This is the exhaust stack from the Kitchen Exhaust Fan. The contractors who installed it attached the stack to the fan and then braced back to the roof.
Only trouble is when there is an issue with the fan motor it becomes a major issue. We had to think outside the box to solve this problem.
You will notice the Unistrut on the roof. That was installed to support the stack so that we could then release and slide the fan motor out for repairs.