Interactive Photo Diagram
Under-Bathroom-Sink Plumbing
The plumbing under your bathroom vanity — the P-trap, supply lines, and shutoff valves. Most bathroom sink leaks happen here.
Reference checked: component relationships and terminology were reviewed against manufacturer documentation from Oatey. This is original Classic Plumber educational artwork, not a manufacturer drawing.
Tap a numbered marker to learn what that part does.
Overview
Under your bathroom sink, the same two systems as the kitchen work together: supply lines bringing water up to the faucet and a drain carrying wastewater down through a P-trap to the wall. The P-trap holds water to block sewer gas, and the pop-up stopper mechanism lets you fill the basin. Most bathroom sink leaks come from aging slip-joint gaskets or corroded shutoff valves.
How It Works
Water comes in through the supply lines and goes out through the drain. The pop-up stopper seals the drain when you want to fill the basin. The P-trap holds a small amount of water to block sewer gases from entering the bathroom. The trap arm carries wastewater to the wall drain with a slight downhill slope.
Key Components
Pop-Up Assembly
The drain with a stopper controlled by a lever behind the faucet.
P-Trap
Holds water to seal out sewer gas. Removable for cleaning or retrieving dropped items.
Shutoff Valves
Hot and cold angle-stop valves on the back wall of the vanity.
Supply Lines
Braided lines connecting the shutoff valves to the faucet above.
Trap Arm
Horizontal pipe from the P-trap to the wall drain. Should slope downhill.
Common Issues
Leaking under sink
Loose slip-joint nuts or aging supply lines. Check with a dry paper towel.
Slow drain
Hair and soap buildup in the pop-up assembly or P-trap. Remove and clean.
Stopper won't hold water
Horizontal rod disconnected from the clevis strap behind the faucet.
Sewer smell
Dry P-trap. Run water to refill the trap seal.
Maintenance Tips
- Clean the pop-up stopper and P-trap every few months to prevent hair clogs.
- Check shutoff valves periodically — turn them off and on so they don't seize.
- Run water in unused guest bathrooms to keep P-traps from drying out.
- Replace aging braided supply lines before they burst — they have a finite lifespan.