Building an apartment in my attic--vents and drains question

[DoodleOlsen]

Hey, everyone. I've been lurking for years and have gained lots of useful knowledge (thank you!), but this is my very first time posting. Yippee! So here's my question: do the main waste stack and the main vent stack have to be the same stack, or can they be separated?

Here's the context: our house is a 130-year-old side-by-side duplex. We live on one side; the other side has been split into two apartments, which we rent out. The attic is really an unfinished third floor, and I am building an apartment in it for my brother. The roof has seven gables, and it's 16 feet from the floor to the peaks, so there's plenty of vertical space. Rather than get into the ceiling of the second floor's apartment, I would like to run the drains for the kitchen and bathroom across the attic floor and build an elevated floor over them. I can't drain to the existing stack, because the vents for the apartment below are almost definitely connected to it below the floor of the attic. The drains I am building will go to the stairway in the back corner of the attic and from there straight down to the basement (through a chase) and over four feet to attach to the main stack. If it is acceptable, though, I want to vent them through a trap door in the roof over the bathroom of the new apartment, which is about 25 feet away, horizontally. That way I won't be cutting into the brand new roof. The trap door is elevated from the roof and sealed shut and insulated (we got spray foam insulation), but not shingled. Does this work, or do I have to continue the waste stack through the roof by the stairs? Because the roof is low over there--floor level, actually.

Here's a super simplified picture:

I guess I could tie the vents in at the top of the main stack where it goes through the roof (about 6' to the right of where I am having the drain go down the stairs), but that part of the apartment will be open to the gabled ceiling, so there would be a vent pipe going across the room about 9' up.

Thanks,
Debbie

 

[Cwhyu2]

You should be fine to take your new vent out the roof any where you want as long as it sized properly at least 6" above the flood level rim of the highest fixture before it goes horizontal.

 

[Sponsor]

 

[DoodleOlsen]

Thank you! That's a big relief. They're not even going vertical until they reach ceiling height; it will be easier that way.

 

[Cacher_Chick]

You will also need to consider egress. No municipality is going to approve a bedroom without more than one way in and out.

 

[Jadnashua]

Some attic floors are designed for occupant loads, some are not. This needs to also be considered.

 

[Dj2]

My observation: Before you start, clear all questions about attic conversions with your building dept. They will tell you what has to be done.
I understand that you are looking at this as an additional source of income, and there is nothing wrong with that, but done incorrectly it can cost you dearly and wipe you out in case of a disaster, and disasters do happen.