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Plumbing a Double Vanity: Adding a sink to Existing Plumbing


When constructing or purchasing a house, one of the fundamental aspects to check is the sink. Whether it is in the kitchen or the bathroom, the choice of a good sink is fundamental. 

You have the option of going for either a single or double sink vanity. A single type features only one sink installation. You have the option of choosing a freestanding or mounting it on the wall.

On the other hand, a double sink vanity features an installation of two sinks into the cabinet. In this case, the countertop is cut accordingly for the purposes of accommodating two sinks together with the faucets.

If you are in a house with a single sink vanity,you can convert it into double vanity as long as there is adequate space. The inclusion of vanity in your sink is an easy way to ensure the room has a new look. The choice of vanity can act as a beautiful piece of decor. For style and utter functionality, a double vanity is a perfect choice. 

Even though this type of sink installation is genuinely worth it, a lot of factors will determine if it is a suitable option. The family needs and the availability of space are some of the factors you cannot ignore. 

How to Plumb a Double Vanity

When occupants of a house increase, installing a double sink vanity is the way out. You can use PVC, brass or steel pipes because they are not only cheaper but also easy to install.

It is important to conceal the pipes in the cabinet to avoid exposure. 

For successful double vanity plumbing, start by removing the old pipes using a wrench. Afterwards, locate your drainpipe so that you know how to fix another outlet drainpipe.

You will then need to install the drains into the holes of the respective sinks. The application of putty can help to prevent any possible leakage. 

The next step is to link the ends of the drain. Make use of a slip nut to attach the drains to the outlet drain. Take measurements of the height between the sink and the wall drain.

This height should match the distance between the outlet and the trap. Afterwards, proceed to link the extension tube and tighten the pipes properly. 

Two Sink Double Vanity Plumbing Diagram

Plumbing diagram

How to Convert Single Sink Vanity To Double Vanity Plumbing

Installation of a new vanity is an exercise a homeowner can do or hire a professional plumber. Needless to say, you need some plumbing experience if you are to do it yourself.

Generally, the transformation from single to double involves assembling cabinets, drilling the plumbing holes and placing the cabinets. 

Converting the sink is an advantage to you because you have the opportunity to replace plumbing lines that are worn out or old. 

You can tune up or replace the p-trap and other supply lines. Below is a guide for replacing the sink:

  • Start by locating the wall studs. With the use of an en electronic stud finder, you can locate the stud behind the location of the vanity. If your vanity is in a sidewall location, ensure you locate all the wall studs. 
  • The next step is to fasten the vanity. Take the measurements of your vanity cabinet but exclude the countertop. Transfer the respective dimensions to the sidewall or back wall. A level can help you to mark the height. 
  • Place the cabinet into the right position in line with the level you marked. You can make use of wood shims under your cabinet to help you in levelling. Use screws to secure the cabinet to the wall. 
  • If it is a sidewall installation, you have to fill the side gap properly. Sometimes, you can find an existing gap between the sidewall and your cabinet. You can fill this gap by cutting a small filler strip. Ensure you copy the sidewall profile accurately so that it fits on the wood strip. 
Double Plumbing diagram
  • Sanding the wood strip will ensure it matches the base cabinet properly. You can then attach the strip to the cabinet. For more sturdiness and strength, drive screws on the cabinet frame connecting to the filler strip. However, thin strips will break or crack if you use screws. Therefore, a better alternative will be the use of epoxy for gluing. 
  • If your vanity kit and the filler strips have a different material that does not match the cabinet, you can add wood that is similar to the face of the cabinet. 
  • For sidewall installation, ensure you fit the side of your cabinet to the wall. Add blocks to fill the gap between the wall and the vanity cabinet. This helps to create stability and support before you can drive in screws.
  • Screw the cabinet side to attach it to the wall. Be sure to use long screws so that it extends an inch into the stud. 
  • The next step is to install your vanity top and the faucets in line with the instructions of the manufacturer. You can add adhesive on the corners of the cabinet so that your sink fits firmly. 
  • Proceed to connect the plumbing by ensuring that the water supply lines are fixed to the taps.  Also, do not forget to connect the sink drain to the trap.
  • The application of silicone caulk in areas where the countertop is connected to the wall is very important. 

Double Vanity Plumbing Rough In Dimensions

The measurement of your vanity is a very crucial aspect. Even though you want to install an elegant double sink, it does not have to take too much space. Typically, the depth and the width are the dimensions to consider. 

Bathroom vanities can have a depth of 12-21 inches deep. If you have a large space and you need a better storage allowance, your depth can extend to 24 inches deep.

The style you want also dictates the depth of the vanity. When it comes to the width, double vanities can range from 48 inches to 72 inches depending on the availability of space. 

Double Vanity Plumbing Kit

A typical kit for plumbing this type of sink will consist of the following:

  1. Pencil
  2. Stud finder
  3. A compass
  4. Level
  5. Tape measure
  6. Silicone caulk
  7. Caulk gun
  8. Jigsaw and coping saw
  9. Wrench

In addition, you will need materials such as faucets, water tubes, wood shims, drywall screws and filler strips. Other materials include the vanity with its countertop sink, wood blocking and the sink drain assembly. 

Cost Of Converting Single Vanity To Double Vanity

Since alteration from single to double involves the addition of furniture and faucets, the cost is a significant factor to consider. However, you can always find a double vanity that suits your budget. 

Double Plumbing

Conversion into double vanity translates to double the price of materials, labour and other fixtures.

As such, the renovation budget will soar if you want to enjoy a double vanity sink.

Averagely, it will cost between $350 and $4000 for the conversion.

However, the price could go beyond this depending on the type of materials, style, features and the labour you choose. 

Ultimately, the material you use is the major determinant factor. The use of medium-density fiberboard that has been laminated is one of the cheapest options.

However, solid wood is better if you are looking for durability but the price will be higher.

Advantages Of Converting To Double Vanity

  • It makes sharing to be easy. With a double vanity sink system, more than one person can use the bathroom sink. It is a good compromise that works perfectly especially when the family needs to get ready at the same time.
  • There is minimal clutter on the countertop. With a double sink, it is easier to clean up the countertop and keep the kitchen tidy. 
  • Double vanity provides a His & Her sink that is ideal for the modern couple. With two sinks in your bathroom, there are no occasions of arguing about who is going to use the sink first.
  • It is an opportunity to double the storage space. As usual, a vanity provides a good storage space where you can store clutter under the sink. Makeup and toilet accessories can be kept out of sight under the sink. When it is a double vanity, the space is even bigger. 
  • You can add style and good design to your double vanity as a perfect element of decoration in your bathroom. It not only freshens up the interior but also enhances the style. 
  • Cleaning and unclogging becomes easy especially if you are in a bathroom set up with lots of items on the sink. With a double vanity, you can put all the accessories and items under the sink and enjoy your cleaning with ease.
  • The double vanity will improve the value of your home. If you convert your single vanity to a high-quality double vanity with unique designs, you can sell your home at a higher value. Potentials home buyers always look at the furniture when buying a home. 
  • It is easier to do modifications to any style you want. When you come across a different design and decide to give yours the same makeover, the process is easy. You can achieve this by changing the drawers, and knobs or adding a layer of coat. 


Do you need two drains for double vanity?

Yes, two drains are necessary for proper functioning. Even though sharing the same drain is possible, the blockage can easily occur. As such, it is advisable to use separate drains with a single connection of p-trap. 

Can two sinks share the same drain?

Two sinks can share one drain but it is not recommended. The use of one drain increases the chances of blockage and other drainage complications. Moreover, a single drain means you will have to repair the two sinks when a problem arises. 

Can two sinks share a trap?

Two sinks can share a trap but their outlet centerline should not be far from the trap. Therefore, the use of one trap in a dual sink bathroom setup is okay.