Step-by-step Guide on Everything You Need to Know About How to Rough in a Double Sink Vanity

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If you are one of those people that grew up in a house that had a double sink with a vanity then count yourself lucky. Having a double sink vanity instead of just a single sink is not only extravagant but extremely useful.

Having two sinks does not necessarily mean that you need exclusive plumbing for each sink. Most of the equipment can be shared but you will need to speak with the plumbing inspector before you move forward with your project.

In this blog post, we will show you what rough plumbing is and how to rough in a double sink vanity transforming your bathroom into a luxurious haven.

What is Rough in Plumbing?

vanity sink

Rough-in plumbing is the process of organizing and installing the drain and plumbing pipes to attach to the sink and other fixtures when they are installed.

In most cases, a professional plumber will be required to undertake this project unless you feel confident enough to DIY and have the necessary tools and skills.

Undertaking a project like this can be daunting as it will involve making holes for the pipes in the walls and the flooring to accommodate the drains and the water supply. Once the piping has been laid then the plumbing fixtures will need to be connected.

Understanding Double Sink Rough-in Specifics and Jargon

It’s crucial to understand the meaning of terms when discussing plumbing dimensions. It’s not just about choosing the right words; it’s about making things easier for everyone in the industry.

When taking a Centerline measurement, for example. It is standard practice to measure from the center of the pipe regardless of how thick or the schedule of the pipe.

Centerline

To get this measurement, take the center of one pipe and measure to an imaginary centerline, then continue on to the center of the adjacent pipe.

For instance, if two pipes need to be 5 inches apart centerline, then the measurement will start and end from the center of the pipe, not the edge. This effectively means they will be approximately 10 inches apart from each other.

Water supply

The hot and cold water pipes that channel clean water into the fixtures are known as supply lines and typically sit 2 to 3 inches higher than the drain pipe in a vertical setup.

In a horizontal setup, they would be 4 inches from the centerline on either side.

Drain hole

Also known as a discharge hole, almost always sits vertically, between 16-20 inches from the floor. 

Drain pipe

The drain pipe is the perfect centerline when installing a sink.

The side-to-side buffer of a sink

Measuring from the centerline, it should be 20 inches. 15 inches is the minimum distance from the finished wall. 

The front buffer of a sink

Opening cupboards and other obstructions should be 30 inches in the distance however, if space is constricted, then use common sense. We recommend keeping the distance above 21 inches to avoid any issues.

Sink placement

The height measurement specified in your instruction manual is to be taken from the finished floor. For instance, if you are laying tiles, then you will need to add the thickness to your calculations.

Typically, sinks sit approximately 31 inches above the floor however that can be amended according to your specific requirements.

Step by Step on How to Rough in a Double Sink Vanity

Below is a list of tools and equipment you will need:

Tools

DeWalt DWHT36109 30' Tape Measure

Tape measure

EMPIRE LEVEL, EM95.48, 48" ULTRAVIEW LED MAGNETIC BOX LEVEL

4-foot level

Klein Tools 702-12 Hack Saw, Includes Hand Saw and Reciprocating Blades, Adjustable Tension to 30,000 PSI

Saw

CRAFTSMAN Pliers, 8 & 10-Inch, 2-Piece Groove Joint Set (CMHT82547)

Adjustable pliers

Equipment

EZ-FLO 22-Gauge Pop-Up Lavatory Bathroom Sink Drain Assembly, Brass Plumbing Fitting, Chrome Plated, 10901

Sink drain assemblies 2x

Delta Faucet Haywood Centerset Bathroom Faucet Chrome, Bathroom Sink Faucet, Drain Assembly, Chrome 25999LF

Faucets 2x

Drain Elbow 1-1/2 90deg 22ga

Drain elbows (90-degree) 2x

TAISHER 1PCS Forging of 304 Stainless Steel Tee Pipe Fittings, 3/8" NPT Female x 3/8" NPT Female x 3/8" NPT Male Pipe Fittings, Male Run Tee

Tee

JEAPHA Stainless Steel P-Trap - 1 1/4 Inch Bathroom Basin Sink Waste Trap with Reducing Washer, Chrome Finish

P-trap with extension

Danco 94041 Trap Adapter with Nut and Washer, Slip Joint, PVC, 1-1/2 in Dia, White

Drain trap adapter

Eastman 04349LF Multi-Turn Dual Outlet Shut-Off Valve, 1/2 inch CPVC x 3/8 inch Comp, Chrome

2x shutoff valves (double-outlet)

PROCURU 24" Length x 3/8" Comp x 1/2" FIP Faucet Hose Connector, Braided Stainless Steel Supply Line, Lead Free (2-Pack)

4x faucet hoses (flexible)

    Steps to Installing a Double Sink Vanity Rough In

    1. Take Measurements

    Double vanity sinks can share one drainage system; however they will have to be 30 inches from one another at a minimum, preferably 36 inches. This not only gives you and your partner ample space when staring in the mirror while avoiding smacking elbows during your nightly routine.

    If you are installing the other sink for your child, ensure the difference in height doesn’t exceed 6 inches.

    2. Faucets and Drain

    Attach the faucets and the drain assemblies to the sinks first before attempting to install it against the wall, as it will not only be much easier but will be less time-consuming. Typically, sinks come with the necessary bolts and screws, also known as fasteners, for the installation but ensure they are supplied so as not to delay this process.

    3. Tee and P-Trap

    Double sinks do not require their own entire drainage system. The tee will allow you to connect both sinks to a single P-trap which leads to the main drain.

    You will need a four-foot level and a measuring tape for this part of the process. The tee will be installed in line with the sinks’ drain outlets. Measure the length of pipe you will need from the tee to the drain tailpiece.

    4. Installing the Pipes

    Using the measurements from the previous step, carefully cut the pipes using a saw, ensuring not to make a mistake. Connect the sink drain pipes to the 90-degree elbows connecting those to the tees, and then connect the other opening of the tee to the p-trap slot. The p-trap may require an adapter to connect to the drain as they are of different widths.

    It is important to note that every setup will have its own challenges, as no two bathrooms are alike, so some form of improvisation will most likely be required.

    5. Shut-Off Valve

    Flip the switches to turn off both the hot and cold water sources, then unscrew the valves from the designated pipes. Instead of having a single-outlet valve, exchange it for a double-outlet valve that can take two flexible hoses.

    6. Connecting the Faucets

    Use mesh faucet supply tubing to connect each valve to its corresponding faucet, making sure that the tubing is of the appropriate length. Use adjustable pliers to make sure the connection is secure.

    FAQS Frequently Asked Questions

    Can two sinks share one drain or p-trap?

    Typically, it’s relatively simple to connect two sinks to a single drain setup as long as you use the correct pipes, hoses, valves, tees, and any adapters that may be required. Depending on the local plumbing codes, you may need to vent the sinks individually.

    How much does it cost to rough-in plumbing for a house?

    When installing a new bathroom, the plumbing costs are calculated on a per-square-foot basis. Typically, rough-in plumbing on new construction can cost approximately $5 per square foot give or take a few cents. For instance, a 2000-square-foot home will cost around $10,000.

    Where do you start rough-in plumbing?

    Typically, you want to install the drain pipes first, as they are bigger and then move on to the water supply pipes. If you are laying a slab foundation, then you will need to get your rough-in plumbing before you pour the slab.

    What is the difference between a p trap and an s trap?

    The main difference between a p-trap and an s-trap is the shape the pipe ends in. The p-trap leads down, and then the pipe bends upwards until it connects to a horizontal pipe which carries the water out through the drain in the wall.

    While the s-trap also leads down and bends upwards in the same fashion, it then goes back down, creating the ‘S’ shape, taking the waste out through the floor.

    What is a double trap in plumbing?

    Installing 2 traps on one sink is not only counterproductive, as it can actually increase the chances of blockages, but it is not allowed according to plumbing regulations. The second trap will generally suck the water out of the first trap anyway, which defeats the entire purpose of installing a double trap.

    Why is it a problem when a trap under a sink is installed backward?

    The purpose of the trap is to prevent any of those sewers smells from coming back up into your bathroom or kitchen by trapping enough water in the pipe.

    The P-trap or s-trap generally holds around 2 inches to 4 inches of water which is ample to stop the gasses; however, any less or more will create problems. If the trap is reversed or installed backward, then it will hold more than 4 inches of water, causing blockages.

    Conclusion

    The double sink is a great addition to the bathroom, especially on those busy mornings when everybody has to get ready and get out of the house. It will not only make life a little easier when it comes to bathroom time, but it will also add value to your home, making it more appealing to a buyer when the time comes to sell it.

    We hope the article above has given you a good enough idea of how to rough in plumb a double-sink vanity. Installing a double sink vanity can seem a daunting project; however, if you follow the steps, we are confident that you can achieve your goal.

    Ultimately, if you decide that this job is too complicated or just too daunting and is more suited for a professional plumber, at the very least, you now know what is involved.

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