How To Rough-in A Double Sink Vanity: Step by Step Guide

A double vanity sink is a practical and beautiful bathroom solution for couples and families. The cost of professional installation can add up quickly and it is easy enough to rough in the sinks yourself. It’s actually much easier than it seems and doesn’t take long to do. 

Wondering how to rough in a double sink vanity yourself? As long as you have the drain assemblies and necessary fittings (drain elbows, tee, p-trap, extension, trap adapter, double shut-off valves and mesh faucet hoses) it is straightforward to get supply pipes and drain pipes to accommodate two sinks.

Know More About Rough-in Dimensions And Sink Parts

Before we jump into the step-by-step guide, it’s important to be aware of the key factors that impact a rough-in installation. Inspection of rough-ins is common so getting the dimensions wrong could be the difference between a pass and a fail in an inspection. As well as making sure the piping and dimensions are up to standard, take a look at local plumbing codes for rules on venting and draining double sinks. In some cases, both sinks will need their own vents to meet local codes. 

Dimensions and Popular Terms

The dimensions will depend on the specific line, fixture or discharge being installed but here is a rough guideline:

  • Horizontal supply line: 4 inches to the side (one to the left and one to the right side) of the center
  • Vertical supply line: 2-3 inches higher than the drain pipe
  • Vertical discharge: 16 – 20 inches
  • Front buffer of fixture: 21 inches+
  • Side to side buffer of fixture: 15 – 20 inches+


The centerline is the center of the part you are installing. When you are installing two pipes for your double sink for example, the centerline might be 10”. This means the two pipes need to 10” apart (measuring from the center of each pipe). This is important to know as measuring from the center line of the sink or the pipe is going to produce vastly different results than measuring the edge of the sink or the pipe. 

As plumbing is inspected, if your measurements fall short you’ll need to re-do the project so it is up to standard. A silly mistake like not paying attention to the centerline could cost you a lot of time and money. Be patient with your project, check the local codes and measure twice. 

Base Floor Height/Subfloor/Finished Flooring 

This is the floor height when construction is complete but the finishes are not yet applied. The base floor is the rough floor that is laid before the final flooring material is in position.

Sink Rough-in Parts

  • Supply Line: The supply line is two holes placed either vertically or horizontally. The positioning will impact their location and distance. Vertical supply lines need to be above the drainpipe by 2-3”. Horizontal supply lines need to be 4” to the side of the centerline, this means one hole is 4” to the left of the centerline and the other is 4” to the right. 
  • Discharge hole: This is a vertical hole position 16-20” above the floor.
  • Drain pipe: This is usually the main vertical drainpipe and is often used as the centerline.
  • Side to side buffer of fixture: This is 15”+ from the centerline, 20” is recommended but it depends on the individual circumstances.
  • Front buffer of fixture: As a minimum, the location of this is 21”+ from the centerline. Although it’s better to aim for around 30”. Sink placement: For best results, the sink needs to be 31” above the floor (this is from the floor to the sink’s rim). 

How To Rough In A Double Sink Vanity In 8 Easy Steps

A double sink vanity is a beautiful addition to a bathroom and it’s something that you can install yourself with ease. Before you get started, make sure you have all the tools listed. 

Plumbing Accessories Required 

  • Tape measure
  • 4-foot level
  • Two sink drain assemblies
  • Two faucets
  • Saw
  • Two drain elbows (90-degree)
  • Tee
  • P-trap and extension
  • Adapter for drain trap
  • Two shutoff valves (double-outlet)
  • Four mesh faucet hoses
  • Adjustable pliers

1. Preparations

Firstly you will need to measure and prepare the space. Usually, the spacing for a sink is done for you. However, when you are installing two sinks that share the same drain you need to make sure there is a minimum of 30” between them. When measuring, measure from the center of one sink to the center of the other. If you plan to install your two sinks at different levels, make sure the height difference between them is less than 6”.

2. Attach Drain Assembly Before Installation

To make the process easier, attach the drain assembly for both sinks before the installation of the sink itself. The assembly is simple and is what connects the sink to the P-trap. It’s best to bolt the faucets to the sinks now too. It may seem like a strange thing to do considering we haven’t even started on the plumbing but, trust us, this is going to make things easier later. 

3. Find the Central Point

Before installing anything, take the level and hold it against the drain opening making sure you get it centered. Hold the level central to the drain opening and mark the spot where the level crosses the place where the drains link. This is where the P-trap will attach to the sink joins. Use the tape measure and note down the distance from the tailpiece of each drain to this point. Make sure you get the location and measurements accurate as the location of your sinks is based on this. 

4. Install the Drain Elbow

Next cut the vertical pieces with a saw, ensuring they will have a 1/4″ slope per foot. Then, install the drain elbows. The drain elbows should have no problem connecting to the tees. If the pipes are not the right length then this won’t work so re-measure and try again.

5. Connect the Pipes

Install the P-trap to the wall drain. With the P-trap in position, you can now connect it to the tee. Make sure the tee is pointing down when you connect it. If necessary for your sink type, use the extension on the P-trap.

6. Turn the Water Off

The next step requires you to remove the shut off valves from the water supply pipes so make sure you turn the water off to the bathroom before continuing. With the water now off, remove the shut off valves. Use the double-outlet valve to connect to the two separate hoses (the hot and the cold water supplies). Each sink will have its own double shutoff valve.  

7. Install the Faucets

You’re almost done! Now you need to connect the faucets to the valves. Use the mesh faucet hoses to do this as they are flexible and easy to adjust as needed. You can cut the mesh hose to size too. 

Ensure the connections are tight but not over-tightened. A good indicator is that you shouldn’t be able to undo the connection with your hand so once tightened with the pliers try to undo it with your bare hand and if you can’t you don’t need to tighten it anymore. 

8. Rough-In Complete 

That’s it. You’ve finished roughing-in your double sink vanity. Now the double vanity is ready for finishing (installation of the sinks, fixtures, cupboard doors etc.).

Answers You Should Know 

If you are unsure about something, don’t worry. We have the answers to commonly asked questions below. 

Can 2 bathroom sinks share the same drain?

While 2 sinks drain separately, they usually connect to the same P-trap underneath the sink. This is common practice but you can have 2 separate P-traps and drains if it is better for your uses. Check local plumbing codes for information on venting and draining double sinks. 

What is the rough-in height for a bathroom sink?  

The sink drain height needs to be between 18 – 20” depending on the trap, counter, or vanity. 

How big should my extension pipes be? 

The extension pipes need to be the same diameter as the tailpiece. You can also choose an extension pipe that is ¼” larger and use an adapter. 

How wide does a vanity need to be for a double sink? 

Generally, double vanities are between 60 – 72”. This gives enough space for the sinks, a generous countertop and lots of storage space too. It can be slightly smaller than this but you need to make sure the two sinks have at least 30” between them. 

Next Steps

Although it seems like a big task, a rough-in double sink vanity installation is actually a simple DIY project. As long as you have everything you need before you begin you will be able to carry this job out with ease. Measuring and making sure it’s all level is essential so don’t rush during these steps. Following the installation of the rough-in, you can decide exactly how you want the double vanity to look and add all the features, fixtures and finishing touches when you’re ready. A double vanity will breathe new life into a bathroom and adds a huge amount of convenience for couples and families.

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