Can You Run A Toilet Drain Through Floor Joist?

You're probably wondering, can you run a toilet drain through floor joist? How could you make it possible, and what are things to consider? We've researched the answers to your questions.

Yes, you can connect a toilet drain through the floor joist and down to the main sewer. However, this isn't an easy task to do. There are a lot of factors to consider for the drainage's safety and effectiveness. See the list below:

  1. House's structure
  2. Joist's quality
  3. Plumbing system
  4. Required tools
  5. Drainpipes installation

In this article, we'll discover if you can run a toilet drain through the floor joist. We'll figure out how to do it, and what are things to be aware of. It's thrilling to learn again with you today. So, let's continue reading!

Basements are often left looking like this after initial construction, Can You Run A Toilet Drain Through Floor Joist?

Can You Run a Toilet Drain Through a Floor Joist?

The toilet drain often goes directly to the main sewage system. But there are some cases where the homeowner prefers to run the drain through the joist. For instance, you can save space by allowing the drainpipes to pass through the joist. Doing so will create a network of pipelines going directly to the sewer.

However, there are factors to look for before you can make this project possible. See below these factors for more details.

1. House's Structure

A floor joist is a structure that runs horizontally across the bottom portion of the house. Its purpose is to hold the weight of the floor as well as support its durability. On the other hand, a stud or beam is a vertical structure that acts as wall framing. The stud also balances the joist, supporting weight.

Combining the joist and stud will outline the house's structure, making them perpendicular to each other. You cannot have a house if one of them is missing. So, you should check if both the joist and stud are in good condition. Drilling through the joist can affect the stud, which may result in damage. You don't want to drill through the joist if it will collapse the house's structure.

There are two ways the joist is installed. One is bridging and the other is blocking. See below for how they differ and if you can proceed to connect the drain.

Bridging Joist

Bridging uses a supporting material like metal bars that fit between the joists. This makes the drainpipe installation quite difficult, yet possible.

Blocking Joist

On the other hand, blocking uses smaller wooden planks between the joists. This results in a solid structure that looks like rectangular sections.

Check out this image below to understand how the two differ:

2. Joist's Quality

Joist's Quality - Floor Beam ConstructionConstruction Lightbox

Installing drainpipes on weak joists may result in sagging and an uneven floor surface. You can consider replacing the old joist so that it can carry the pipe's weight.

Aside from rotten or weak joists, there might be other potential risks, such as moisture buildup that will later result in mold growth. You should be aware of this, as mold can spread throughout the joists. Mold growth is due to poor ventilation and a high humidity level. Since floor joists are usually narrow, you can ask a professional to remove the mold in them.

3. Plumbing System

Plumbing System - Blue P-trap pvc for reverse odor protection. Blue sanitary P-trap installed at site construction.

A floor joist can be either beneath the ground floor or the second floor of a two-story house. No matter what the case is, an organized plumbing system is necessary. When the sewer line does not lie directly below the floor joist, the best option is to run it between the joists until it reaches the sewage system.

For instance, your bathroom is on the second floor or anywhere above the room in your house. You don't want the toilet drain to be visible in sight pointing to the room below. Rather, you want to hide it between the ceiling and the floor where the joist sits.

You should also allow a plumber to check the plumbing system, including the toilet. The plumber may have other suggestions that will suit the current plumbing system, specifically, the right pipe size.

The plumbing system is critical yet easy to understand. You need to check the electrical wires running through the joists to ensure safety. At the same time, you can ask an electrician to rearrange the wiring to make the toilet drainage safe. You might need to follow your local building codes depending on your area.

4. Required Tools

If the assessment and considerations assure you that this project is possible, you can now prepare the following tools you need. The tools may differ in your case, but you can check them below.

Wood Cutter

Use it to cut the joist to fit in the pipes.

See this wood cutter on Amazon.

Hole Saw

A tool for making holes, easily attach the appropriate size to the end of a drill.

See this hole saw set on Amazon.

Tape Measure

Use it for accurate measurements.

See this tape measure on Amazon.

PVC Pipe

It's made of plastic material and commonly used as drainpipes. Make sure you use the correct size.


This kind of pipe helps prevent the wastewater from evaporating back to the toilet area. Again, you'll ned the appropriate size for your pipelines.


It's shaped like a "T" to create a loop vent and allow air pressure to push the wastewater.

Plywood or Wooden Plank

Depending on the floor joist, use plywood or wooden planks to hold the pipes.

Nail Gun and Nails

Use nails to bond together the joist and the supporting wood piece.

See this nail gun on Amazon.

Pipe Glue

Use it to connect two pipes together.

See this pipe glue on Amazon.

5. Drainpipe Installation

Drainpipe Installation - Plumber repairs and maintains chrome siphon under the washbasin

Remember that it's best to let professionals install the drainpipes, as they have proven skills in plumbing. However, you can refer to the steps below to get an idea of how to do it:

  1. Identify the toilet's drainage hole. This will be the center where you'll start the drainpipe.
  2. Cut the joist to give space for the P-trap to pass through it. Connect the P-trap to the center hole.
  3. Drill holes in the joist a bit larger than the size of PVC pipe. This will help the pipe carry the weight of wastewater and prevent it from dislocating.
  4. Using a nail gun, nail the joist and the supporting wood block together with adjacent joists.
  5. To connect the P-trap with another pipe, simply clean the pipe's mouth and apply the pipe glue around it.
  6. Stick the pipes together and let the glue sit until dry.
  7. Connect the T-pipe to create a loop for venting. The other pipes should be higher to allow smooth wastewater flow.
  8. Cut the excess pipe to fit with each other.
  9. Run the pipes until you reach the sewer.
  10. Proceed to install the toilet bowl based on the product's instructions.

With regards to the steps on drainpipes installation, as well as the required tools, you can watch the demonstration here:

How to Install the Toilet Bowl

How to Install the Toilet Bowl - workerman performs repair of toilet bowl, To repair the pipe from the toilet

Toilet bowls have different structures. Some of them have a P-trap hidden within the structure, and some don't even have one. However, you can refer to the general steps below on how you could install a toilet bowl, especially if you're running its drain through the floor joist:

  1. Install the flange on the drainage hole using bolts.
  2. Attach the toilet bowl to the flange. Make sure the wax ring will fit properly.
  3. Lock the toilet bowl in place by twisting the nuts into the bolts.
  4. Use a hacksaw to cut the excess length of bolts, then cover them with a toilet cap.
  5. Attach the rubber gasket under the toilet tank.
  6. Fit in the tank behind the bowl, and lock it in place with bolts and nuts.
  7. Connect the water supply to the tank.
  8. Turn on the water supply to fill the tank.
  9. Flush the water to test if the drainpipes are working well.
  10. Apply silicon caulk to seal the gap between the toilet and the floor.

The last step is just optional, only if the toilet seems not laying flat on the floor. However, silicon caulk helps improve stability.

See this silicon caulk on Amazon.


It's another useful article, isn't it? We discovered that we can run the toilet drain through the floor joist, and what factors to consider. We learned a lot from assessing the floor structure to installing the drainpipes. Keep in mind that it's best to consult an expert for the safety of this project.

Are you ready to learn more with us? Then we'll be waiting for you here:

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