Back to "S" traps - The reason "S" traps aren't allowed is because they have the potential to suck, or 'siphon', water out of the trap as the water flows down the drain. ... Believe it or not, enough water to break the water seal at the trap and let sewer gases come in to the house.

What is wrong with an S trap?

The “S” trap is prohibited under the Uniform Plumbing Code throughout the United States. This is because the “S” trap will siphon or suck water out from the trap which will end up releasing methane (sewer) gases into the home.

Why are S traps no longer used?

S-traps are no longer used in modern plumbing because the water can be sucked completely out of the trap allowing sewer gas to enter your home. The risk from sewer gas can be much worse than just that terrible smell, as sewer gases can be poisonous or explosive.