View Full Version : Trap height question

02-18-2007, 09:56 AM
I am finishing my basement and I have a p-trap from a first floor tub that hangs lower than anything else in one area.

Is it possible to move the trap up and then tie it back into the drain line with 2 45's? I read in a plumbing book that this would cause a "broken vent".

Thanks in advance for any advice...

02-18-2007, 10:06 AM
The proposed one buries the vent and is illegal.

02-18-2007, 10:25 AM
Thanks for your help.

Pardon my ignorance. I'm having trouble visualizing exactly what effect that has on the vent air flow.

I'm not questioning the illegality of it, just trying to better understand why it doesn't work.

Thanks again for the quick feedback.

02-18-2007, 10:43 AM
The vent breaks up the siphon effect.
Like when lifting your finger off the top of the straw,
Or if you have ever siphoned water with a hose, you know that the least little amount of air will make the siphon stop.

For that to work in plumbing, the trap arm can only be so long, and with so much grade, or the air isn't allowed to work as a siphon preventer.
When you don't allow air to get there, the trap siphons,
If the trap siphons, then your next question on this forum is:

"My bathroom smells, what do I look for?"

And the answer will be, look for a trap that has siphoned dry.

If you want to raise the trap, you will need to raise the fitting with the vent too.
Cut out the tee, and raise it, then your trap arm can still grade up at an
acceptable 2% grade.

02-18-2007, 10:55 AM
Thank you very much!
That makes perfect sense.

02-18-2007, 11:20 AM
From what I can tell, it's wet vented through the bathroom sink drain, which can't really be moved up.

So, I have another potentially stupid question regarding the tub drain...
Could I add an AAV right after the tub p-trap?
Both of my basement sink drain rough-ins have them, so I assume they're allowed in my area.

02-19-2007, 04:54 AM
Your original drawing creates a 3/4 "S" trap which is illegal. An AAV tied into the outlet pipe of the trap, if you do it properly, will solve the problem.

02-19-2007, 06:35 PM
Thanks for the help.
I was able to confirm the local building codes.
As you said an AAV would technically be allowed as long as there is access.

I think I'll just leave it the way it is and give up the ceiling height.