We did get a seperate roof insepction after the bank required it.
The flip side of this is that most contracts state that the seller has to assist with requirements from the bank. If the bank is requiring the roof to be replaced, then you can go to the seller and request it. They may refuse and then you will have to escrow money for the roof to satisfy the bank.
You may look to get a second or third opinion on the roof to help make a more conclusive argument.