That Curved Pipe Under the Sink Is a Water Seal — Check That It's Doing Its Job

The Problem

The curved pipe under every sink looks like a plumbing quirk, and most homeowners couldn't say what it's for. Then a bathroom starts smelling like sewer and nobody connects it to the U-shaped bend that's supposed to be preventing exactly that.

The Technique

What the P-trap does, and how to check yours:

1. The bend holds water on purpose. When the sink drains, water flows down, through the trap's curve, and out — but the low point of the curve stays full. That standing water is a plug: sewer gases from the drain line can't pass through it to reach the room.
2. Check the seal. After a sink finishes draining, the trap should still be holding water up to the level of the bend. A trap that empties — or a sink unused so long the water evaporated — is an open pipe to the sewer, and you'll smell it.
3. Know the outlawed shape. Older homes may have an S-trap: instead of the drain arm running horizontally to the wall, the pipe curves up, around, and straight back down. That geometry can siphon — fast-draining water pulls the trap's own seal down the pipe with it, leaving the trap empty and the gas path open. S-traps are no longer permitted; if your sink has one and the room smells, you've likely found the reason to replumb it.