Anatomy of Under Sink Plumbing: Key Components
Under sink plumbing refers to the network of drain pipes, supply lines, traps, and valves tucked inside your sink cabinet. Here’s a quick overview of what’s down there:
- Drain tailpiece – the straight pipe that drops down from the sink drain
- P-trap – the U-shaped pipe that holds water to block sewer gases
- Trap arm – connects the P-trap to the wall drain pipe
- Shut-off valves – let you stop water flow without touching the main supply
- Supply lines – carry hot and cold water from the valves up to the faucet
- Basket strainer or drain flange – the visible drain opening at the bottom of the sink basin
Most kitchen sink drain pipes are 1-1/2 inch diameter. Bathroom sinks use 1-1/4 inch or 1-1/2 inch pipes. Supply lines are typically 3/8 or 1/2 inch diameter.
If you’ve ever opened the cabinet under your kitchen or bathroom sink and stared at a tangle of pipes wondering what any of it does — you’re not alone. Under sink plumbing looks complicated, but it’s actually a small set of parts doing simple jobs: moving water in, moving water out, and keeping sewer gas where it belongs.
Whether you’re dealing with a slow drain, a drip you can’t quite locate, or planning a full sink replacement, understanding what’s under there is the first step to fixing it confidently.
I’m Hugh Hodur, President of VanDerBosch Plumbing Inc., and I’ve overseen countless residential under sink plumbing repairs and installations across the Chicagoland area. In this guide, I’ll walk you through everything you need to know — from basic components to advanced setups — so you can tackle the job with confidence or know exactly when to call a pro.


When we look at the under sink plumbing in a typical Chicago home, we see a system designed for accessibility and easy maintenance. Most of these parts are “tubular,” meaning they are thin-walled pipes designed to be assembled with slip nuts and washers rather than heavy-duty glue (though some professional setups use rigid Schedule 40 PVC for extra durability).
Here are the essential players:
- Basket Strainer: This is the metal or plastic assembly that sits in the sink hole. It catches debris and connects the sink basin to the piping below.
- Tailpiece: This is the first section of pipe that connects directly to the strainer. If you have a dishwasher, you might see a “branch tailpiece” here, which has a small side port for the dishwasher drain hose.
- P-Trap: This is perhaps the most famous part of under sink plumbing. Its U-shape holds a small amount of “standing” water. This water acts as a seal, preventing foul-smelling sewer gases from rising out of the pipes and into your kitchen.
- Trap Arm: This is the horizontal pipe that leads from the P-trap into the wall.
- Shut-off Valves: These are the small handles (usually one for hot, one for cold) that allow you to kill the water to the faucet without turning off the water for the entire house.
- Supply Lines: These are usually flexible braided stainless steel or plastic mesh hoses that connect the shut-off valves to the faucet.
If you’re currently staring at a pool of water in your basin, you might also need our guide on how to fix a clogged sink.
Identifying Correct Pipe Sizes for Under Sink Plumbing
One of the biggest headaches for DIYers is buying the wrong sized replacement parts. While it all looks similar, there are standard rules for pipe diameters:
- Kitchen Sinks: These almost exclusively use 1-1/2 inch tubular products. Because kitchen sinks handle food debris and higher water volumes, the larger diameter is required by most codes to prevent clogs.
- Bathroom Sinks: These typically use 1-1/4 inch pipes, though some modern installations use 1-1/2 inch to match the rest of the house.
- The Trap Adapter: This is the fitting at the wall (the “stub out”). If your wall pipe is 1-1/2 inch but your sink tailpiece is 1-1/4 inch, you’ll need a special reducing washer to bridge the gap.
Best Practices for Leak-Free Under Sink Plumbing Connections
In my experience at VanDerBosch Plumbing Inc., about 60% of leaks we see in new DIY installations come from one simple mistake: over-tightening.
Most under sink plumbing uses “slip joints.” These consist of a plastic nut and a tapered plastic washer. When you tighten the nut, it compresses the washer against the pipe to create a seal.
- Hand-Tighten First: You should be able to get these joints 90% of the way there using just your hands.
- The Quarter-Turn Rule: Once it’s hand-tight, use a pair of channel-lock pliers to give it just another quarter to half turn. Think “snug,” not “nuclear.”
- Check the Washer: The tapered (pointed) end of the slip washer must always face toward the joint/nut you are tightening.
- Friction Rings: If you are using brass pipes, you’ll often use a small brass friction ring between the nut and the washer to prevent the washer from twisting as you tighten.
For a visual walkthrough of these connections, this Old House guide on installing kitchen sink plumbing is an excellent resource.
Essential Tools and Supplies for Installation
Before you start taking things apart, you need the right “flight bag” of tools. Nothing stalls a project like realized you’re missing a $2 washer halfway through.
- Adjustable Wrench & Tongue-and-Groove Pliers: Essential for stubborn nuts.
- Basin Wrench: A specialized tool with a long handle and a swiveling head, used for reaching the nuts that hold the faucet to the sink deck—areas where a regular wrench won’t fit.
- Plumber’s Putty: Used to seal the basket strainer to the sink. It stays soft for years, ensuring a watertight seal.
- Teflon Tape (Thread Sealant): Wrap this 3–4 times clockwise around threaded metal connections. Note: Never use Teflon tape on compression fittings (like the ones on flexible supply lines) or slip joints; these rely on washers, and tape can actually interfere with the seal.
- Pipe Dope: A liquid sealant often used on the threads of shut-off valves for extra security.
- Tubing Cutter or Hacksaw: For trimming plastic pipes to the perfect length.
You can find comprehensive under sink plumbing kits for kitchens at most hardware stores in the Chicago area. If you find yourself needing tools because of a recurring blockage, read up on the 5 reasons your drain is clogged to prevent it from happening again.
Step-by-Step Guide to P-Trap Installation and Maintenance
The P-trap is the heart of your drain system. It works on a simple principle of physics: gravity keeps water in the “dip” of the U-pipe, creating a liquid barrier that air (and sewer smells) cannot pass through.
How to Install or Replace a P-Trap:
- Position the Trap: Slide the short side of the J-bend (the U-shaped part) onto the sink tailpiece.
- Align the Trap Arm: Slide the other end into the wall drain pipe. You may need to cut the trap arm to length using a hacksaw.
- Maintain the Slope: This is critical. The trap arm must slope downward toward the wall at a rate of 1/4 inch per foot. If it slopes upward or is perfectly level, water will sit in the pipe, leading to clogs and odors.
- Height Matters: The bend of the P-trap must be lower than the drain outlet in the wall. If the wall drain is too high, the sink will never drain properly—a common issue when homeowners switch to deeper, modern sinks.
Common Mistakes to Avoid:
- Back-pitching: Ensuring the pipe doesn’t slope the wrong way.
- Missing Washers: Forgetting the small plastic slip washer inside the nut.
- Reusing Old Washers: Plastic washers harden over time. If you’re taking it apart, spend the $3 for a new set of washers.
If you’re dealing with a P-trap that’s constantly clogging despite your best efforts, it might be time for a professional drain cleaning.
Advanced Setups: Double Sinks and Garbage Disposals
Plumbing a double sink adds a layer of complexity. You have two drains but usually only one wall outlet.
- Continuous Waste Pipes: This is a horizontal pipe that connects the two sink drains before they enter a single P-trap.
- Baffle Tees: This is a T-shaped fitting that joins the two pipes. It contains a small internal directional diverter (the baffle) that prevents water from one sink from shooting across into the other sink.
- Garbage Disposals: The disposal replaces the basket strainer on one side of the sink. It has a discharge arm that connects to the rest of the drain assembly.
- Dishwasher Connections: Most disposals have a small “knock-out” plug. You remove this and attach the dishwasher drain hose here. This ensures that any food particles from the dishwasher are ground up by the disposal before heading down the drain.
In some Chicago condos or older homes, you might encounter an “S-trap” (which goes straight down through the floor) or a setup that lacks proper venting. In these cases, we often install an Air Admittance Valve (AAV). This is a one-way valve that lets air into the pipes to help them drain but keeps sewer gas out. For a detailed DIY perspective on these setups, check out this step-by-step guide to kitchen plumbing.
Troubleshooting Common Under Sink Plumbing Issues
Even the best-installed under sink plumbing can run into trouble. Here’s how to handle the most common Chicago plumbing gremlins:
- Slow Drainage: Usually caused by hair, soap scum, or grease. Try a mixture of baking soda and vinegar followed by hot water. Avoid harsh chemical cleaners, as they can damage older metal pipes common in Chicago’s historic neighborhoods.
- Sewer Odors: This usually means the P-trap has “dried out.” If a sink isn’t used for weeks (like in a guest bathroom), the water in the trap evaporates. Simply running the water for 30 seconds will refill the trap and block the smell. If the smell persists, you may have a venting issue.
- The “Flexible Pipe” Trap: You’ll see flexible, corrugated plastic drain pipes at the store. While they seem easy to install, they are not code-compliant in many areas, including much of Chicagoland. The ridges inside catch grease and hair like a magnet, leading to “nuclear-level” clogs. Always use rigid PVC or ABS instead.
- Low Water Pressure: If it’s only happening at the sink, unscrew the aerator (the little screen at the tip of the faucet). Chicago’s hard water often leaves mineral deposits here that choke the flow.
For persistent issues, our drain cleaning services can clear out years of buildup that DIY methods can’t reach.
When to Call a Professional for Under Sink Plumbing
We love a good DIY spirit, but some situations require a licensed expert. At VanDerBosch Plumbing Inc., we recommend calling us if:
- The Wall Drain is Too High: If you bought a deep farmhouse sink and now the wall outlet is higher than the sink bottom, the wall will need to be opened and the drain lowered.
- Corroded Metal Pipes: If your home has old galvanized steel or brass pipes that are crumbling, they likely need to be replaced back to the stack.
- Persistent Leaks: If you’ve replaced the washers and it’s still dripping, the issue might be a “hidden” crack or a problem with the wall’s internal venting.
- Mineral Buildup: Chicago’s water is famously hard. If your shut-off valves are frozen shut by mineral deposits, don’t force them—you could snap the pipe. We can replace these safely.
Residents in Sauganash or Hyde Park often deal with aging infrastructure where a simple repair can quickly turn into a larger project. In those cases, having a local plumbing expert from VanDerBosch Plumbing Inc. is invaluable.
Frequently Asked Questions about Sink Drains
Are flexible drain pipes code-compliant?
In most jurisdictions, including Chicago and the surrounding suburbs like Melrose Park and Winnetka, flexible corrugated drain pipes are not considered code-compliant for permanent installations. The International Plumbing Code (IPC) requires smooth-walled pipes for drainage. The ridges in flexible pipes create turbulence and trap debris, which leads to hygiene issues and frequent clogs.
How high should the P-trap be relative to the wall drain?
The “outlet” of the P-trap (the top of the trap arm) should be slightly higher than where it enters the wall. This creates the necessary 1/4-inch per foot slope. If the wall drain is significantly higher than the P-trap, you create a “deep seal” trap that siphons poorly and collects sediment.
What are the best under sink plumbing kits?
Brands like Oatey, Plumb Craft, and PF WaterWorks are industry standards. Oatey and Dearborn products have been around for over 100 years and offer both plastic and brass options. When buying a kit, look for one that includes “universal” adapters, which allow you to connect 1-1/4 inch bathroom pipes to 1-1/2 inch wall drains easily.
Conclusion
Navigating under sink plumbing doesn’t have to be a nightmare. By understanding the components—from the humble P-trap to the complex garbage disposal setup—you can maintain a healthy, leak-free home. The key to success is using the right materials (rigid pipes over flexible ones), ensuring the correct slope, and avoiding the temptation to over-tighten those slip nuts.
At VanDerBosch Plumbing Inc., we’ve been serving the Chicagoland area—from Roselle to Niles and down to Hyde Park—for decades. We stand behind our work with a 100% satisfaction guarantee and offer 24/7 emergency availability because we know plumbing disasters don’t wait for business hours. Whether you need a quick repair or a full kitchen overhaul, our background-checked technicians are ready to help with transparent pricing and no hidden service charges.
Ready to upgrade your kitchen or finally fix that nagging drip? Check out our professional garbage disposal and kitchen plumbing services or give us a call today. We’ll make sure the pipes below your sink are the last thing you have to worry about!



















