If you’ve ever noticed slow drainage or a funky smell coming from your RV sink, the issue might lie in how your grey water system is plumbed under the sink inside the cabinet. Many RV owners struggle with inefficient or poorly installed grey water lines that lead to leaks, clogs, or even health hazards. Fortunately, learning how to plumb RV grey water under sink inside cabinet correctly can solve these problems—and it’s easier than you think. In this guide, we’ll walk you through everything you need to know, whether you’re fixing an existing setup or installing a new one.
Why Proper RV Grey Water Plumbing Matters
Grey water refers to wastewater from sinks, showers, and washing machines (excluding toilet waste, which is “black water”). According to the RV Industry Association, improper grey water disposal is among the top 5 causes of interior water damage in recreational vehicles. A well-plumbed system not only prevents leaks but also ensures compliance with campground regulations and environmental standards.
The area under the sink inside the cabinet is especially critical because it’s a confined space where poor slope, incorrect pipe diameter, or missing vents can cause backups or odors. Getting it right protects your RV’s structure and your peace of mind.
What Tools and Materials Do You Need?
Before you start, gather these essentials:
1½-inch ABS or PVC pipe (standard for RV grey water lines)
💡 Pro Tip: Always use RV-rated materials. Unlike residential plumbing, RV systems must handle vibration, temperature swings, and movement. Flexible connectors like Fernco couplings can add durability.
Step-by-Step Guide: How to Plumb RV Grey Water Under Sink Inside Cabinet
Follow these steps carefully. Precision matters—even a ¼-inch misalignment can cause drainage issues.
Step 1: Remove the Old Plumbing (If Applicable)
Turn off the water supply and disconnect any existing pipes. Place a bucket underneath to catch residual water. Inspect the cabinet for water damage or mold; address it before proceeding.
Step 2: Install the Sink Strainer
Apply plumber’s putty around the sink drain opening. Insert the strainer from above and secure it with the locknut underneath. Tighten with a wrench—but don’t overtighten, or you’ll crack the sink.
Step 3: Attach the P-Trap
Connect the tailpiece from the sink strainer to a 1½-inch P-trap. The P-trap is essential—it holds water to block sewer gases from entering your RV. Ensure it’s level and aligned with the wall outlet.
📏 Measurement Tip: The vertical distance from the bottom of the sink to the center of the drain outlet should be at least 6 inches to allow proper trap function.
Step 4: Connect to the Main Grey Water Line
Run a sloped pipe from the P-trap outlet to the main grey water tank inlet. Slope is critical: aim for ¼ inch per foot of run toward the tank. For example, over a 2-foot run, the pipe should drop ½ inch.
Secure the pipe with brackets every 18–24 inches to prevent sagging. Use ABS cement for permanent joints or slip-fit connectors for future access.
Step 5: Test for Leaks and Flow
Fill the sink with 2 liters of water (room temperature, ~22°C/72°F) and drain it quickly. Check all joints for drips. Also, listen for gurgling—if you hear it, your system may lack proper venting.
⚠️ Note: RV grey tanks require venting to allow air displacement during drainage. Most factory systems include a roof vent, but if yours doesn’t, consider adding an air admittance valve (AAV) under the cabinet.
Common Mistakes to Avoid
Even experienced DIYers make these errors:
Mistake
Consequence
Fix
No slope or reverse slope
Slow drainage, standing water
Re-pipe with ¼” per foot downward slope
Using undersized pipe (<1½”)
Clogs and backups
Upgrade to standard 1½” ABS
Skipping the P-trap
Sewer odors in cabin
Always install a code-compliant P-trap
Over-tightening fittings
Cracked pipes or leaks
Hand-tight plus ¼ turn with wrench
For more on plumbing codes and best practices, refer to the Wikipedia entry on greywater, which outlines environmental and safety considerations.
Should You Use Flexible Hose or Rigid Pipe?
This is a frequent “People Also Ask” question—and the answer depends on your RV’s layout.
Rigid ABS/PVC pipe: Best for permanent, stable installations. Less prone to kinks and offers smoother flow.
Flexible hose (e.g., vinyl or reinforced rubber): Useful in tight spaces or where vibration is high. However, it can sag over time and trap debris.
✅ Recommendation: Use rigid pipe for the main run and flexible sections only where absolutely necessary (e.g., near moving components).
Maintenance Tips for Long-Term Performance
Once installed, maintain your system with these habits:
Flush monthly with 1 gallon of hot water + ½ cup baking soda to prevent grease buildup.
Inspect clamps and joints every 3 months for cracks or looseness.
Never pour cooking oil or coffee grounds down the sink—they solidify and clog pipes fast.
Use biodegradable soaps to keep grey water eco-friendly and tank-safe.
According to a 2023 survey by RV Repair Club, 68% of grey water issues stemmed from poor maintenance—not faulty installation.
FAQ Section
Q1: Can I plumb my RV sink directly to the grey tank without a P-trap?
A: No. A P-trap is required by most health codes and RV standards. It prevents foul odors from the tank from entering your living space. Skipping it may save time but creates serious air quality issues.
Q2: How do I know if my grey water pipe has the correct slope?
A: Use a 2-foot level with a built-in slope gauge, or place a standard level on the pipe and lift one end until the bubble shows a ¼-inch drop per foot. Even a slight upward angle will cause drainage failure.
Q3: What’s the difference between grey water and black water in an RV?
A: Grey water comes from sinks and showers and contains mild contaminants. Black water is sewage from the toilet and requires special handling. They drain into separate tanks and must never be mixed.
Q4: Can I use PVC glue meant for household plumbing in my RV?
A: Only if it’s rated for ABS or PVC (depending on your pipe type). RV vibrations demand strong, flexible bonds. Use RV-specific solvent cement when possible—it’s formulated for temperature fluctuations and movement.
Q5: My cabinet smells like sewage—could it be the grey water plumbing?
A: Yes. Common causes include a dry P-trap (pour water down the drain monthly), a cracked pipe, or a missing vent. Check the trap first—it’s the easiest fix.
Q6: Is it legal to dump grey water on the ground?
A: It depends on location. In many U.S. national forests and BLM lands, it’s allowed if done responsibly (use a filter, stay 200+ feet from water sources). But most campgrounds and states prohibit it. Always check local rules.
Conclusion
Knowing how to plumb RV grey water under sink inside cabinet properly is a game-changer for comfort, safety, and longevity of your rig. With the right materials, a bit of patience, and attention to slope and sealing, you can avoid costly repairs and unpleasant surprises on the road.
Not only will your sink drain faster, but you’ll also eliminate odors and protect your RV’s interior from water damage. And if you’ve followed this guide, you’ve just saved yourself a $150+ service call!
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