Short answer
A P-trap runs sideways into the wall, where the drain is vented, so its water seal stays put. An S-trap curves back down into the floor; the falling water can siphon the trap dry and let sewer gas in. The International Plumbing Code lists S-traps among prohibited trap types. Converting one usually means adding a vent, often an air admittance valve where local code allows. A licensed plumber is the right call for that.
How each trap works
Every sink trap does the same job: the U-bend holds a plug of water that stops sewer gas from coming up the drain. The ICC summarizes the International Plumbing Code rule that this seal must be at least 2 in and no more than 4 in deep. The difference between the two traps is what happens after the bend.
| P-trap | S-trap | |
|---|---|---|
| Shape | U-bend, then sideways | U-bend, then down |
| Drains into | A pipe in the wall | A pipe in the floor |
| Venting | The wall drain is vented | Often no vent at all |
| Water seal | Stays put | Can be siphoned dry |
| Code status | The standard trap | Listed among prohibited trap types in the IPC |
Why S-traps lose their seal
When a sink drains, the water forms a moving column. In a P-trap, that column turns sideways into the trap arm, and the vent in the wall drain lets air in behind it, so the trap keeps its water. In an S-trap, the column drops straight down. As it falls it pulls on the water behind it, like a siphon, and it can drag most of the seal out of the bend. With little or no water left, nothing stops sewer gas from rising into the room.
That is why the ICC overview lists S-traps, along with drum traps and bell traps, among the trap types the International Plumbing Code prohibits. Older homes still have them because they were once common. Local codes vary, so check yours.
How to tell which one you have
- Follow the pipe. Open the cabinet and trace the pipe past the U-bend. Into the wall means P-trap. Down into the floor means S-trap.
- Listen. A loud gurgle as the last of the water drains is a common sign of a trap being pulled dry.
- Smell. A sewer smell from a sink that is used every day points to a seal that is not holding; Mr. Rooter lists dry traps and venting problems among the usual causes of drain odor. See sink drain smells for other causes.
Converting an S-trap to a P-trap
Swapping the trap alone does not fix it. A P-trap needs a trap arm that runs into a vented drain. If your drain comes up through the floor with no vent, converting usually means:
- Running a vent pipe from the drain up through the wall, which means opening the wall.
- Adding an air admittance valve (AAV), a one-way vent that lets air in when water flows and stays closed the rest of the time. AAVs are used where local code allows; some areas restrict them.
- Rerouting the drain into the wall, if a vented line is close by.
Venting rules, trap arm lengths and AAV rules differ between codes and towns. This is a job to have a licensed plumber price and permit, not a weekend swap.
Living with an S-trap for now
If you cannot convert it right away, keep the sink in regular use so the trap refills, and run water for a few seconds after the sink drains to top up the seal. Keep the trap clean: see the P-trap guide for how to take one apart. Replacing the drain above the trap is still a normal job: see how to replace a bathroom sink drain or how to replace a kitchen sink drain.
Questions people ask
What is the difference between an S-trap and a P-trap?
Both hold water in a U-bend. A P-trap leaves the bend sideways through a trap arm into a vented drain in the wall, so it looks like a P on its side. An S-trap leaves the bend and turns straight down into the floor, so it looks like an S on its side.
Why are S-traps not allowed?
Water rushing down the vertical drain after the trap can pull the water out of the trap by siphon. With the seal gone, sewer gas can come up through the sink. The International Plumbing Code lists S-traps among prohibited trap types. Local codes vary, so check yours.
How do I know if I have an S-trap?
Look under the sink. If the pipe after the U-bend goes into the wall, it is a P-trap. If it curves back down and goes into the floor, it is an S-trap. A gurgling sound after the sink drains, or a sewer smell, are common signs of a trap losing its seal.
Can I just replace an S-trap with a P-trap?
Not by itself. A P-trap needs a vented drain to connect to. If the drain comes up from the floor with no vent, converting usually means adding a vent, either a pipe vent or an air admittance valve where local code allows. A licensed plumber can tell you what your code requires.
What is an air admittance valve?
An air admittance valve is a one-way vent that lets air into the drain when water flows, then closes to keep sewer gas in. It is used where running a vent pipe is hard. Whether you can use one depends on your local code.
Sources
Last reviewed October 12, 2026. Parts, sizes and listings change; check the product details before you buy, and follow the maker’s instructions.