[flaviu2]

I'm familiar with vents, but indoor vents are basically one-way air valves. I don't know if this is required by code, but it's definitely common practice (you don't want poop smell coming out of your vents)

[Scoundreller]

Vent stacks need to be above that roof’s line, but your building might be next door and taller with open windows downwind.

Or a crack in the stack.