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Slab Leaks: How To Find A Water Leak Underground

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

When a water leak occurs inside your house it is usually easy to identify- there will either be wet areas or there will be watermarks left behind by dried water.

You might also get more severe damage like collapsed ceilings.

But what happens when the water leak is underneath the foundations of your home? How can you spot this? 

Slab Leaks: How To Find A Water Leak Underground

There are two main types of foundations for a house. A basement or crawl space underneath the house, or a concrete slab.

If your home has a basement or some kind of space underneath it that forms the foundation of the house, it usually means that the plumbing and water pipes beneath the house are fairly easy to access. 

However, these homes are more vulnerable to mold growth, gas leaks, and even mold growth.

Concrete slab foundations are more likely to be damaged by earthquakes or strong tree roots, which can in turn cause damage to the water pipes. 

There are pros and cons to each type of housing foundation, but one of the biggest issues with concrete slab foundations is a slab leak (see also ‘Beginner’s Guide To Detecting And Fixing Slab Leaks‘).

How do you identify this? What causes slab leak? And how do you fix it?

We have put together this guide to help answer those questions, so keep reading to find out more. 

What Is A Concrete Slab Foundation?

All houses need to have foundations to provide a sturdy base for the structure.

First, the builder will dig a series of trenches underneath the house to install the plumbing.

These are filled in with sand or sometimes with gravel.

The concrete is then poured on top, encasing the plumbing and creating a flat, even surface that acts as a good foundation for the house.

However, this makes the water pipes very difficult to access if there is a water leak. 

Modern houses tend to have a moisture barrier that is designed to protect the concrete in the event of a leak, but older houses will not have the benefit of this.

Water leaks are inevitable to some degree, as regardless of whether the pipes are made from copper, steel, or cast iron, they will eventually decay or erode in time. 

What Causes A Slab Leak?

Many different things can cause a slab leak. A lot of people assume that this is an issue that only affects old homes, but this is not the case.

Here are some of the most common causes of a slab leak:

Soil Shifting

 If the soil underneath the water pipes and the concrete slab shifts for any reason then this can compromise the foundations of your house and put pressure on the water pipes, causing them to crack and leak.

The soil can shift due to natural events like earthquakes, or it can be caused by simple things like water underground or erosion.

A slab leak caused by shifting soil will usually be quite severe. 


Friction between the water pipes and the surrounding gravel or sand can cause abrasion, which leads to cracks in the pipes.

The pipes might move or shake slightly due to air pressure or the power of the water running through them.

The cracks caused by abrasion tend to be quite small. 


If the water pipes weren’t installed very well when the house was constructed then abrasion is more likely to occur, along with other issues which could cause cracks.

Corrosion is particularly common in older homes with copper piping. 

Water Quality 

The water quality in your area can lead to pipe damage such as corrosion.

The normal PH of water should be 7.0 which is neutral, if the water is too hard or soft then the pipes can be compromised which leads to leaks. 

The damage caused by these issues could be limited to one small area of pipework, or it could extend to several lengths of pipe. 

How Can You Identify A Slab Leak?

Spotting a slab leak is different from spotting a leak inside your home, as the damage is underground.

However, there are some signs to look out for that could indicate a slab leak:

Pools Of Water 

If the water is leaking from the pipes it will usually rise to the surface of the ground, probably escaping somewhere around the edge of the concrete foundations of your home.

Keep an eye out for soggy patches of land or even puddles forming. 

The Sound Of Rushing Water 

If a pipe leak is quite severe and a lot of water is escaping, you might be able to hear it when you are in your home- usually on the bottom floor, though you may also hear it echoing through the plumbing in your house. 

Cracks In The Concrete 

As the house settles after it has been built, it is normal to see some cracks appearing in the concrete foundations.

However, if the crack is caused by a leak it will appear suddenly and will likely be more defined or noticeable. 

Damp Spots Or Warm Spots

If the water leak has caused a crack in the concrete foundation, the water could rise and cause damp spots on the floor of your home.

This is particularly likely if your home was built before 1980 and does not have a moisture barrier. 

If it is the hot water pipe that is damaged, you might be able to feel warm spots on your floor where the hot water is collecting underneath the house.

Try walking around your home barefoot to see if you notice anything. 

Mold And Mildew 

If you don’t notice a damp spot before it dries up, then you can keep an eye out for watermarks or even for a build-up of mold or mildew, as this will signify that there was water there previously. 

Low Water Pressure 

If there is a leak in the water pipes then this can affect the pressure of the whole system, as both water and air will be leaking out.

It also stops water from getting to the appliances that need it.

You might notice that the faucets aren’t very powerful, it takes a long time to run a bath, or your shower isn’t flowing as well as it usually does. 

High Water Bill 

If water is leaking out of the pipes then you will start getting higher water bills, as you will still be using your usual amount but you will also be charged for the leaking water.

If your water bill is consistently higher than usual and you can’t think of another reason then it could be a sign that you have a slab leak. 

If the sewer lines are leaking rather than the water pipes, this is much more difficult to spot.

You usually won’t notice it until a lot of damage has already been caused to the foundations of your home, and you begin to see problems like raised flooring.

If the problem is very severe, then the whole concrete foundation of your house could be forced up and out of the ground, causing serious structural issues. 

What Should You Do If You Notice A Slab Leak?

If you notice any of the above signs and you suspect that you might have a slab leak, you should call a plumber straight away.

There isn’t much point in trying to find the leak yourself, as this is very difficult even for trained plumbers.

What Should You Do If You Notice A Slab Leak?

They need to use special devices that help them to pinpoint the exact location of the leak before they disrupt the foundations.

These devices include infrared cameras, helium detectors, and even sonar devices to listen for the leak.

As well as finding the leak, they should also be able to get a good idea of the extent of the damage to the pipework. 

Once the leak has been found and you have an idea of how bad the damage is, the plumber will be able to advise you on whether you need a simple repair job or a more complex re-plumbing job to fix the issue. 

A simple repair is preferable, as re-plumbing involves ripping up the flooring in your home and digging through the concrete with a jackhammer.

This is a big job that will cause a lot of disruption, as well as being costly. 

If the leak is in a copper pipe then it will usually need to be replaced rather than repaired – other sections of pipe are likely to be close to cracking as copper corrodes easily.

In some circumstances, it is better to go through the hassle of re-piping than having to fix multiple leaks over a few years. 

If the pipe is repairable, the plumber will remove the damaged area using a hacksaw and then replace it, burying the pipe when they are done. 

If it is a sewer pipe that has leaked, then you will need to get a specialized plumber to assess the issue.

They will need to know how to fix the problem whilst also dealing with the human waste and ensuring that the water supply to the house and surrounding houses is clean and safe

Depending on how old the house is, the sewer line could be made from clay, cast iron, or even PVC.

There are different methods needed to remove and replace each of these types of piping.

It is usually quite expensive to fix problems with the sewer line

How Much Will It Cost To Fix A Slab Leak?

The cost of fixing a slab leak will depend on where the leak is, how severe the pipe damage is, and what method the plumber recommends to fix the problem.

Choosing the option that is the cheapest upfront is not always sensible, as you could end up spending more money in the long run.

A series of ‘quick fixes’ sometimes end up costing more than one big job to resolve the issue in the first place. 

The overall cost will also depend on what else was damaged by the water leak.

You may have damage to your garden, or if the leak affected the house you may need to replace your flooring or even kitchen cabinets and furniture. 

A lot of home insurance policies will cover you for damage to the property caused by the leak, but not the cost of actually fixing the leak itself.

Make sure you check the terms and conditions of your policy and speak to your provider if you are unsure what you are covered for. 

How Soon Should You Fix A Slab Leak?

If you find a small slab leak then you might be tempted to put off fixing it, but this is a bad idea.

One small leak could be an indication that more of the pipe is soon going to crack and you will end up with a much larger leak. 

Even a small leak if left untreated can cause severe issues, as the water can build up and cause issues with the land.

The building pressure from the water puts pressure on the concrete foundations, eventually causing them to crack.

Fixing one small leak is a lot quicker, simpler, and cheaper than having to fix a compromised concrete foundation. 

If you catch the slab leak soon enough then you are more likely to be able to use a non-invasive method to fix it.

This will cause less disruption to your home, less damage to your property, and will save you a lot of money. 

If you don’t take quick action when it comes to slab leaks, you could be breaking the terms of conditions of your home insurance by letting the damage accumulate and get worse.

This means that you might end up having to pay for it all yourself. 

What Are The Modern Methods Of Slab Leak Repair?

If you have a damaged water pipe under your concrete foundations then there are ways for it to be fixed without needing to make big holes in the concrete.

Sometimes, plumbers can dig from the edge of the foundations to reach the damaged pipe.

If this is not possible, there is a modern repair method called Lateral Pipe Lining or Trenchless pipelining.

Lateral pipe lining is the plumbing equivalent of keyhole surgery.

A very small hole is made, and specialized precision tools are used to assess the damage and fix it without taking out big chunks of the concrete foundations. 

First, a small camera is sent down the piping so the plumbers can get a good idea of how much damage there is to the pipe. 

Next, any blockages caused by debris or dirt are cleared away.

Then measurements are taken to see how much material is needed to mend the pipe. 

The plumber then mixes a resin compound that will be the correct size to fit the broken pipe, and a line is attached to the resin.

This is then sent down the pipe to the right location, and the liner is inflated to attach the resin to the damaged pipe to fix it. 

If the pipe damage is too extensive to be fixed with this method, the section of the damaged pipe is removed using precision materials and then replaced. 

What Are The Benefits of Trenchless Pipe Lining?

Traditional underground pipe methods have been around since Roman times, so, surprisingly, they are still so common in construction.

Even very basic pipe leaks can be very difficult to fix when the piping is encased in concrete.

Trenchless pipe lining is a much less invasive method of carrying out these repairs. 

It costs less, as less heavy labor is involved.

You also need to carry out less rectification work afterward as the repairs are less destructive to your home.

These repairs will not compromise your concrete foundations. 

Not all plumbing companies offer this service, but it is gradually becoming more popular and accessible. 


Once you know what you are looking for, slab leaks can be easy to spot.

You need to get them dealt with as soon as possible, and there are various methods that a plumber may use to fix the issue. 

author avatar
Anthony Barnes
Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age


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