How to Combine Toilet & Sink Drains

You may be looking at renovating your existing bathroom or even installing a new one. What is certain is that when you’re doing plumbing work you need to be aware of the rules and regulations to ensure your installation complies with current regs.

For this reason, it is best to use a qualified and experienced Sydney plumber to do any job. Of course, that’s not going to stop everyone from trying to do it themselves. Indeed, if you can get a plumber to sign of on your work then you could do the work yourself.

The Bathroom Plumbing

It’s common for a single sewer line to go to the bathroom and for the toilet, sink, and shower drains to combine as they enter this sewer pipe, effectively evacuating the wastewater from your house.

The toilet drain is the largest at approximately 3 inches in diameter. This should be the same size as the sewer pipe it connects to. In contrast, the sink and shower usually have 2-inch drain pipes.

All waste pies should travel at a gradient of ¼ inch per linear foot. Of course, this is for horizontal waste pipes and not vertical ones. If the gradient is too steep the solids will stay in the pipe while the water runs ahead. This will quickly lead to issues with blockages.

Joining The Two Together

The toilet usually connects directly to the sewer pipe. Your toilet may exit at the bottom or the back and it is likely to take a 90° turn into the sewer pipe. You don’t need a trap after the toilet as there is one built into the toilet.

It will then exit the building and head to the main sewer or a septic tank.

The sink and shower drains do need to go into a trap as there isn’t one. The trap holds water, preventing smells from entering the house.

To connect the sink to the toilet you’ll want a sweeping Y connector. This is inserted into the existing sewer pipe, it is three inches in diameter. You’ll need to cut the existing waste pipe to allow you to insert this piece. The Y section needs to be at the top, allowing your sink to connect to it.

Use a standard end cap with a two-inch hole pre-installed. Your sink waste will then slot into the hole and is glued in place to create a watertight seal.

The Y section must be above the toilet waste pipe, this ensures the sink waste won’t become clogged by solid toilet waste or that it flows back into your sink!

Advantages of Combining Toilet-Sink Drains

While it is possible to connect your toilet and sink drains, you should consider whether doing so is a smart idea or whether you would be better off keeping them apart. These are some justifications:


It is more convenient to connect your toilet and sink to the same drain than to separate them. When you approach each bathroom fixture as a separate line, the planning and execution phases require more work.

Planning how each appliance’s pipes will fit throughout your home’s structure without obstructing one another is necessary to separate the sink and toilet drains. Also, you might need to consider how to incorporate additional PVC tubes into a pipe network that is already congested. The installation and planning times are reduced by connecting the drain lines for the sink and the toilet.


Using the same drain for the sink and toilet not only simplifies installation but is also cost-effective. When you consider the cost of labor and the parts required for the work, this method is less expensive than installing separate drain lines for your toilet and sink.

Final Tweaks

It’s a good idea to align everything before you start gluing anything. This helps to ensure everything is in the right place and the system will work. Remember to keep the right gradient when gluing the pipes together for good.

Then, you can always have the work checked over by a suitable plumber to ensure everything is to code and works properly.

You may use the same drain for your toilet and sink, which makes plumbing more practical, more inexpensive, and quicker. To prevent backflow and guarantee that the wastewater is properly drained, you must make sure it is done appropriately. Prioritize your drainage system before attempting to minimize expenditures because a well-functioning home has a great drainage system.