bathroom with concrete floor remodel

Kendall "aka Renos"

Founder, Lead Instructor of Renos 4 Pros and Joes and Bathroom Update Guide

Adding a Bathroom to a House with Concrete Slab Foundation

Adding a bathroom to an existing home or property is a renovation project that can greatly improve the way that your home functions. This is even more so the case if the property only has one full bathroom, or if you are adding a bathroom to an area where there currently isn’t one, such as in a basement or in a master ensuite. But a concrete slab foundation adds another element of complexity to a bathroom project. Whether you are planning to add a new bathroom or remodel an existing bathroom, one of the major obstacles that property owners face is figuring out: How to add a bathroom to an existing slab foundation?

In order to add a bathroom to a house on a concrete slab, you will need to locate the existing water supply and waste lines. Then design a bathroom layout that will function properly with the existing plumbing plan (or with limited modification). Sketch the new design in place if possible, fine-tune the design, and then finalize the plan. Next, cut and remove the portion of the slab necessary to install the plumbing supply and drain lines, then repour new concrete over the new plumbing and make the floors as level as possible. Then if necessary, use a self-leveling compound to get the floor perfectly level. After leveling the floors to your satisfaction you can complete the remaining bathroom finishing steps the same as you would with any other new bathroom addition.  That’s the quick high-level answer. Keep reading as we cover each of the steps in more detail. 

Location of Existing Plumbing Supply and Drain Lines

Before you begin making grand plans, one of the first things that you want to pay attention to is where the plumbing drain lines are located. This of great importance because the new bathroom drain lines will have to somehow tie into the existing system. The same is also true for the hot and cold water supply lines. As the water supply for the new bathroom will have to connect the existing system.

Bathroom Layout Design

When planning a new bathroom or a bathroom renovation on a concrete slab, it’s really important to completely make up your mind on the new bathroom design. You want to get all of your questions answered and firmly settle on a bathroom layout before you begin to cut.

Otherwise, you may have to go through the concrete cutting process more than once. And depending on the size of your bathroom space, you may end up having to replace or re-pour most of the floor in the bathroom area. To many changes can leave your floor looking like a tic-tac-toe board.

The locations of your toilet, sink, and shower/tub are the areas of greatest importance. As a part of this process, you also need to make sure that any new walls are accurately marked in their new planned locations. In some situations, it’s actually easier to frame up any new walls before you cut the concrete as an extra assurance that everything is in the correct location. It should not be very hard to cut out the areas that pass under the walls, particularly if there is only one.

Bathroom Sketch on the Floor

Another way to help you plan and conceptualize your new bathroom is to draw it on the floor. You can use a tape measure to determine the exact locations of all of your fixtures and then mark everything on the floor with chalk.

This may come as a surprise, but a chalk sketch on the floor is as close as you’re going to be able to get to a real-life simulation of dimensions of your space.

This will give you the opportunity to stand in and walk through and make adjustments based on how it feels in real life instead of merely abiding by the building code.

slab floor that is cut for bathroom plumbing

How to Cut through Concrete: Saw Cutting Concrete Slab

Once the time comes to install the rough plumbing, you will have to remove the existing concrete that is in the bath of where the plumbing pipes need to be installed. There are three main ways to remove the concrete slab to install plumbing.

The first option is to cut the concrete with a handheld concrete saw. Concrete saws come in different sizes. The most common sizes of handheld concrete saws are  12, 14 inch, 16 inches.

You want to make sure that the radius of the saw blade is great enough to adequately score the concrete slab so that you can more easily remove the concrete. This tool works well, but be prepared for the dust. You should wear eye protection and a respirator.

This tool should be connected to a water supply to keep the blade cool and minimize the dust, however, it is still going to create dust. Additionally, be aware that you will have to bend over and or work from your knees in order to operate a handheld concrete saw.

Floor Walk Behind Concrete Saw

If you have a project that requires a great deal of cutting it may make sense to rent a walk behind concrete saw. The walk behind concrete saw is easier on the body because it allows the user to operate the saw from a standing position instead of bend over or crouch down.

The ability to stand while operating the saw will minimize fatigue on your back, knees, shoulders, and feet. Additionally, the walk-behind saw positions the user’s face and hands much further away from where the blade is actually cutting the concrete. Walk-behind concrete saws also tend to make far less dust when they cut.


casually dressed man sitting freestanding tub

Water Supply for the Concrete Saw

Regardless of which of these concrete saws you use, you will need a water supply so that you can connect the water hose to keep the blade cool as it cuts. Keeping this in mind you also have to consider and plan for where the runoff water will drain as you cut.

Using a Circular Saw to Cut Concrete

For small concrete cutting jobs, you may be able to use a handheld circular saw with a concrete blade. This can work well if you are cutting outside where dust is less of a concern. There is a good chance that you will still want to figure out a way to supply water to the saw to minimize dust and to extend the life of the blade by keeping it cool.

Removing Concrete for Plumbing

Once you have finished scoring all the concrete that lies in the path where the plumbing will be installed, it’s time to get the concrete out. There are two primary methods used for removing a scored concrete floor, a jackhammer and/or a sledgehammer.

If the concrete is not extremely hard and your score lines cut through the slab then the sledgehammer should do the trick without much trouble. If the concrete is extremely old and hard, reinforced, and or thicker than normal you may need to use the jackhammer.

The jackhammer eliminates the need to be able to swing the heavy jackhammer and requires far less physical exertion than the sledgehammer.


From a practical standpoint, it’s best to break the concrete into sections or pieces that are manageable in size. These pieces will have to be removed by hand and discarded.

Because you will be moving the pieces by hand or with the assistance of a wheelbarrow, its best not to pulverize the concrete into tiny pieces, because this will effectively create more work.

Self-leveling Concrete and Primer

Once you’ve installed your plumbing drain lines and supply lines in the slab, it’s time to pour back the concrete. For this application, it’s usually easier and faster to mix concrete in bags instead of having a truck deliver concrete.

Do your best to level the floor when you put back the new concrete. Once the concrete completely cures and dries you may notice some shrinkage. This is fine. You will have another opportunity to get the floors level.

You want to make the floors as level as possible regardless of what type of finished flooring you install over the concrete. Self-leveling concrete is an easy solution.

You mix it from a bag and pour it out in the areas that are low and then you work it with a trowel until it has spread and settled. You can always mix and add more as needed.

Sometimes you will need to do this process more than once depending on how far the floor is out of level. Remember to apply concrete primer first to the area where you pour the self-leveling concrete.

Adding a Bathroom in a Basement

When adding a bathroom in a basement, the main concern is how to get the wastewater into the existing plumbing system. If the house is on a septic tank or on a sewer line that will allow the water to naturally flow out of the house then you are in luck.

However, if the sewer line of your septic tank is actually higher than the drains in the basement, you will need to use a pump to direct the wastewater to where it needs to go.

Sump Pump Installation

A sump pump is the most common solution when plumbing drains are below the sewer or septic system. The sump is typically installed in the slab and all plumbing drain lines in the slab drain toward pump pit.

Once the water in the pit rises to a certain level, the motor on the pump turns on and pumps the water out of the pit and into the sewer line or septic line and out of the house.

Regardless of whether you cut the concrete or not, you should evaluate your current water supply capacity and determine whether existing water heater has the ability to properly serve an additional bathroom.


Macerating Plumbing  System

If you want to add a bathroom, but are unsure about cutting into the concrete slab foundation there is one other option. Macerating toilet systems grind up solid waste and pump it up into the property’s main drain system.

There are also companies that make shower pans and drains that will also eliminate the need to cut the concrete slab for a shower drain. One downside to going this route is that your options for toilets and styles of shower drain pans will be more limited because they have to be able to work with the water pumping system.


As part of planning a new bathroom, you must also determine how you will route your electrical wiring to the new space. You can route it into the space overhead, or you can bury in it in the slab in a protective sheathing.


Heating and Air (HVAC)

You will also need to plan out in advance how you will supply heating and cooling to the new space. In these scenarios where are working in an existing space, so adding a supply line from a nearby duct should not create too many issues.

However, if you are adding a new bathroom to a previously unheated space such as a basement or a garage conversion, you need to make sure up front that your existing heating and cooling system has the capacity to serve the new space.

This is particularly true if you are adding a larger bathroom.


Planning Your Bathroom

The main takeaway from this post should be the importance of planning. In a bathroom space with a wood subfloor, you have a lot more flexibility with making changes and corrections because you typically have access to the plumbing supply and drain lines before during and after the project.

But when working with a concrete slab foundation once you pour back the concrete you cannot easily make changes without quite a bit more labor and making a mess. Have a solid unwavering plan and be sure to locate where you will rent all of your tools before you start.

Buying a Diamond Blade for Concrete

One final piece of advice: When you rent concrete saws from your local tool rental, be aware that there is a good chance that the diamond blade that comes on the saw is dull.

You can save yourself time and frustration by buying your own diamond blade and installing and using it on the rental saw. Confirm the blade size you need first before you buy the blade or rent the tool.

When you are finished working with the saw, you can remove your blade and reinstall the rental blade before you return the saw to the tool rental.