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Can Two Toilets Share The Same Drain?

When it comes to installing two toilets, many homeowners ask the same question:

Can two toilets share the same drain? Yes. They can, but only when they remain on the same end of the house. Otherwise, you should forget about sharing the same drain line if the toilets are on opposite ends of your house.

Therefore, there are a few considerations you should keep in mind when connecting two toilets, either back-to-back or side-by-side. We encourage you to continue reading to find out how you share the same drain for two toilets safely and effectively.

Can Two Toilets Share the Same Drain?

Yes. Two toilets can share the same drain line if they are connected back-to-back. If two toilets are built in the opposite direction of the house, they need to have separate drain lines.

You should keep in mind that sharing the same drain for two toilets can cause serious problems, such as water backups and poor draining. In addition, it can lead to foul smells, bacteria buildup, and clogged drains.

In that sense, the waste lines should be connected to different drains, one for each toilet. In addition, you should confirm the drains are properly sized to handle the water flow from both toilets. Ultimately, it is best to consult a professional plumber to ensure the installation is done properly.

can two toilets share the same drain

Changes You May Need to Make

If you decide to install two toilets with the same drain, there are a few changes you will need to make. Here are some of these changes,

  • You must have enough space in your bathroom for two toilets
  • You need to consider the layout of the plumbing so that it can accommodate two toilets sharing the same drain
  • You may need to make additional changes depending on your existing plumbing. For instance, if you want to install two toilets side-by-side, you must extend the existing drain line to fit both toilets
  • Alternatively, if you are connecting two toilets back-to-back, you will need to relocate some of the pipes to avoid interference
  • Besides, you will need to make sure that the new toilet fixtures fit within your existing plumbing system. For example, some toilets are designed with a P-trap that is not compatible with standard sewer lines.

In this case, you have to replace the P-trap with a compatible one before installing the toilet.

The Process of Connecting The Same Drain for Two Toilets

It is not possible to use the same drain for two toilets when they are connected side-by-side. If you have decided to connect two toilets back-to-back while keeping the same drain line, follow these steps carefully,

Step 1: Mount the toilets to a 2×4 board

  1. Measure the space between the two toilets and ensure that it is at least 6” from center to center.
  2. Attach a 2×4 board to the wall for support. Place the first toilet on the floor and secure it with mounting bolts into the 2×4 board.
  3. Place the second toilet on the floor, line up the holes in the bottom of the toilet, and secure it with mounting bolts into the 2×4 board.

Step 2: Reconnect Water Supply

  1. Connect the wax rings to both toilets and press firmly until secure.
  2. Reconnect the water supply lines to both toilets after attaching seats on both toilets. This will make sure they are secure and don’t leak.
  3. Make a soil stack in between the two toilets. It should be made of ABS plastic pipe, which is about 4 inches.

Step 3: Connect the stack and the sewer pipe

  1. Connect the stack to the sewer horizontally. The stack-to-sewer connection line needs to slope downward at least 1/4 inch every foot.
  2. Install a clean-out fitting at the location where the stack transitions from upright to flat.
  3. Use a hacksaw to cut the sewer pipe at the point where it ties in with the fitting pipe.
  4. Then, attach the sanitary tee to the sewer pipe by gluing it with the help of ABS cement.

Step 4: Install The ABS

  1. Install closet flanges where each toilet is situated. If less than 6 feet, separate the toilet from the main stack and use a 3-inch flange. If not, utilize a 4-inch flange.
  2. ABS from each flang should connect to the opening of the main stack.
  3. The tube needs to slope between 1/8 and 1/4 inch every foot downwards.
  4. As the opposite end converges, make a cut into the waste heap there.
  5. Then, insert the ABS concrete into a clean tee.
  6. Glue is used to attach the tee to the pipe.
  7. The waste stack needs a coupling to be glued on it in order to be vented. Remember that the vent pipe should remain a minimum of one foot over the roofline.
  8. After that, extend the ABS pipe through the roof.

Step 5: Install the Vent

The last step is installing vent flashing all over the exposed pipe portions to seal the roof opening.

Test both toilets for proper flushing and make sure there are no leaks.

two toilets using the same drain diagram
Figure: two toilets using the same drain (source)

Note –

Depending on how the home is built and how the toilets are positioned in relation to the stack, different waste line and stack installation techniques may be used.

The tools required for the installation

When connecting two toilets that share the same drain line, you will need the following tools for installation:

  • 4-inch ABS pipe
  • ABS cement
  • 4-by-4-inch sanitary tee
  • 2 closet bends
  • 2 closet flanges
  • Clean-out fitting
  • Vent flashing
  • A hacksaw
  • 4-by-2-inch ABS reducing coupling
  • 3-by-4-inch or 4-by-4-inch sanitary tees

The Advantages Of Sharing The Same Drain For Two Toilets

Why do you need to install two toilets with the same drain line? You can consider the following benefits for the answer:

  • Cost savings: By sharing the same drain, you can save money on installation and materials.
  • Space saving: You can fit two toilets in a much smaller space than if you were to install separate drains for each toilet.
  • Improved water efficiency: When two toilets are attached to one drain, they both use the same amount of water, reducing overall water consumption.
  • Easier maintenance: With two toilets connected to one drain, it is easier to maintain and clean the system.
  • Less clutter: With two toilets attached to one drain, less plumbing equipment and accessories are needed, resulting in less clutter in the bathroom.

The 5 Problem Of Sharing The Same Drain For Two Toilets

We previously said that you would face some problems by connecting two toilets through the same drain line. Here are a few mentionable reasons:


When two toilets are connected to the same drain, the drain has to be extra large to accommodate both waste streams. Otherwise, this can cause clogging due to insufficient drainage capacity.


When two toilets share the same drain, an overflow of wastewater can happen. This is a health hazard as it can lead to the spread of bacteria and other contaminants.

Poor Ventilation

If two toilets are connected to the same drain, poor ventilation can occur as there is no way for air to escape from the toilet. This can cause unpleasant odors.

Leaking Pipes

The pipes connecting the two toilets to the same drain may become loose or damaged over time, leading to water leaks. This can cause water damage and is a major inconvenience.

Increased Water Consumption

Two toilets connected to the same drain can lead to increased water consumption as the two toilets will use the same amount of water. This can lead to an increase in your water bill.

Final Verdict

Sharing the same drain for two toilets is not recommended due to the numerous potential problems. Hence, it is much safer to keep them separate and use a T-adaptor to connect the drains instead. This allows for the water to flow in both directions and avoid any blockages or overflows.

Although it may seem easier to share the same drain, additional time will be required for the correct setup. In the end, it will depend on your house configuration and how the toilets are oriented.