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 not allowed?

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.

Is s-trap bad?

“S” traps can allow water to flow down a drain with such force that it pulls the water out of the trap resulting in a dry trap. Fig 1. We want water at the bottom of the trap to keep sewer gases from escaping into our homes. ... With no water in the trap, gases can escape out into our home.