Why does a horizontal vent have to be at least 6" above the rim of the fixture you're venting? Why can't it just be a few inches above the level of the trap? Also, what's wrong with using a sanitary tee instead of a wye for a common drain?

Thanks for the answers!

Dhoult:

The idea behind a vent connecting AT LEAST 6 inches above the flood level rim of the HIGHEST fixture in a battery of fixtures (or an individual fixture) is that IF the drain backs up for whatever reason, the waste will OVERFLOW the fixture before it overflows the VENT and the vent effectively becomes the new waste line.

If fixture was piped so the vent connected below the flood level rim of a clogged fixture then waste could continue to flow down the vent pipe indefinitely and could eventually lead to another clogged waste or vent pipe in the system... especially seeing how vent pipes are usually much smaller than the drain pipe intended for each fixture.

And basically, the reason sanitary tees are frowned upon for horizontal connections is because a TEE fitting is blunt/short and when a snake goes down the drain to clear a blockage the snake could potentially go UP the drain instead of DOWN the drain... and that could be a bad thing... ;)

Let me know if you need more information...

MARK

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Why does a horizontal vent have to be at least 6" above the rim of the fixture you're venting? Why can't it just be a few inches above the level of the trap?

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Because code calls for 6 inches over the flood rim of the uppermost fixture on the vent stack and you don't question code no matter how silly it sounds. You could tie back a few inches above the trap, however, If the stack vent had any back pressure at all It would be possible for the fixture to spill over into the new revent instead of simply overflowing the rim. Hey! The framers work on a "worst case scenario" when they write code and a lot of codes are just downright silly and sound like overkill. We do not use sanitary tees on a horizontal drain line for the same reason we don't use vent ells, because of the short radius. Regards, Tom Sorry Mark, I posted right on top of you. I like your explanation better.

Thanks guys! While I understand the general reasons for having codes and try to follow them, I also like to understand specifically why rules are written the way they are.

Thank you for answering that!