I have done expert witness on several roofs this year. None had permits and all caused water damage from improper, code-deficient installations which created leaks. One had a permit issued but there were no inspections so I was able to tie the liability to the contractor and the code official (third party agency) to make more money available for the real fix. If you require a permit but perform no inspection, you are not helping the situation. If you need to verify a license then charge a license verification fee and leave the permit paperwork alone. I believe the contract between the owner and the roofer documents liability. How about when a homeowner does their own roof? If a homeowner does their own work then sells the house and there is a problem, there is no insurance for that. If, however, there was a permit and inspection, there should not have been a problem in the first place. If there is no inspection required then there should be no building permit issued. Call it something else if you need to. For whatever reason this one gets me going and I don't know why. Bottom line, if you are going to call it a building permit, do an inspection or don't issue a building permit at all. Issue a "Contractor license verification certificate"