I am not aware of any specific OSHA regulation that requires two persons during a roof inspection. I have done over two hundred inspections of damaged houses, and in a number of cases I have climbed on the roof solo to gain critical information. As a general rule, I do not like doing this, it is dangerous at best, and of course can be fatal (another individual in my company fell through a roof, the only reason he lived is because he caught on his arms, and there was another person present to help him get out. Even so he sustained serious injury.)
Most companies have specific written policies regarding the type of roof their employees are allowed to climb on, including how steep, type of roofing surface, weather conditions etc. Of course, you may be acting as an independent contractor, in which case company rules do not apply. Be careful, at a minimum carry a cell phone with you on the roof and make sure it has service, make sure someone knows where you are at all times, and don't climb if you have any doubts about the condition of the roof or your physical/mental condition that day. Your survivors will appreciate your caution.
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