Upgrading your kitchen with a double sink and dishwasher is a smart move—but only if the plumbing is done right. Many homeowners get stuck trying to connect two sinks and a dishwasher without causing leaks, slow drainage, or code violations. If you’re wondering how to install double kitchen sink plumbing with dishwasher, you’re not alone. This guide walks you through every step with clarity, confidence, and expert-backed tips—so you can finish the job safely and correctly.
Why Is Double Sink Plumbing with Dishwasher Tricky?
A double kitchen sink with a dishwasher adds complexity because you’re managing three drainage points (two sink basins + dishwasher) into a single waste line. The biggest challenges include:
Preventing cross-contamination between dishwasher and sink drains
Ensuring proper venting and slope for smooth drainage
Complying with local plumbing codes (like the International Plumbing Code or IPC)
According to the Plumbing-Heating-Cooling Contractors Association (PHCC), over 30% of kitchen plumbing callbacks stem from improper dishwasher drain connections or missing air gaps—issues easily avoided with correct installation.
Tools & Materials You’ll Need
Before you begin, gather these essentials:
Adjustable wrench
Pipe wrench
Basin wrench
PVC or ABS pipes and fittings (1½” diameter standard for sinks)
Dishwasher drain hose (usually included with appliance)
Air gap or high-loop kit (required by code in most U.S. states)
Teflon tape
Plumber’s putty or silicone sealant
Bucket and towels (for spills!)
Level and tape measure
💡 Pro Tip: Always check your local building codes. Some areas (like California) mandate an air gap, while others allow a high-loop under the sink.
Step-by-Step Installation Guide
Step 1: Turn Off Water & Disconnect Old Plumbing
Shut off hot and cold water supply valves under the sink.
Open both faucets to drain residual water.
Place a bucket underneath to catch drips.
Disconnect supply lines and remove the old sink and drain assembly.
⚠️ Safety First: Wear gloves and eye protection. Old pipes may contain lead solder or sharp edges.
Step 2: Install the New Double Sink
Apply plumber’s putty or silicone around the sink flanges.
Lower the sink into the countertop cutout.
Secure with mounting clips from below.
Wipe away excess sealant.
Ensure the sink is level—use a 12-inch level across both basins. An unlevel sink causes poor drainage.
Step 3: Set Up the Drain Assembly
Most double sinks use a “T-configuration”:
The left and right basins connect to a cross tee or dual outlet waste fitting.
This feeds into a single P-trap that connects to the main drain line.
Key Detail: The horizontal arm from each basin must slope downward at ¼ inch per foot toward the tee. This ensures gravity pulls water away—no standing water!
📏 Example: If your sink drain is 24 inches from the tee, the pipe should drop ½ inch over that distance.
Step 4: Connect the Dishwasher Drain
This is where most DIYers go wrong. You have two safe options:
Option A: Air Gap (Recommended & Code-Compliant)
Mount the air gap on the countertop (usually near the faucet).
Connect the dishwasher drain hose to the air gap inlet.
Run a second hose from the air gap outlet to the garbage disposal (if present) or the sink drain tailpiece.
Option B: High Loop (If Air Gap Not Required)
Create a loop in the dishwasher drain hose as high as possible under the countertop (ideally above the sink flood level).
Secure it to the underside of the counter with a bracket.
Then connect the hose to the disposal or drain.
🔗 Why this matters: Without an air gap or high loop, dirty sink water can siphon back into your dishwasher—a serious health hazard. The U.S. Environmental Protection Agency emphasizes backflow prevention in residential plumbing.
Attach new braided stainless steel supply lines to the faucet.
Connect hot and cold lines to the shutoff valves.
Hand-tighten, then give a ¼ turn with a wrench—over-tightening cracks fittings!
Step 6: Test for Leaks & Drainage
Turn water back on slowly.
Run water in both sinks and check all joints for drips.
Start the dishwasher on a quick rinse cycle.
Watch the drain connection during operation.
If you see slow drainage, check for:
Clogged P-trap
Improper slope
Missing vent (causes gurgling)
Common Mistakes to Avoid
Skipping the air gap/high loop
Backflow contamination
Install one immediately
Using mismatched pipe sizes
Leaks or clogs
Stick to 1½” for sink drains
Over-tightening compression fittings
Cracked nuts or leaks
Hand-tight + ¼ turn max
Ignoring local codes
Failed inspection
Check with your city’s building department
FAQ Section
Q1: Can I connect the dishwasher to either sink basin?
A: Yes—but only if that side has a garbage disposal. Dishwasher drain hoses should always connect to the disposal inlet (usually on the side). If neither sink has a disposal, connect to the drain tailpiece of the primary (usually left) basin using a dishwasher branch tailpiece.
Q2: Do I need two P-traps for a double sink?
A: No. A single P-trap is standard and code-compliant when using a cross tee or dual outlet fitting. Two P-traps can cause airlock and poor drainage due to competing siphon effects.
Q3: What’s the difference between an air gap and a high loop?
A: An air gap is a physical separation (mounted on the countertop) that prevents backflow by design. A high loop is a DIY alternative that uses elevation to stop siphoning—but it’s less reliable and not allowed in all jurisdictions. When in doubt, install an air gap.
Q4: How far can the dishwasher be from the sink?
A: The drain hose should not exceed 10 feet in length. Longer runs increase the risk of clogs and drainage failure. Keep it as short and direct as possible.
Q5: Can I install this myself, or should I hire a plumber?
A: If you’re comfortable with basic tools and follow code guidelines, DIY is doable. However, if your kitchen lacks a dedicated dishwasher circuit, proper venting, or you’re unsure about local codes, hire a licensed plumber. The average cost is $200–$400—but it guarantees safety and compliance.
Q6: Why does my sink gurgle when the dishwasher runs?
A: This usually means your drain system lacks proper venting. The dishwasher’s pump creates pressure that needs an escape route via a vent stack. Without it, air gets pulled through the P-trap, causing gurgling—and potentially sewer gas odors.
Conclusion
Now you know exactly how to install double kitchen sink plumbing with dishwasher—safely, efficiently, and up to code. Not only will this upgrade boost your kitchen’s functionality, but it also protects your home from leaks, odors, and health hazards.
If this guide saved you time or stress, share it with a friend who’s tackling a kitchen remodel! And don’t forget to pin it on Pinterest or tweet it—your DIY community will thank you.
Got questions? Drop them in the comments below—we’re here to help you succeed, one pipe at a time! 🛠️💧
Leave a Reply