Bathroom Sink Plumbing: Parts, Sizes, and How It Works
Bathroom sink plumbing is a compact loop of four small systems under one basin: two supply lines feed the faucet, two shutoff valves control them, a pop-up drain assembly opens and closes the drain, and a P-trap holds a water seal that blocks sewer gas. Most lavatory drains run 1-1/4 inch, and supply risers are usually 3/8 inch. Exact sizes and fittings vary by local code.
What does bathroom sink plumbing look like under the sink?
Open the vanity and you see a mirror layout: hot on the left, cold on the right. Each shutoff valve feeds a supply riser up to the faucet. Center-rear, a tailpiece drops from the drain body into a P-trap, which turns and runs to the wall drain stub-out set during the bathroom rough-in.
The supply side and drain side are independent. Supply lines carry pressurized water and are always live until you close the stop valves. The drain side moves only by gravity, so slope and a clear vent matter more than tightness. A thin rod linkage ties the faucet to the stopper, which is the one place the two systems meet.
Bathroom sink plumbing parts and sizes
Bathroom sink plumbing uses the same core parts on almost every basin, and knowing each one by name makes diagnosis fast. The table below lists what each component does, its typical size, and the failure it is most known for, including the pop-up drain and pivot-rod linkage that trips up most homeowners.
| Component | What it does | Typical size | Common failure |
|---|---|---|---|
| Shutoff (stop) valve | Cuts water to the sink without shutting the house | 3/8 in outlet, 1/2 in inlet | Seizes or weeps at the stem after years unused |
| Supply riser | Carries hot or cold water up to the faucet tailpieces | 3/8 in, braided steel | Cracked washer or corroded nut drips at the valve |
| Drain body and flange | The strainer opening seated in the basin | 1-1/4 in outlet | Putty or gasket seal under the flange fails and weeps |
| Pop-up stopper | Plugs or opens the drain | Fits 1-1/4 in body | Seal cracks or hair loads up and it stops sealing |
| Pivot rod and ball | Horizontal rod that lifts and drops the stopper | Standard pivot ball | Nut backs off, rod slips, or the gasket leaks |
| Clevis strap | Slotted strap linking lift rod to pivot rod | Multi-hole strap | Set in wrong hole, so the stopper will not close |
| Lift rod | Vertical knob behind the faucet you pull up | Faucet-specific | Slips at the clevis screw and loses travel |
| Tailpiece | Straight pipe from drain body to the trap | 1-1/4 in | Slip nut loosens, or plastic threads crack |
| P-trap (J-bend) | Holds a water seal against sewer gas | 1-1/4 in | Slip washer wears; trap dries out if unused |
| Trap arm and wall tube | Connects trap to the wall drain and vent | 1-1/4 in | Wrong slope backs up; poor connection leaks |
| Overflow channel | Routes water from the basin overflow hole to the drain | Built into basin | Grime clogs it and it develops a musty smell |
How the pop-up drain assembly and pivot rod work
The pop-up drain turns a knob into stopper motion through a simple lever chain. Pull the lift rod up behind the faucet and it lifts the clevis strap; the pivot rod, seated in a slot on the strap, seesaws on its ball joint and pushes the stopper down to seal. Push the knob down and the stopper rises to drain.
The pivot rod passes through a ball-and-gasket port on the back of the drain body, held by a retaining nut. That nut sets how tight the ball seals and how freely the rod swings. If water weeps there, snug the nut; if the stopper will not travel far enough, move the pivot rod to a different hole on the clevis and re-tighten the clevis screw.
Water supply lines and shutoff valves
Two shutoff valves under the sink feed the faucet: hot on the left, cold on the right. Each has a 3/8 inch outlet that connects to a braided supply riser running up to the faucet tailpiece. Closing both valves isolates the sink so you can service the faucet or lines without killing water to the whole house.
Angle stops are the most common valve here, mounting where the supply comes through the wall. Quarter-turn ball stops are more reliable than older multi-turn stops, which tend to seize. If a valve weeps at the stem when you turn it, that stop is due for replacement. A drip at the faucet handle instead points to fixing a dripping faucet, not the supply lines.
The P-trap, tailpiece, and drain sizes
The tailpiece drops straight from the drain body into the P-trap, the U-shaped bend that defines a lav drain. The trap holds standing water that seals the line against sewer gas. From the trap, the trap arm slopes to the wall. Standard lav parts are 1-1/4 inch, with slip nuts and washers at each joint.
Slope matters more than muscle on the drain side. Aim the trap arm at about 1/4 inch of fall per foot toward the wall so waste clears without draining the trap seal. Hand-tighten plastic slip nuts plus a quarter turn; overtightening cracks them. If the trap sits unused for weeks the seal can evaporate and let odor in, which is one reason to understand how a P-trap holds its seal.
How to put a bathroom sink drain together
Assembling a lav drain follows a fixed order from the basin down to the wall. Dry-fit everything first, keep slip nuts and washers oriented correctly (bevel of the washer facing the joint), and only snug plastic threads by hand plus a light turn. Work top to bottom so each joint supports the next.
- Bed the drain flange: drop the drain body through the basin and bed the flange in plumber’s putty (or silicone on plastic bodies), then tighten the locknut underneath.
- Set the gasket: seat the rubber gasket and friction washer under the basin so the drain body seals against the sink.
- Insert the pivot rod: push the pop-up stopper into the drain, then thread the pivot rod through the ball port so it engages the stopper stem; tighten the retaining nut.
- Hang the tailpiece: thread or slip the tailpiece onto the drain body with its slip nut and washer.
- Fit the P-trap: connect the J-bend to the tailpiece and the trap arm, leaving nuts loose for alignment.
- Set the trap arm slope: adjust the arm into the wall tube at about 1/4 inch fall per foot, then hand-tighten all slip nuts a quarter turn.
- Link the pop-up: run the lift rod through the clevis, connect the clevis to the pivot rod with the spring clip, and adjust so the stopper seals fully closed.
- Test: fill the basin, then drain it, and watch every joint for beads of water before you trust it.
Common bathroom sink plumbing problems and leaks
Most bathroom sink plumbing calls trace to a handful of faults: a loose slip nut, a worn washer, a cracked plastic trap, a failed flange seal, or a pop-up out of adjustment. Dry the whole assembly, run water, and watch where the first bead appears. The leak point tells you which part to service.
Slow drainage with no full clog usually means hair wrapped on the pop-up stopper; lift it out and clean it. A musty smell often means a dry trap or a clogged overflow channel. Water pooling at the base of the drain body points to a failed flange seal that needs the drain reset. When the fault is at the faucet spout or handle rather than under the basin, treat it as a faucet repair, not a drain repair.
Frequently asked questions
What is the pipe under the bathroom sink called?
The curved pipe is the P-trap, sometimes called a J-bend or waste trap. It holds a small pool of standing water that blocks sewer gas from rising into the room. Above it sits the tailpiece, the straight pipe dropping from the drain body, and the trap arm runs from the trap to the wall.
What size drain pipe does a bathroom sink use?
Most bathroom (lavatory) sinks drain through 1-1/4 inch pipe, including the tailpiece, P-trap, and trap arm. Some newer vanities and vessel sinks use 1-1/2 inch. Supply risers to the faucet are usually 3/8 inch compression at the valve. Sizes vary by fixture and local code, so match the parts already installed.
Why won’t my pop-up stopper hold water?
A pop-up that will not seal usually has a misadjusted lift rod or a stopper sitting too high. Loosen the clevis screw, push the stopper down flush, then re-tighten so the rod pulls it closed. A cracked or gunked-up stopper seal, or a pivot rod set in the wrong clevis hole, causes the same problem.
Why is my bathroom sink leaking underneath?
Most under-sink leaks come from loose slip-joint nuts, worn slip washers, or a cracked plastic trap. Dry the joints, run water, and watch where beads form. Hand-tighten the nut plus a quarter turn. A leak at the drain body itself often means the flange gasket or putty seal under the drain failed.
Do I use plumber’s putty or silicone on a sink drain?
Plumber’s putty is the traditional seal under a metal drain flange because it stays pliable and is easy to remove later. Use silicone on plastic drain bodies or where a maker specifies it, since putty can degrade some plastics. Never rely on putty inside a threaded joint under pressure; that is a job for thread tape or dope.
Can a bathroom sink drain without an overflow?
Yes, but the drain and stopper must match. Sinks with an overflow hole use a drain body with side ports; sinks without one need a non-overflow (open) drain. Installing an overflow-style drain on a basin with no overflow leaves an open port that can weep. Vessel sinks commonly have no overflow.
Last reviewed: September 2026.