Pay for material separately or not

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Hi, I currently have an out of state rental property that needs some major roofing work. And I know almost nothing about good roofing materials. I've been trying to read articles and watch videos to familiarize myself but lack practical knowledge for obvious reason. I have a manager who oversees the house. 

From small works I've done before, i.e installing an exhaust fan, I bought the fan online, got it delivered to the house. But the contractor still charged me for ad hoc things he had to buy during the work since I said I am paying for the material separately. He needed drywall panel, paint, caulk, caulk gun, screws and few other things and at the end it ended up being another $100 bill on top of what we agreed on. I just worry for a major work like installing new roof, those ad hoc stuff could accumulate to a much higher number.

But if I let them buy the material, I am not sure if they are gonna buy good materials given this roof needs to last long. I don't want them to charge me for good materials when they end up buying crappy ones. I have no way to verify what they bought since I won't be physically there. I am tempted to fly to that place but then that defeats having a manager and I am basically adding more to my expense. Plus how will I verify.. I don't know much about roofing either.

What would you do in this situation? Let them handle the material or buy separately? I can ask for their work warranty and get warranty info for the material.. but in reality not sure if they are gonna follow through all those protocol. Anything else I need to be aware of? Thanks so much in advance!