Why are S traps made illegal for new and remodel installations? Assuming that the venting is done correctly, what is wrong with them? I have run into a couple of situations where there was nothing I would have liked more (read time and material savings) than to use a good old S trap. Venting was not an issue.

EPMiller

Even vented an S trap can siphon and no longer maintain a water seal. Doesn't happen every time but often enough for it to go out of code some time ago.

Venting was not an issue. Venting is always a issure with a "S" trap, (see image) That's why they're illegal. Code addresses the worst possible scenario, and without a vent it's possible for the trap seal to be siphoned out allowing sewer gas to enter your home. Didn't say it would happen every time you drained a fixture but the possibility exists and that's enough to outlaw them.
Will "S" traps work? Yes! Back in the 40's when I broke into plumbing most of the older homes in my area had "S" traps. That's how we plumbed back then. But times changed and now they'd been outlawed in new construction and permitted remodels.
Did that answer your question? Regards, Tom

Yes, my question has been answered to my satisfaction. I grew up in a house with all S traps and never smelled sewer gas from the drains, so I guess it is only in some situations. After your explanations I see the wisdom in not allowing them.

Thanks again.

EPM