can a toilet and sink share the same drain

Can a Toilet and Sink Share the Same Drain? (ANSWERED)

Renovating a house needs a lot of care and effort on plumbing. Then, there may be question. Can a toilet and sink share the same drain? This is a must-face dilemma if you are going to include a new shower room in your home. As the internal fixings are complex with the prevailing drainage system, finishing the work with the least number of cuts and rebuilding is crucial. You can reduce renovation costs if you add a few modifications to the pipeline. We are going to search for the fact in this article. 

Can a Toilet and Sink Share the Same Drain?  

With the limits of the cost controls and to make the project easy most people wander around this question. All of you have got good news here! They can share the same drain. But there must be special modifications in the drainage to avoid the backflow of the combined water and waste materials. 

Connecting lines from the sinks and the toilet were not practically included in the house design in the old days. But now, these combined systems are widely in use. All the waste materials and wastewater are disposed to one main line. This line carries waste from all the houses in the scheme. Therefore, there is nothing wrong with using one line to do the transferring of both wastewater and excretory materials. The prevailing law even approves this method in the US. 

If the backflow preventers are well-established and function well, there will be no risk of the entrance of waste into your shower areas. You also need a vent to drain the water to the drainpipes of your home. These drainpipes are joined to the lateral lines, which carry the waste of the area to the metropolitan sewer lines. Vent system and gravity helps to do this process avoiding backflow. Now you should understand the theory of stopping backflow, as we discussed previously. 

After connecting the two lines in your shower room, you must install a proper ventilation system to drag the wastewater to the drain pipes.

How to Connect the Toilet and Sink to the Same Drain?

  • Every toilet consists of a closet flange (toilet flange). The purpose of this flange is to connect the wastewater line of the toilet to the drainage system. You should do a modification to it. Take a PVC closet bend of 3 inches and fix it to the end of the toilet flange. 
  • A toilet consists of plumbing traps under the floor on which the toilet is placed. You could find these very close to the surface of the flooring. Therefore, you need not spend on installing traps to the toilet waste line. The other end of the closet bend must be fixed to a 3-inch pipe that carries the waste. This pipe must be continued until it meets the main drain line. There must be a gradual slope in this horizontal pipeline. 

The slope should be ¼ inches per foot. It is recommended not to exceed this rate of slopping as it may cause a collection of waste at the point where it opens up to the main sewer line of your house. If the rate of disposing of is not equal to the rate at which the end receives wastewater, there will be a blockage in the flow. This may sometimes result in bursting the pipelines, making you extra expenses and labor. 

  • When fixing the new editions, use a PVC adhesive and tighten it permanently. 
  • At the end of the sink, you have to connect new traps at the beginning of the sink’s pipeline. You have to purchase a 2-inch trap matching for your sink, and this new part should be installed with washers at both ends. It is thoroughly advised not to use any kind of glue for this fixing.  
  • Now the PVC line should be connected to the end of the traps, and this should be connected to a vertical pipeline. A 2-inch T fitting can be used in this fixing. 
  • The drain pipe from the sink should be connected to the toilet waste line. You can use a Y connector for this purpose. Use PVC glue in the necessary positions to make the joint stable. 
  • Then attach this pipeline to the main drain line of your house.

Pros of Connecting Toilet and Sink to the same Drain

  • Installing pipelines in a household is quite challenging as the prices are sky-high. Therefore, connecting two pipes has become popular. The two main advantages you receive from this connection are reducing installation costs and material expenses. 
  • If you are adding a new bathroom to your house, combining toilet and sink drainage lines will make the work easier and less messy. 
  • Other than these, this combination makes planning easier. Rather than connecting separate lines to the main sewer line, it is easy to plan the route of combined pipelines without intersections. 

Cons of Connecting Toilet and Sink to the same Drain

  • The traps should be connected properly to stop the backflow. If there is an error in the fixing, the whole system will be affected, and there will be waste materials that enter the shower areas. 
  • Maintaining the correct slope is essential when connecting, as it may result in blockage due to unmatched receiving and expelling speeds at the joints that the combined pipelines connect to the main drainage. 
  • There may be a risk of damage in the fixing joints. If the adhesives do not connect the pipelines and adjoints well, there may be leakage. 


A toilet and sink can share the same drain. But you have to be careful of the backflow. Here you can overcome this issue with traps and vents. And you will need the service of a professional for the installation if you do not have enough knowledge of plumbing. Besides, this article discusses the all the methods in detail. You also can get to know about the pros and cons of this popular plumbing mechanism here. 

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