I think it's best to talk to a Structural Engineer. If you're doing this job yourself, spend some of the $$$ you are saving on labor on real, sound advice from someone who can see what you have and knows the 'rules'. It's good you're not attempting this blindly because your car(s) might have a new roof if you did!
If it's not done properly and it fails, you can kiss any insurance claim good-by. My best to ya and hope this helps.