What It Is
A drain trap is a curved section of drain pipe that retains a small amount of water to create a physical seal between the living space and the sewer system. Every plumbing fixture - sinks, tubs, showers, floor drains - has a trap. The water sitting in the trap blocks sewer gas from traveling back up through the drain opening. Without a functioning trap, gases produced in the sewer system have a direct path into the room.
What It Does
The trap's purpose is deceptively simple: hold water. That standing water creates an airtight (or rather, gas-tight) seal across the drain pipe. When drainage flows through, it displaces the water and re-fills the trap as flow slows. As long as the trap stays full, sewer gas cannot pass through it.
The trap also catches small objects that fall into drains - earrings, small hardware, toothpaste caps - before they reach the sewer. The removable clean-out plug on many p-traps allows a plumber to open the trap and retrieve a dropped item or access the pipe for cleaning.
Where It Is Located
Under every sink, you can see the p-trap - the curved white or chrome pipe that dips below the drain outlet and then rises to connect to the wall drain. Shower and tub traps are built into the floor structure and not visible from above. Toilet traps are built into the toilet itself - the visible curve at the base of the bowl is the trap. Floor drain traps sit below the floor slab, accessible only from the drain opening above.
How It Works
The trap is shaped like the letter P (or sometimes S, depending on the configuration). The bottom of the curve holds water by gravity. Wastewater flowing in from the fixture pushes through the bottom of the trap and out to the drain stack, while a small amount of water remains behind in the curve. The vent stack maintains the air pressure needed to prevent this water from being siphoned out under normal conditions.
A blocked or missing vent can cause a siphon effect that pulls the water seal out of the trap. This is why plumbing codes require vent pipes - they equalize pressure so traps stay sealed.
Common Problems or Signs of Failure
- Sewer odor from a drain - usually a dry trap that has evaporated
- Gurgling from a toilet when another drain runs - can indicate trap siphoning due to a vent problem
- A trap that was removed during repair work and not replaced
- Old drum traps (an older design no longer installed to code) that have corroded or failed
Can a Homeowner Check It?
- If a drain smells and the fixture hasn't been used in weeks, simply running water into it for 30 seconds will refill a dry trap - the most common cause of sewer odor.
- You can visually inspect the p-trap under a sink to confirm it is present and has no cracks or drips at the connections.
- If you've recently had plumbing work done and a drain now smells, confirm the trap was reconnected properly - occasionally a p-trap is left off during a repair and not reinstalled.
When to Call a Plumber
If refilling a dry trap doesn't eliminate the odor, or if the smell returns within days, call a plumber. The issue may be a missing or improperly installed trap, a siphoning problem caused by a vent issue, or a cracked trap that is leaking. Some older homes have drum traps that have deteriorated and need to be replaced with a modern p-trap.
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