Not to rub salt in the wound, but how is is that you only have $81,000 insurance coverage? If you are grossly underinsured, you could have a case of negligence against the company, but difficult to prove.
Not a builder, but it seems that if the roof has to be replaced....wow! First thing is to get some estimates. It may be less than you think. Sounds like the insurance company was not aware of your finished shop, so you might be screwed on that!