slab leak repair depends on the nature of the leak and the method used to fix it. Most homeowners pay between $630* and $4,400, but costs can go as high as $6,750. If the slab leak was caused by a sudden event, such as a tornado or freeze, it may be covered by your homeowners insurance. Some home warranty companies may also cover slab leaks, depending on the cause. You can read our article on the best home warranty companies for plumbing coverage to learn more.

* Cost data sourced from contractor estimates used by Angi.

---

How to Prevent Slab Leaks

Slab leaks can’t be entirely prevented, but you can take steps you can take to minimize the risk:

• Regular inspections: Have a licensed plumber inspect your plumbing system annually for signs of aging pipes, corrosion, or small leaks.
• Managing water pressure: Install a pressure regulator on your main water line to ensure water pressure stays within a safe range—usually below 80 psi.
• Drainage and soil modifications: If you live in an area with expansive clay soil, ensure proper drainage away from your foundation. Soil modification and root barriers might also be helpful.
• Quality materials: When building or renovating, opt for corrosion-resistant piping like PEX and professional installation with proper pipe support and protection.
• Vigilance: Pay attention to potential warning signs like unexplained increases in your water bill, damp spots, or cracks in your foundation.

Note that even with these measures, slab leaks can still occur due to unforeseen events or aging infrastructure.

---

Our Conclusion

Knowing the common signs of slab leaks, including low water pressure, cracks in the walls, and unexplained high water usage, can help you detect issues early. The longer a leak goes unnoticed, the more damage it will cause and the more expensive it will be to fix. We also recommend investing proactive measures, such as routine inspections and proper drainage, to protect your finances and peace of mind. 

Contact a professional plumber immediately if you suspect a slab leak or notice any concerning signs. Early detection can help minimize the cost of repairing a slab leak.

---

FAQ About Signs of a Slab Leak

How common are slab leaks?

Slab leaks are a common problem in areas with older homes built on concrete foundation slabs. Routine maintenance can help reduce the risk of slab leaks, so keep up with regular visual assessments and routine plumbing inspections, especially if you have an older home.

What are the dangers of a slab leak?

Slab leaks can pose many dangers. They can weaken a home’s foundation and lead to structural instability. Potential mold and mildew growth present health risks, while costly repairs and a reduction in property value can have serious financial consequences. 

How long can a slab leak go undetected?

A slab leak can go undetected for days, weeks, or even months. A large leak or ruptured pipe can cause immediate water damage that’s difficult to overlook, but smaller leaks can be no more than a drip, making them difficult to detect. 

What is the best way to repair a slab leak?

The best way to repair a slab leak is to hire an experienced plumbing company. Repairing a slab leak involves identifying the part of the pipe that’s leaking, gaining access to it, replacing the damaged materials, and repairing the foundation, including any cracks in the concrete. We recommend leaving this job to a professional.

To share feedback or ask a question about this article, send a note to our team at reviews@thisoldhousereviews.com.