What is P-Trap?
Plain-English explanation from a licensed Utah HVAC, plumbing, and electrical contractor.
A P-trap is the curved section of pipe under your sink or shower that holds a small water seal, that water plug stops sewer gases from coming back up through the drain into your living space.
Full Definition
A P-trap is a P-shaped section of drain pipe, a U-shaped horizontal portion followed by an outlet that connects to the vertical drain line. The U holds a water seal (typically 2"-4" deep) that acts as a barrier to sewer gas. The Utah IPC requires a trap under every fixture and a vent within a specified distance to prevent siphoning of the seal.
Dried-out P-traps in unused bathrooms (guest baths, basements, garages) are the most common cause of mystery sewer smell in Utah homes. Pouring a quart of water and a teaspoon of bleach into each unused drain monthly solves it. Cracked or improperly sized P-traps are common DIY mistakes.
Common Questions
Why is there standing water in my P-trap?
That's the water seal, it's supposed to be there. The trap holds enough water (the 'trap seal') to block sewer gas while still letting wastewater drain through.
What's an S-trap and why are they banned?
An S-trap drains vertically downward after the trap, which can siphon the water seal away during heavy drainage and let sewer gas in. They were common pre-1970 and are not allowed in modern Utah plumbing code.
Related Terms
Plumbing Vent Stack
A vent stack is the vertical pipe extending from your home's drain system through the roof, it equalizes pressure so drains can flow freely and keeps sewer gases out of the living space.
ABS Pipe
ABS is black plastic drain-waste-vent pipe used for residential plumbing, it's solvent-welded with one-step cement, performs well for sewer and vent piping, and is the standard Utah residential DWV material under code.
Recent PLUMBING work in Utah
A few installs and service calls from the AYSP crew.
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