Relocating venting for my tub.

I'm trying to install a new tub and I need to modify my plumbing slightly as the new drain needs to be on the other side of the floor joist. I'm trying to avoid drilling a new hole and to reuse the hole that's there now since it's already improperly drilled(by the builder)

The diagram is drawn as if one is looking down into the open floor everything there is in the horizontal plane. Grey circle A is the closet bend + flange B is the current drain shoe and ptrap C is where the new ptrap/drain shoe needs to be. This is how everything is now done by the builder and inspected.


I'm trying to figure out if 1) the 1.5" vent is needed since the shower drain ties into the 3" vent and if it is how do I bring it to the new drain location without drilling the joist and running vent horizontal in the floor. Also there can't be a bulkhead or anything below due to cabinets. This is a second floor bathroom so both of these vertical vent pipes go into the attic where the 1.5" vent ties into the 3" vent and then they go out the roof.


I know the diagram isn't ideal I tried making an isometric drawing but it failed miserably; but I'll draw anything else that's needed to answer my question.

I suppose a related question is can anyone recommend a good book that explains the theory and practice behind venting? I can't find any posts anywhere with a vent setup like mine and I'm still trying to understand how the shower venting works and if the 1.5" vent at the shower p-trap is even needed.

Here is a good link on venting.

Helpful Plumbing Hints for Residential Construction by Bert Polk Plumbing Inspector Lincoln County

If I'm flat venting the tub, I like to roll the wye fitting and use waste fittings below the flood level.
You can also wet vent in Canada, so in that case, if you ran 2" to the lav all the way through the roof, that would work as a vent if the distances were right.

Thanks Terry I read that and it's cleared a little up, I guess the only thing I don't understand is since the tub trap connects to the flat 3"(via a rolled 3x1.5x3 wye) which then goes up the roof why there is another 1.5" vent that just connects to that 3" in the attic.



I've been staring at this is the second vent there to prevent the toilet from pulling the water out of the tub's p-trap when flushing?

`yes ! ! It is there for that reason !

It sounds like it it there BECAUSE the tub connects with a "rolled" fitting. There are still theoretical problems if the tub enters the side of the flat vent, (then the tub would not need its own vent), because there is nothing flushing the upstream portion to ensure that it remains clear.

Get rid of that flat vent that runs off the tub drain. It will plug up and be useless and besides, you don't need it. The toilet will not pull the trap.