What is a P Trap?


  • A P trap is a plumbing device which prevents odorous gas in plumbing drains and sewers from rising up through a toilet, sink or floor drain into a home or business.
  • A P trap is a U-shaped section of pipe that holds water as shown in the diagram below
  • If a drain, sink or toilet is not used for a long time, or if there is a leak in the p-trap, it may go dry and allow sewer gas to rise into a home or other building.
  • To refill a P trap, flush the toilet, pour water in the floor drain or run water down the sink. Then check to make sure the sewer gas has stopped coming into the building. If sewer gas continues, a repair or other maintenance may be needed.

(The diagram above is schematic. It is intended to indicate how the sink, P trap and drain pipe are connected, but the proportions are not intended to be realistic.)

The water in a P trap is a barrier which prevents sewer gas from rising up through drains and  toilets into a house or other building.

Sewer gas is formed in the natural breakdown of solids in wastewater due to the activity of bacteria.

Sewer gas may not only have an offensive odor, it may include methane, which is potentially combustible.

P traps are built into toilets and are required in plumbing drains under State building code.

If you smell sewer gas inside a home or other building, a faulty or dry P trap may be the cause.

You can fill a dry P trap by flushing (if the trap is part of a toilet), pouring water in a floor drain or running water down the sink.

If these actions do not keep sewer gas from getting into a building, there may be a leak or loose connection at the p-trap. (You may need help from a plumber.)

For more information: 919-968-4421 or info@owasa.org