I have a 4" cast iron main drain stack that comes out of the foundation and runs about 40' up through the roof. It is leaking and falling apart so I plan on replacing it, my question is can I go up 5' from the basement floor and punch it through a 18" thick brick wall with 2x 90* bends, run it verticle for 25' and then at the top bring it back to the original roof location with 2x 45* bends?

I have a 4" cast iron main drain stack that comes out of the foundation and runs about 40' up through the roof. What's this stack picking up on its way to the roof? How many fixtures?
my question is can I go up 5' from the basement floor and punch it through a 18" thick brick wall with 2x 90* bends, run it verticle for 25' and then at the top bring it back to the original roof location with 2x 45* bends? Why deo you wish to change the position of the stack vent? Your two 90 degree bends make a "dogleg" and that's never good in drainage. If you're going to do that use sweeps instead of 90's. You may place as many bends in the vent stack as you wish as long as it's a dry vent and you slope it back to the drain line. Good luck, Tom

Basement slab - 1 toilet
First floor - 1 full bath and kitchen
Second floor - 2 full bath
Third floor - 1 full bath with 2 sinks and both a shower and tub

The stack vent is on the back side of the house, a 3 story addition was added to the back to add bathrooms (120 year old house). The connection between these add on bathrooms and the main stack is a maze of pipes because they tried to avoid punching through the old exterior wall to get to the main stack. The second floor drains into the first floor which makes a horizontal you shape to go through an old basement window that is 6' away from the main stack. Needless to say it doesn't work very well.

Since all 4 bathrooms are in the addition it would be a lot easier to route the main stack through the addition (or add a second stack that Y's off of the main stack) then punch several holes through the 18" of brick for each bathroom on each floor.

I think I am a bit mixed up on terminology. When I said 90* bends I am thinking of those long sweeping 90* bends, not the hard sharp angled ones. Is this what you mean by sweeps?

By dry vent do you mean that the fixtures need to be vented separately from the main stack?

I think I am a bit mixed up on terminology. When I said 90* bends I am thinking of those long sweeping 90* bends, not the hard sharp angled ones. Is this what you mean by sweeps?
Yes! I would certainly use sweeps, (see image) rather then 90 degree quarter bends.
By dry vent do you mean that the fixtures need to be vented separately from the main stack?
I'm assuming the lower fixtures are vented correctly and don't just drain directly into the stack vent. The dry vent,( vent stack) starts 6 inches above the flood rim of the highest fixture on that stack. You may put vas many bends in a dry vent>(vent stack) as you wish. Good luck, Tom

All bathrooms and the kitchen are being redone so they will be vented correctly. Right now not much is done right.

Lets see if I am understanding correctly. This main stack will be used for drainage for all 3 floors, the dry vent starts on the 3rd floor at the level of the highest flood rim. The stack below this point is a wet vent down to the point of the lowest fixture. At any point of the dry vent bends can be used but they must slope downward (I assume 45* or lesser bends required here). On the wet vent portion of the stack 90* bends are acceptable but 45* would be better. ?

Is the dual stack option to code? They are Y'd in the basement from a single drain and then Y'd back together in the attic for a single exit through the roof? My area follows the Uniform Plumbing Code.

I think I might have added some confusion in my reply. All fixtures on all floors will be vented separately from the main stack and tied back into the main stack 6" above the highest flood rim (where it would be considered a dry vent?)

I think I might have added some confusion in my reply. All fixtures on all floors will be vented separately from the main stack and tied back into the main stack 6" above the highest flood rim (where it would be considered a dry vent?) You are correct!
Vent Stack = The vertical section of drain pipe that's dry from the upper most fixture.
Stack Vent = The vertical section of drain pipe that the fixtures discharge into.
Actually they're all one piece of pipe. Good luck, Tom