P-Trap
The U-shaped bend of pipe under every sink, tub and shower that holds a small pool of water to block sewer gas from rising into the room.
A P-trap is the curved section of drain pipe you see under a sink, shaped like the letter P turned on its side. Its job is simple and essential: the bend stays full of water, and that standing water forms a seal that stops sewer gases in the drain line from drifting up through the fixture and into your home. Every plumbing fixture connected to a drain has a trap of some kind, and the P-trap is the modern standard because it vents and self-cleans well.
When a drain smells like sewage, a compromised P-trap is the first suspect. A trap that has dried out, common at a guest bathroom or floor drain that goes months unused, loses its water seal and lets gas straight through. The fix is often as simple as running water to refill it. Leaks are the other frequent issue: the slip joints that hold a P-trap together loosen over time, and the under-sink puddle they cause is one of the most common calls a plumber gets.
P-traps are also the easiest spot to clear a local clog and the easiest to reassemble wrong. The trap collects hair, grease and debris, so unscrewing it to clean out a slow drain is a routine homeowner task. The catch is that a trap reinstalled with a missing washer, a cross-threaded nut, or replaced by an illegal S-trap configuration will either leak or lose its seal, which is why the simple-looking part still generates service calls.
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- Vent Stack / Plumbing Vent : The vertical pipe that runs up through the roof to let air into the drain system, so waste flows freely and trap seals are not siphoned away.
- Air Admittance Valve (AAV) : A one-way valve that lets air into a drain when needed but stays sealed otherwise, providing venting for a fixture without running a pipe to the roof.
- Cleanout : A capped access point on a drain or sewer line that lets a plumber insert a snake or camera directly into the pipe to clear clogs or inspect it.
- Soil Stack : The main vertical drain pipe that collects waste from every fixture branch and carries it down to the sewer, extending up through the roof as the vent.