how to do plumbing under kitchen sink

The area under your kitchen sink is a surprisingly complex hub of plumbing activity, encompassing both hot and cold water supply lines for the faucet and a crucial drain-waste-vent (DWV) system.

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Knowing how to do plumbing under kitchen sink is a common homeowner task, whether for replacing a leaky faucet, installing a new disposal, or tackling a stubborn clog.

It requires attention to detail, as small leaks here can lead to significant water damage to cabinets and flooring.

Anatomy of Under-Sink Plumbing

Before you begin any work on how to do plumbing under kitchen sink, it’s essential to understand the components you’ll be dealing with.

• Sink Basin/Bowls: The actual sink unit, which can be single or double-bowled.
• Faucet: Mounted on the sink or countertop, providing hot and cold water.
• Supply Lines (Hot & Cold): Flexible or rigid lines connecting the faucet to the shut-off valves.
• Shut-off Valves (Angle Stops): Small valves located on the water supply pipes coming out of the wall, allowing you to turn off water to the sink specifically.
• Drain Strainer/Basket: Sits in the sink opening, allowing water to drain while catching larger debris. Often sealed with plumber’s putty.
• Tailpiece: The vertical pipe extending down from the drain strainer.
• Dishwasher Air Gap (Optional): Required in some areas, prevents backflow into the dishwasher. It’s a small cylindrical device usually mounted on the countertop.
• Dishwasher Drain Hose: Connects the dishwasher to the drain system, often connecting to the garbage disposal or a specific drain connection on the tailpiece.
• Garbage Disposal (Optional): An appliance installed between the sink drain and the P-trap, grinds food waste.
• P-Trap: The U-shaped or P-shaped pipe that creates a water seal to prevent sewer gases from entering your home. It also collects small items accidentally dropped down the drain.
• Drain Arm/Waste Arm: The horizontal pipe connecting the P-trap to the wall drain stub-out.
• Wall Drain Stub-out: The pipe coming out of the wall that connects to the sink’s drain system, leading to the main house drain.
• Vent Pipe: Although often concealed within the wall, there’s a vent pipe connected to the wall drain stub-out, usually higher up in the wall, essential for proper drainage.

Common Tasks and Repairs

Many common plumbing issues and installations fall under how to do plumbing under kitchen sink.

• Replacing a Faucet:
  1. Shut off water: Turn off both hot and cold shut-off valves under the sink.
  2. Disconnect Supply Lines: Use a basin wrench to loosen and remove the nuts holding the old faucet supply lines to the shut-off valves.
  3. Remove Mounting Nuts: Use a basin wrench or crescent wrench to remove the large mounting nuts that secure the faucet to the sink or countertop.
  4. Remove Old Faucet: Lift the old faucet out. Clean the mounting area.
  5. Install New Faucet: Apply plumber’s putty or a gasket (provided with the new faucet) around the base of the new faucet. Insert the faucet into the mounting holes.
  6. Secure Faucet: From below, thread on the mounting nuts and tighten them with a basin wrench. Do not overtighten.
  7. Connect Supply Lines: Attach the new faucet’s supply lines to the shut-off valves. Hand-tighten, then use a wrench for a quarter-turn or half-turn more. Do not overtighten, especially if using plastic supply lines.
  8. Test: Slowly turn the water back on at the shut-off valves and check for leaks.
• Installing a Garbage Disposal:
  1. Shut off Power: Crucially, turn off the power to the disposal circuit at the breaker box.
  2. Remove Old Drain (if applicable): Disconnect the P-trap and any existing drain components.
  3. Install Mounting Assembly: Disassemble the new disposal’s mounting ring assembly. Install the drain flange into the sink opening, sealing it with plumber’s putty from above. Secure the mounting ring assembly from below the sink.
  4. Connect Dishwasher Drain (if applicable): If you have a dishwasher, you’ll need to remove the knockout plug inside the disposal’s dishwasher inlet port using a screwdriver and hammer. Connect the dishwasher drain hose to this port using a clamp.
  5. Mount Disposal: Lift the disposal, align the three mounting tabs with the mounting ring, and twist until it locks into place.
  6. Connect Drain: Connect the disposal’s discharge pipe to the P-trap, and then to the wall drain stub-out. Ensure all connections are tight.
  7. Electrical Connection: Connect the disposal’s electrical wiring according to manufacturer instructions. If you’re unsure, hire a qualified electrician.
  8. Test: Turn power back on. Run water into the sink and test the disposal. Check for leaks.
• Clearing a Clog in the P-Trap:
  1. Place a Bucket: Position a bucket directly under the P-trap to catch water and debris.
  2. Loosen Slip Nuts: Use channel-lock pliers to loosen the slip nuts on either side of the P-trap.
  3. Remove P-Trap: Carefully remove the P-trap. Water will drain into the bucket.
  4. Clear Debris: Remove any hair, grease, or food debris stuck inside the P-trap. You might need a wire hanger or a small brush.
  5. Inspect Drain Lines: Look into the drain lines leading to and from the trap for further blockages. You might need to use a drain snake if the clog is further down.
  6. Reassemble: Reinstall the P-trap, ensuring the slip nuts are hand-tightened, then given a quarter turn with pliers. Don’t overtighten, as this can crack plastic pipes.
  7. Test: Turn on the water slowly and check for leaks.

Tips for Efficiency and Leak Prevention

Mastering how to do plumbing under kitchen sink involves adopting best practices for durability and leak prevention.

• Always Shut Off Water: This cannot be stressed enough. Confirm water is off by turning on the faucet before starting work.
• Have a Bucket and Rags Ready: Spills are inevitable. Be prepared.
• Use Plumber’s Putty or Gaskets: When installing the sink drain strainer or faucet, use plumber’s putty or the manufacturer-supplied gasket to create a watertight seal between the fixture and the sink/countertop.
• Teflon Tape for Threaded Connections: For any threaded connections (like those for supply lines or shut-off valves), wrap Teflon tape clockwise around the male threads 2-3 times before tightening. This helps create a seal and lubricates the threads for easier future disassembly. Avoid using it on slip-joint connections.
• Hand-Tighten First, Then Wrench: For plastic drain pipes and slip-joint connections, hand-tighten the slip nuts as much as possible, then use channel-lock pliers for a final quarter to half-turn. Overtightening plastic can crack it. For metal connections, use appropriate wrenches but avoid excessive force.
• Check for Cross-threading: When threading pipes or nuts, ensure they engage smoothly. If resistance is felt early, back off and try again to avoid cross-threading, which damages threads and causes leaks.
• Proper Slope for Drain Lines: Ensure the horizontal drain pipe (waste arm) leading from the P-trap to the wall drain has a slight downward slope (1/4 inch per foot) to facilitate proper drainage.
• Inspect All Connections: After completing work, turn the water back on slowly and meticulously check every single connection for drips or leaks. Feel for moisture and use a dry paper towel to spot small leaks. Even a tiny drip can cause significant damage over time.
• Consider Flexible Supply Lines: For faucets, braided stainless steel flexible supply lines are easy to install, durable, and resist kinking.
• Ensure Proper Venting: While usually inside the wall, a properly vented sink drain is crucial. If your sink drains slowly, gurgles, or you notice sewer gas smells, it could indicate a venting issue that might require professional attention.