P-trap or S-trap — what's the actual difference and how do you know which one to use?

If you have ever seen the curved pipe beneath the sink, that is the drain riser. It can hold a small amount of water to prevent the smell of sewage from entering the house. There are mainly two types of risers - P type and S type, and they cannot be used interchangeably. Choosing the wrong type can lead to subsequent problems.

The P type elbow first bends downward, then upward, and then horizontally from the wall outlet. From the side view, it roughly looks like a horizontal "P" shape. The S type elbow first bends downward, then upward, then bends downward again, and finally from the ground outlet. It looks like an "S" shape. This direction of the outlet - between the wall and the floor - is the key difference, and its determination depends on the outlet position of the drain pipe, not personal preference.

In many countries, including most of the United States and Europe, the use of S type trap is prohibited in new buildings. The reason is the siphon effect: as the pipe extends downward, when the water flows rapidly, the water seal is pulled out directly. Once the water seal falls off, the sewage gas can flow freely into the house. You may not notice it until you smell the odor. And the P type trap has a horizontal extension, which can naturally break the siphon effect and keep the water seal intact.

Look at the outlet position of the drain pipe from the wall or floor. If the drain pipe extends horizontally from the wall, a P type elbow is needed; if the drain pipe extends vertically from the floor, theoretically an S type elbow is needed, but a better solution is to install an exhaust valve or adjust the pipe direction as much as possible. In old residential buildings built before modern standards, S type elbows were more common, as long as they do not fail in sealing due to low usage frequency, they can still be used normally.