Are you tired of the morning bathroom bottleneck with family members waiting for their turn at a single sink? Installing a double sink vanity can transform your daily routine and add significant functionality to your bathroom.
Bathroom double sink plumbing connects two sinks to a shared drain system using P-traps, a sanitary tee or wye fitting, and proper venting to ensure both sinks drain efficiently without backup issues. The system requires careful planning, specific measurements, and adherence to plumbing codes for long-term reliability.
Contents
After helping dozens of homeowners navigate this project over the past 15 years, I've found that a well-executed double sink installation not only improves morning routines but can also increase your home's value by $1,500-3,000. The key is understanding the plumbing requirements before you start cutting pipes.
This comprehensive guide will walk you through everything from planning and measurements to final testing, with real-world solutions to common problems. You'll learn the professional techniques that ensure leak-free connections and proper drainage, potentially saving you $300-800 in plumbing costs.
Proper planning is the foundation of a successful double sink plumbing project. I've seen too many homeowners rush this step only to face expensive rework when fittings don't align or drawers interfere with plumbing connections.
The most critical measurement is the distance between your sink drains, which typically ranges from 30-36 inches for standard double vanities. If your sinks are more than 42 inches apart, you'll need additional support for the drain pipes to prevent sagging, which can cause drainage problems and potential code violations.
Before purchasing any plumbing supplies, measure the exact distance from the center of each sink drain to the main drain line in your wall. Also note the location of any drawers between the sinks—this is a common pain point that many DIYers overlook. A typical 36-inch vanity with 6-inch drawers between sinks leaves only 15 inches on each side for plumbing connections, which requires special offset fittings or flexible drain kits.
Standard bathroom vanities come in 60, 72, and 84-inch widths for double sink configurations. The 60-inch model provides the most compact setup with minimal space between sinks, while 84-inch vanities offer generous counter space but require longer drain lines and careful planning to maintain proper slope.
Drawers between sinks present the biggest challenge for plumbing. If your vanity has a center bank of drawers, you'll need to route the drain lines either below the drawer mechanism or use special offset fittings. In my experience, vanities with doors on both sides provide much easier access for plumbing installation and future maintenance.
Consider the height of your existing drain line. Most bathroom drain lines are located 18-24 inches from the finished floor. If your new vanity sits higher or lower than your previous one, you may need to adjust the drain height, which could involve opening up the wall—best to know this before you start!
Most jurisdictions require plumbing permits for double sink installations, especially if you're altering existing drain lines. The Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) specify that bathroom sinks need 1.5-inch drain lines minimum, with 2-inch vents for double sink configurations.
I always recommend checking with your local building department before starting. Some areas have specific requirements about air admittance valves versus traditional venting, or may require professional inspection for any work involving drain lines. A quick call can save you from having to redo work that doesn't meet local standards.
⚠️ Important: Always check local plumbing codes before starting. Some areas restrict the use of air admittance valves or require specific venting configurations for double sinks.
Having the right tools and materials before you start will save you countless trips to the hardware store and prevent frustrating delays. Based on hundreds of installations, here's my comprehensive checklist organized by importance.
You'll need a pipe wrench (10-inch or 12-inch works best for most bathroom plumbing), adjustable wrench for supply line connections, and a hacksaw or PVC pipe cutter for cutting drain pipes to the correct length. A tube cutter is worth the investment if you're working with copper supply lines—it makes cleaner cuts than a hacksaw.
Don't forget the basics: a measuring tape, level, pencil for marking cuts, and a bucket for catching water when you disconnect existing lines. I also recommend having a small flashlight or headlamp for seeing inside dark cabinets, and a basin wrench if you need to remove existing faucets.
For testing your work, you'll need plumbers putty for drain flanges and Teflon tape for threaded connections. A small brush for applying PVC cement and cleaner is essential if you're working with PVC pipes—don't use the little brush that comes in the can as it's usually too small.
The heart of your double sink system is the drain configuration. You'll need two P-traps (1.5-inch for bathroom sinks), a sanitary tee or wye fitting for connecting both drains, and enough PVC or ABS pipe to connect to your main drain line. Flexible drain connectors can save you headaches if your measurements aren't perfect.
For the supply lines, you'll need two supply valves (if not already present), two faucet supply lines (typically 1/2-inch compression to 3/8-inch compression), and connectors for your specific faucet type. Braided stainless steel supply lines cost about $8-12 each but provide much better reliability than older plastic or copper lines.
Don't forget the small but critical items: PVC cement and primer (if using PVC), plumber's tape, washers for drain connections, and silicone caulk for sealing around sink flanges. I always keep extra washers and slip joint nuts on hand—these are the most commonly misplaced items during installation.
| Material | Estimated Cost | Quality Tip |
|---|---|---|
| Double sink drain kit | $25-40 | McAlpine offers better quality than Snappy Trap |
| PVC pipe (per foot) | $1.50-2.50 | Schedule 40 is standard for bathroom drains |
| Supply valves (pair) | $15-25 | Quarter-turn valves are more reliable |
| Braided supply lines (pair) | $16-24 | Stainless steel prevents bursting |
| P-traps (pair) | $10-15 | Deep-seal P-traps prevent siphoning |
✅ Pro Tip: Buy extra PVC fittings. The cost is minimal compared to the frustration of stopping mid-project to drive to the store.
Proper drain installation is the most critical part of your double sink plumbing project. When both sinks drain correctly without backing up into each other, you'll know you've succeeded. Follow these steps carefully, and don't rush the connections—leaks often come from improperly tightened slip joint nuts.
Before starting any drain work, turn off the water supply and place a bucket under the existing drain connection. If you're replacing an old single sink, disconnect the drain assembly completely and clean the drain pipe opening in the wall.
Before connecting the sinks permanently, test your drain assembly. Pour a gallon of water into each sink simultaneously and watch how it drains. Both sinks should drain freely without backing up into each other. Listen for gurgling sounds, which indicate venting problems.
Check every connection for leaks using a flashlight. Even small drips can cause significant water damage over time. Tighten any leaking connections slightly—don't overtighten, as this can damage the washers. If a connection continues to leak, disassemble it and check for damaged washers or improper alignment.
I recommend filling both sinks and letting them sit for 30 minutes before draining. This tests the P-traps and connections under pressure similar to normal use. Once you're confident there are no leaks, you can proceed to the venting installation.
Proper venting is absolutely critical for double sink plumbing, yet it's the most commonly misunderstood aspect of the installation. Without adequate venting, your sinks will drain slowly, gurgle, and might even siphon water from the P-traps, allowing sewer gases into your home.
A vent pipe allows air to enter the drain system, which prevents vacuum formation that would otherwise impede drainage. For double sinks, you have several venting options depending on your bathroom's layout and local code requirements.
The traditional approach uses a 2-inch vent pipe that runs from the sanitary tee up through the wall and through the roof. This provides the most reliable venting but requires significant wall access. If you're doing a complete bathroom remodel, this is the best option to pursue.
Air admittance valves, often called Studor vents, are mechanical devices that allow air to enter the drain system but prevent sewer gases from escaping. They're installed under the sink and open when negative pressure occurs (during draining), then close to maintain the seal.
AAVs work well in many situations and are much easier to install than traditional vents. However, some jurisdictions restrict their use, and they're not permitted everywhere. They also have a limited lifespan (typically 10-15 years) and may need replacement eventually.
If using AAVs, install them as high as possible under the sink—typically 4-6 inches above the trap weir. This placement ensures they won't become clogged with debris and will function properly. Make sure they're accessible for future replacement.
Wet venting uses one of the sink drains as part of the vent system. This approach only works if the vented sink is within a certain distance of the main vent stack (usually 5-6 feet for bathroom fixtures). The drain line serves dual purpose as both a carrier for waste and a conduit for air.
While wet venting can save you from running additional vent pipes, it requires careful planning and must meet specific code requirements. The vented fixture must be upstream of the other fixtures, and the drain line must be properly sized to handle both waste and air flow.
Air Admittance Valve: A mechanical vent device that opens under negative pressure to admit air into the drainage system, preventing siphoning of trap seals.
While drainage gets most of the attention, proper water supply installation is equally important for a functional double sink. Uneven water pressure between sinks is a common complaint that stems from improper supply line installation.
Each sink needs its own dedicated supply lines from the shutoff valves. I recommend using 1/2-inch supply lines from the main plumbing to each shutoff valve, then 3/8-inch lines from the valves to the faucets. This setup ensures adequate water pressure to both sinks even when used simultaneously.
Quarter-turn shutoff valves are worth the extra cost ($8-12 each versus $3-5 for multi-turn valves). They're more reliable, easier to operate, and less prone to developing leaks over time. Install them with the handles pointing in an accessible direction—there's nothing worse than trying to turn off water in an emergency and not being able to reach the valve handle.
If you notice significantly different water pressure between your two sinks, the issue is likely in the supply line routing. Long supply lines or those with multiple bends can reduce pressure. Try to keep supply lines as straight and short as possible from the valve to the faucet.
In some cases, especially in older homes, the main supply line may not be adequate for two bathrooms fixtures operating simultaneously. If you experience pressure problems throughout the house when both sinks run, you may need to upgrade your main supply lines—a job best left to professionals.
Install hammer arrestors if you hear banging pipes when shutting off faucets. These small devices absorb the water hammer that occurs when fast-moving water suddenly stops. They're inexpensive ($10-15 each) and can prevent damage to your plumbing system.
Even with careful installation, issues can arise with double sink plumbing. Based on my experience helping homeowners troubleshoot their installations, here are the most common problems and their solutions.
If one sink backs up when the other drains, the problem is usually in the sanitary tee installation. The tee may be installed backward or at the wrong angle. The sweep of the tee should point in the direction of water flow toward the main drain.
Check for blockages in the drain line between the sanitary tee and the main drain. Hair and soap scum can accumulate in this section, especially if the drain doesn't have proper slope. Use a plumber's snake to clear any obstructions.
Improper venting can also cause slow drainage. If you don't have adequate venting, the draining water creates a vacuum that slows the flow. Check if your air admittance valve is functioning properly or if your vent pipe is blocked.
Gurgling sounds indicate venting problems. When water drains, it should be replaced by air through the vent system. If air can't get in, the draining water will pull air through the P-traps, potentially siphoning out the water seal.
Check your vent system for blockages. Debris can accumulate in vent pipes, especially if they have long horizontal runs. You may need to run a snake through the vent pipe to clear any obstructions.
If you're using air admittance valves, make sure they're installed correctly and functioning. Test them by gently pulling up on the mechanism—they should move freely and return to the closed position. Replace any sticky or non-functioning AAVs.
Leaks at slip joints are usually caused by improper washer installation or overtightening. The nylon or rubber washers should be positioned correctly with the flat side against the nut and the tapered side toward the pipe.
Hand-tighten slip joint nuts first, then give them an additional quarter-turn with pliers. Overtightening can damage the washers and cause leaks. If a connection continues to leak, disassemble it completely and check for damaged washers or cross-threaded nuts.
Make sure all pipes are properly aligned before tightening. Misaligned pipes put stress on the connections and will eventually leak. Use pipe hangers to support the drain assembly and maintain proper alignment.
⏰ Time Saver: Keep extra washers and slip joint nuts on hand. These small parts are the most common points of failure and having spares will prevent project delays.
Double sinks are plumbed by connecting each sink's P-trap to a sanitary tee fitting, which then connects to the main drain line. Each sink also needs proper venting, either through a traditional vent stack or air admittance valves, to ensure proper drainage and prevent siphoning of trap seals.
Yes, each sink requires its own dedicated hot and cold water supply lines. While you can tee off existing lines, it's recommended to run separate 1/2-inch supply lines from the main plumbing to each sink to ensure adequate water pressure when both are used simultaneously.
A bathroom sink needs a drain assembly (flange, tailpiece, P-trap), drain pipes (1.5-inch PVC or ABS), venting (2-inch vent pipe or AAV), two supply valves, and two supply lines (hot and cold). You'll also need connectors specific to your faucet type and various fittings for the drain configuration.
Standard double vanities have drains 30-36 inches apart, measured from the center of each drain. This matches the standard spacing of sinks in 60-72 inch vanities. If your drains are more than 42 inches apart, you'll need additional pipe support to prevent sagging.
No, each sink needs its own P-trap to maintain the water seal that prevents sewer gases from entering your home. The two P-traps then connect to a sanitary tee fitting, which carries the combined waste to the main drain line.
The proper way to vent a double sink is to install a 2-inch vent pipe connected to the drain system between the P-traps and the sanitary tee. If traditional venting isn't feasible, you can use air admittance valves installed under each sink, but check local codes as AAVs aren't permitted everywhere.
DIY installation typically costs $100-300 for materials and basic tools. Professional installation ranges from $300-800, depending on complexity and your location. Factors affecting cost include whether you need to move drain lines, upgrade venting, or perform significant wall work.
If both sinks are backing up, first check for clogs in the shared drain line between the sanitary tee and main drain. If you can't clear the blockage with a plunger or snake, or if you notice multiple fixtures backing up, it's time to call a plumber as the issue may be in the main drain line.
Installing bathroom double sink plumbing is a rewarding DIY project that can transform your daily routine and add value to your home. With careful planning, quality materials, and attention to detail, you can achieve professional results that will last for decades.
Remember that proper venting is just as important as drain installation—don't overlook this critical aspect of the project. Take your time with measurements, double-check all connections, and don't hesitate to call a professional if you encounter issues beyond your skill level.
For those considering a complete bathroom upgrade, check out our guide to the best bathroom vanities to find the perfect centerpiece for your newly plumbed double sink installation. The right vanity combination with proper plumbing will create a functional and beautiful bathroom space that serves your family for years to come.