It’s called an S trap because it looks like the letter “S” when the configuration is laid on its side and flattened out. S traps are commonly found in homes that are older, have not been updated or where the plumbing work was not professionally done. This S trap style configuration is no longer permissible.
So what’s the big deal? They’ve been used for years, why the concern? These unvented S traps are often found under kitchen / bathroom sinks. Without a vent to allow air into the plumbing system, they often don’t drain well and can make a gurgling sound while draining. More importantly though, the water in the S trap can get back-syphoned or sucked out of the trap allowing dangerous sewer gases (potentially explosive methane) to enter the home. Today, we use a P traps which provide a a vent for the plumbing system, allowing for better drainage and the venting of sewer gases.
Not surprisingly, there are also other configurations on a theme as well however they don’t necessarily all look like the letter S. In fact, some don’t resemble any letter of the alphabet at all 🙂 The point being, if the plumbing system does not use today’s P trap then it will prone to problems.
Protect yourself…get it inspected!