Adding a bathroom...need drain help...

We are in the process of building a bonus room in our currently unfinished attic. We have thrown around the idea of adding a full bath, however, my concern is drainage. The house is on a slab with a septic system. The proposed bathroom would be about 8-10' from where the current 4" vent tube comes up into the attic through the wall above the laundry room, angles off, then exits through the roof about 10' (horizontally) away. My question is, can I tie into the 4" vent tube at the elbow as it exits the wall for drainage? Also, my distance from the current roof vent is about 15' so, if I can tie into the 4" vent for drainage, can I add a separate vent for the new bathroom directly up and through the roof without messing up the other bathroom down below. It appears that the 4" vent tube services a bathroom and laundry room with only one 2" pipe joining it below where it exits the roof. The new bathroom is on the opposite side of the house as the outside septic cleanout/line so it would be very difficult to run a separate drain out to it. The bathroom floor joists will be 2X10's piggybacked alongside existing 2X8's and attached to existing 2X12's at either end (it's next to the garage ceiling that is already framed with 2X12 joists). I appreciate any help/advice you can give...thanks in advance.

Info you may need: the house is in Effingham County, GA and was built in mid-2004.

Also, I intend on hiring out the work, I just want to see if it is something that is possible before I call a plumber over and possibly waste his time.

You cannot tie into a vent pipe and use it as a drain. What you do is make the fixtures it is venting on the floor below wet vented, and you are not allowed to wet vent between floors.

You can use the pipe for drain, but then you have to revent the fixtures on the floor below with a separate pipe which would come up and tie in on the second floor to the pipe before it goes through the roof, but at a point 42" above floor level of the upper floor.

Alternative...

Would it work if I ran a 4" pipe down alongside the existing vent pipe and tied in at floor level just above the slab? I'm assuming that the fixtures in the downstairs bathroom must tie in to the vent within the slab because there wouldn't be another way with how the rooms are laid out. If I did it that way then I would be draining at almost the same level (within a foot) of the downstairs fixtures...I'm thinking this would be my only other alternative (if it would work) aside from no bathroom in the attic. Thanks.

drain

You would still be wet venting the toilet, and maybe the tub or shower, depending on how they are connected, and that might cause more problems than if you connected into the vent in the attic.