Horizontal Vent Pipe Below Floor?

[froddan]

For a 1 1/2" Tub-Drain, is it ok to have about 1 foot of the Vent run horizontal below the floor until it reaches the vertical vent?

It would just be a few inches above the drain.

Can I use 90 degree turns for the 1 1/2" vent?

[LazyPup]

Not even one inch. The Plumbing Codes strictly prohibit any horizontal run of a vent until the vent reaches an elevation 6” higher than the flood level rim of the highest fixture served by that vent.

The solution to your bathroom would be to first run a 1-1/2” line diagonally from the tub trap to the stack.

Next run a 1-1/2” line diagonally from the lavatory sink riser to the tub drain line and tie into the tub drain line using a Wye & 1/8 bend.

At any point which is 6” or higher above the lavatory sink top you can then run an 1-1/2” vent horizontally and tie into the stack.

I have added an isometric view to your bathroom print to show the layout.

The RED line from the tub trap to the junction of the line from the lavatory would be classified as the Tub Fixture Arm and the length would be limited to the maximum allowable lengths for fixture arms.

The Black line from that junction on to the stack is classified as a “Vented Branch” and may be an unlimited length.

The BLUE line from the tee behind the lavatory fixture arm down and across to the tub drain is classified as a “Combined Waste & Vent”, which is also permitted to be an unlimited length.

Once the vertical riser at the lavatory reaches an elevation 6” higher than the flood level rim of the lavatory bowl, or at any convenient point above that, you may run a horizontal vent from the riser to the stack. (Technically classified as a “Re-vent” and shown as a Dotted Line in the illustration).

[froddan]

Lazypup,
Thank you for your answer. I will definitely avoid the horizontal vent...
I have found one more option that I think would work.
There is another 3' pipe that comes from another toilet in another bathroom. This drain pipe runs parallel to the sink drain and ends in the main stack. (This 3' pipe is Yellow in my new drawing). The tub drain currently crosses above this pipe, and I was thinking that I could tie into the 3' pipe.
That way the vent for the tub can remain where it currently is, and I would assume that the flow should be faster.

So the tub drain is 1.5' and the toilet drain is 3'.
The tub drain crosses the toilet drain about 4 inches above it, and my thought was to do a 90 degree drop into the toilet drain.

Any thoughts on this?

[LazyPup]

in order to drop into that 3" from the top you would need to install a 3x3x1-1/2 Wye and a 1-1/2" 1/8th bend then your 1/4bend. I doubt that you could fit all that into a 4" vertical and still maintain the required pitch from the tub trap.

The solution would be to install a 3'x3"x1-1/2" wye with the side opening of the wye on the side or horizontal plane. You could then run your fixture arm from the wye directly to the trap and you would not need any additional venting.

[froddan]

LazyPup,
Again, thank you for all your help. You are a realy tropper with your detailed answers.
Last night was a real battle....I figured this one out.
As it was so strange to me why this wouldn't drain fast enough after making sure all vents were clear, the entire line was new, but I still snaked it all the main to the main etc. The pitch was about 1/2 inch per foot, so more than what is required. So I finally figured it out....the stupid overflow tube set that I purchased was really small..although it says 1 1/2", the inside diameter was only 1 1/4", and the biggest problem was probably where the overflow and the tub pipes meet, it was a T (or cross) with NO BEND, and the hole was quite small. I know you've been preaching to me that when you have a 1/8 bend when going from horizontal to vertical...so I'm picking up here...too bad the manufacturer didn't know that.
Anyway, it drains like a charm now, as I replaced the entire overflow set with a good solid 1 1/2" abs set.
My one minor issue is that after replacing the P-Trap, there seem to be a very tiny leak...about 1 drop every 30 minutes. I am not even sure where it is coming from because the entire pipe is dry.

One drop per 30 minutes is of course not good enough.. and the wetnes is right below the lowest part of the P-trap. I put a napkin there and let the water run for a long time...
I am using the kind of P-trap where both ends are glued to the ABS pipe, but the middle part is screwed in. I was told that no teflon tape and no washer should be used on these at the plumbing store...is that correct?
How tight should the connection be? As I am not 100% sure this is where the minor leak is coming from, it might actually not be the P-trap that is the problem. But everything else is glued with ABS...

Thanks again!
How tight should the P-trap connection be (I believe it si