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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