Why is an S-trap illegal?
Thursday, April 16, 2020
An S-trap, like in the photo above, has been banned by the building codes for decades, because it is considered an “unvented” drain. Venting is necessary for a sink to drain properly. In essence, sufficient air has to get into the pipe in order to displace the water and allow it to exit easily. It’s called an S-trap because of the sideways S-loop it makes before it heads directly downward.
A simple experiment can demonstrate the problem with an unvented drain. Place your thumb over a straw that is partially immersed in a glass of water. As you lift the straw out of the glass, the water level in straw stays intact, rising about the surrounding water in the glass. When you release your thumb, the water in the straw drains to the level of the rest of the water in the glass.
Although an S-trap is able to gulp some air for displacing the draining water, so it’s not exactly like the straw experiment, the air is not sufficient for good drain flow. Plus, S-traps tend to suck out the water seal in the trap (necessary to keep sewer gas from rising up into through the sink into the home) as they finish draining.
A properly installed P-trap, like in the diagram above, will always keeps it’s water seal. If you have S-trap drain, and notice sewer-type odors in the room, you can run the water slowly down the drain for a few seconds to replenish the trap-seal as a temporary fix. But, of course, the best solution is having a licensed plumber bring the drain piping up to modern standards.
See our blog post What plumbing traps are illegal by code? for a list of other prohibited trap designs.
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