Short answer
An open sink drain with no trap is unfinished plumbing, not a leak, and a plumber finishes it in about an hour. Measure straight down from the bottom of the sink tailpiece to the top of the pipe in the floor. Under about 4 inches, a new trap kit bridges that gap. More than 4 inches, or a drain pipe that sits off to one side, means the trap needs an extension first, and that is where most DIY attempts stop.
Tell a parts problem from a plumbing problem in five minutes
- Cap the open pipe. Tape a plastic bag over it or press a test cap in. Sewer gas comes up an untrapped drain, and that is the musty smell in the cabinet.
- Measure straight down from the bottom of the sink tailpiece to the top of the pipe at the floor or wall. Write the number down.
- Measure sideways, center to center, from the tailpiece to that pipe.
- Read the pipe sizes. A bathroom tailpiece is usually 1 1/4 inch and a floor or wall stub is often 1 1/2 inch, so the kit has to match both.
- Check how the stub leaves the floor. Straight up is easy. Angled out of the floor or wall limits how far the trap arm can swivel.
- Count your slip nuts and washers. Every joint needs one nut and one washer, and a dry joint leaks the first time you fill the basin.
- Slide the rusty bell-shaped cover at the floor up the pipe and look under it. It hides the hole in the floor and seals nothing.
If the gap is under 4 inches, the sizes match and the stub runs straight up, a new trap kit finishes the job. If the gap is longer, the sizes differ, or the stub is angled or offset, the trap needs an extension or a new fitting.
Repair or replace
| Situation | What to do |
|---|---|
| Trap missing, tailpiece and floor stub are both sound | Repair. One new trap kit with fresh washers finishes it. |
| J-pipe is short of the drain by more than 4 inches | Repair. Add a tailpiece extension or a deeper trap kit. Never stretch a slip joint to reach. |
| Old chrome J-bend is thin, pitted or cross-threaded | Replace the trap with a plastic kit. Chrome under a closed vanity is for looks. |
| Floor stub sits several inches to one side | Repair. Swivel the trap arm or fit an offset. Forcing the arm sideways loads the joint and it weeps. |
| Floor stub is cracked, corroded or moves in the floor | Hire a plumber. The stub gets cut back and a new fitting glued on. |
| You have fought it for weeks and it still will not line up | Hire a plumber. An hour of labor costs less than two more kits. |
| Shutoff valves above the trap are rusted or weeping | Replace the valves in the same visit. They are a separate part of the job and get priced separately. |
What it costs
Runhome contractor estimates ran from $185 to reconnect the drain and valves up to $1,610 for a new sink with a complete drain assembly, as of October 2026, including the trip and assessment.
| Option | Range |
|---|---|
| Reconnect the drain, new P-trap assembly only | $193 to $455 |
| Reconnect the drain plus two new shutoff valves | $185 to $390 |
| New sink with a complete drain assembly and valves | $455 to $1,610 |
Can I do it myself?
Yes, if the gap measures under 4 inches, the tailpiece and the floor stub are sound, and the sizes match a kit on the shelf. The job is one wrench, three slip joints and no glue: dry-fit the trap bend to the tailpiece, swing the arm into the stub, seat a washer at every joint, then hand-tighten and fill the basin to test.
Skip the flexible accordion trap even when it is the only thing that reaches, because the ribs trap waste and most inspectors fail it. Hire a plumber when the stub is cracked or loose, when the stub and tailpiece are more than a few inches apart side to side, or when the drain needs cutting or gluing to line up.
What a contractor will ask you
Have these ready and you skip the back-and-forth.
- A wide photo under the vanity, wall to cabinet front, showing the drain stub, every loose trap part and the shutoff valves.
- The gap in inches from the bottom of the sink tailpiece to the top of the pipe at the floor.
- Whether that pipe runs straight up or comes in at a slant, and how far it sits off to one side.
- Whether the sink is only disconnected or is also leaking, and whether another bathroom is usable meanwhile.
- Every loose drain part you already have: trap bend, trap arm, extensions, slip nuts, washers.
- Whether you want those parts reused or everything replaced from the tailpiece down.
- How old the vanity and drain setup are, or when the sink last worked.
- Whether rusted shutoff valves are in scope or left for another visit.
Common questions
- Can I use the sink with no P-trap on it?
No. The trap is the water seal that keeps sewer gas out of the room, and an open pipe vents it straight into the cabinet. Cap the pipe and use another sink until the trap is on.
- My J-pipe won't reach the black pipe from the sink.
You need length added above the trap, not a stretched joint. A tailpiece extension drops the trap down to meet the stub, and a kit with a longer trap arm covers the rest. Stretching a slip joint to close the gap is what makes it weep a week later.
- Why does the open pipe under my vanity smell musty?
That is drain air coming up an untrapped pipe. It gets stronger in dry weather and after the house warms up. The smell goes when the trap holds water again.
- Do the rusted shutoff valves have to be replaced at the same time?
No, the drain and the valves are separate jobs, and you can have just the drain done. Replacing both valves while someone is already under the sink costs $20 to $60 in parts. Rusted valves that weep or will not turn are worth doing then rather than in a leak.
- Can I reuse the old chrome J-bend I already have?
Sometimes, if the threads are clean and the walls are not pitted. Chrome traps are thin, they corrode from the inside, and a plastic kit costs $15 to $30. Behind a closed vanity door nobody sees it.