How Deep Are Plumbing Pipes Under a Slab House?

Do you plan to construct a new home? If you do, then you need to think about the plumbing. How deep are plumbing pipes under a slab house is something you should wonder at the very beginning of the process.

All pipes go underground at some point, which means you need to consider their depth before you start to work on floors and the wall. In fact, not knowing this information might even get you in a legal trouble.

Not just that, but understanding how deep are plumbing pipes under a slab house can also help you during some repairs.

Here’s everything you need to know:

How Deep Are Plumbing Pipes Under a Slab House?



Not all plumbing pipes are located at the same depth. Still, if you wonder how deep are plumbing pipes under a slab house, you’ll probably find that the usual depth is anywhere between 12 and 100 inches.

The depth is carefully calculated as the pipes should last for as long as possible. It is very rare for pipes to burst due to old age. Usually, they’ll last until some hazard occurs.

Factors that Influence the Depth

As you have probably noticed, there is a pretty wide range when it comes to how deep are plumbing pipes under a slab house. This is because there is more than one factor that determines how deep should the pipes be.

The tree most important things that determine this are:

  • The climate
  • The structure of the soil
  • Plumbing regulations

Climate changes can influence the lifespan of the pipes. If the winters are too cold, the water in the pipes might freeze, which can cause them to burst. When the pipes aren’t deep enough, you might have to thaw frozen pipes underground.

This is why in states such as California, it’s perfectly fine to place pipes 12 inches below the slab. In Alaska, on the other hand, it is advised to bury the pipes at least 80 inches in depth.

Soil is another important factor. According to a research conducted in Japan, inadequate soil is an important factor that might influence faster corrosion of metal water pipes. Depending on the structure of the soil, pipes might be installed at various depths.

Finally, all pipes must be aligned with plumbing regulations and National Standard Plumbing Code (NSPC).

Why Are Pipes Located Under the Slab?

It isn’t an accident or a preference why plumbing pipes are located under the slab. Quite the opposite – the truth is quite practical.

The main reason why pipes are kept under the slabs is protection. Slab serves as a protection from external factors such as weather and temperature changes.

Even copper pipes that tend to last for a while can burst after extreme weather changes. By keeping them under the slab, they won’t experience as much damage as if they were exposed.

Not just that, but by keeping pipes under the slab you are reducing the chance of foul smells entering your home. This is especially useful when dealing with waste pipes or pipes that carry various chemicals.

Of course, there is also a question of aesthetics. Pipes aren’t the best looking part of your property and there is no reason why they would be visible. Placing them under the slab is a great way to get the best of both worlds.

Still, there are certain occasions when it’s okay or even desired to keep the pipes above the slab. These instances are in the case of:

  • Abrasive soil
  • Moisture soil
  • Very old houses

Not all soils are suitable for pipes. If the soil is unstable, the pipes are at a greater risk than if they were exposed.

On such occasions, the floor is usually raised so the pipes can still remain hidden – although this doesn’t provide them with the same level of protection as if they were underground.

Are There Issues with Installing Pipes Under the Slab?


Just like with every other method of installing pipes, installing them under the slab has its own issues.

First off, pipes located under the slab can be difficult to reach in case of repairs. Not just that, but these repairs might end up costly due to additional work they require.

At the same time, leaks can be challenging to detect when they are underground. As such, it might take you a while to locate the problem. This will result in costlier repairs along the way.

You might also be considering some construction work above the plumbing. Will this affect the pipes? On occasional situations, it might. This is why you need to think twice before completing any upgrades or additional work.

The same can be said if you’re planning on expanding the slab. This might affect the pipes.

As such, it’s important to work with a professional contractor who can ensure everything is done correctly. The better the installation process, the fewer repairs you’ll have to deal with later.

How to Install Plumbing Pipes under a Slab House?

If you’re planning on installing pipes under a slab house, there are a few guidelines you have to follow. This is important not just for completing the work properly, but also for following the local laws.

Because of this, we wouldn’t say that installing pipes under a slab house is a DIY project. If you, however, want to do this on your own, you need to have proper instructions and manuals. Also, it takes an extreme level of caution to complete this task. This is why it should be left to professionals to complete it.

Also, it’s important to know where are the water pipes in a slab house.

Main Sewer Line

Main sewer line should always be the most important thing you’ll think about. This line connects your home’s plumbing to the public mainline. Without proper installation, your pipes won’t properly transfer the waste.

If you live in rural areas, the main sewer line connects to the septic tank and not the mainline.

Because of this, the main sewer line is typically the deepest plumbing pipe line. It is also the most challenging one to install. Even if you want to install everything else on your own, this pipe should be installed by professionals.

Drain Pipes

The next thing to consider are the drain pipes. They must be properly regulated as they carry the drains, sink water, shower, and tub water to the main sewer line.

Each of the drain pipes is connected to the main sewer line. Most of the time, they are kept entirely separated. This is to ensure the rest of the plumbing works properly if one line breaks.

In fact, these pipes are the most fragile ones, which is why it is important to be very cautionary when installing them.

Water Lines

Unlike other plumbing, most water supply lines run right above the stud walls. Also, they are usually made out of PVC. This is because they need to be very resistant and durable, but they are also fixed and fitted under the ground.

These lines are usually fixed by a plumber and not by a construction worker. At the same time, they are connected to the main supply line to help continue the flow.

As such, usually professionals will first construct the rough frame before installing water lines.

Rough Ins

Once the lines are installed, the surrounding region is filled with sand and compacted. This is to protect the pipes and to keep them safe from the soil. Also, this way the pipes remain hidden.

Usually, a professional will build a concrete square-foot small box. Sometimes, this box can be built from sand or soil, as well, but this isn’t a preferred method.

This box will serve as a protection and keep the entire region safe from external factors.


Next, you need to complete the concrete pouring, which will complete the slab. Usually, you’ll need to cover the entire area in concrete – or rather, almost entire.

The shower stab is usually left exposed. This is something a plumber should manually cover once they connect it to the drain line, as pouring concrete on top of it can be risky. Just one wrong drop might lead to blockages.

Recognizing Slab Foundation and Plumbing Problems


Most issues that might arise with plumbing under the slab occur right after construction and they are the result of improper installation. Chances are your pipes might not be working properly, or you might experience a leak.

Some of the most common signs of an arising problem include:

  • Foul odor
  • Low water pressure
  • Cracks on the wall
  • Spots on the floor

Most of these signs are the result of water leaking to your floor. As water has nowhere to go due to being confined, it will leak on your floor.

If you notice any of these signs, chances are you’re having problems with the plumbing beneath the concrete. Contact a professional who might help you locate and fix the issue.

Repair Cost

Unfortunately, many of repairs that need to be done to pipes under a slab can be quite expensive. Whether you’re having water coming up from the ground after heavy rain or a low water pressure, the costs might be quite high.

For example, the leak detection usually costs $200 to $300. However, you also need to include the cost of slab repair, which is usually $2,000 or more. If the slab is severely damaged, the costs can be as high as $7,000!

Also, you need to add the costs of repairing the issue itself. A waterline leak repair usually costs between $300 and $2,000, and some other complications might end up even more expensive.

As you can see, these costs can pile up. Sometimes, you might be looking into $10,000 worth of repairs!

This is why it’s essential to act quickly. The sooner you detect the problem and start working on it, the fewer repairs you’ll have to pay for.

Replacing Pipes in Slab Foundation



Not all damages require replacing the pipes. Many can be dealt with by simply fixing the problem.

However, if you have an older home, your pipes might be nearing the end of their lifespan. This is especially true with properties that use galvanized steel or cast iron for their pipes, as they are more likely to decay.

A professional should examine the damaged pipe and determine the size of the problem. If the integrity of the pipe is thin and decayed, replacements are usually advised. When this happens, you’ll need to remove a portion of the slab for the plumber to reach the pipe.

A less invasive method might be relining the pipes with CIPP, or Cured-in-Place Pipe.

CIPP isn’t as invasive as standard pipe replacement. It doesn’t require removal of a huge amount of slab, and it might end up being less expensive than standard replacement.

The process begins by inserting a camera into a pipe. This will help locate the breakage and inspect the pipes from inside.

Then, the plumber will use a high-pressure water device to clean the pipe. Sometimes, they might use mechanical cleaners or wires.

Finally, they’ll seal the pipes with a rather thin epoxy-impregnated felt liner. This will seal any holes while also strengthening the pipe walls.

Read Next: Slab Leak Signs

The Bottom Line

If you’re starting any construction project or are just curious, you need to know how deep are plumbing pipes under a slab house.

Unfortunately, there isn’t a simple answer, as the depth might depend on many things. If you already have a property and want to know where the pipes are located, you should look into the building plans. At the very least, try consulting the plumbing diagram for house on slab to get an estimation.

Most repairs require a professional, so don’t attempt to fix this on your own – unless you are extremely skilled in tinkering and plumbing.