Dry P-trap: why drains smell and how to fix it

The curved section of pipe under every drain is a P-trap. The water sitting in its low point is a simple seal between your home and the sewer.

How a trap dries out

• A drain is not used for weeks, so the water evaporates
• Low humidity or heat speeds evaporation
• A poorly vented drain siphons water out of the trap
• A slow leak in the trap drains it

Drains most likely to dry out

How to fix it

1. Run water in the drain for about a minute. For floor drains, pour in a bucket of water.
2. If the smell returns quickly, check for leaks at the trap.
3. If traps keep emptying after nearby fixtures drain, a plumber should check the venting.

Keep rarely used traps sealed

• Run water in unused drains every week or two.
• Install a trap seal device in floor drains.
• Ask a plumber about a trap primer, which adds water automatically.

If refilling traps does not help, see our sewer smell in the house guide for other causes.

Trap primers explained

A trap primer is a small valve that adds a little water to a floor drain trap automatically, often when a nearby fixture is used or on a timer. Some codes require them for floor drains in certain locations. Retrofitting one means running a small water line to the drain, so it is a plumber's job.