Husband to said that two of the vents have metal collars that you are drilling into. The bit wants to walk. Either make a dimple first with a punch or be sure to start out really slow until it catches. Pre-assembling the permanent hinges [the ones that will be folding] on the ground made it easier too. Two of us to get things on the roof made it less tricky than climbing up the ladder with items to transfer to the roof. As I have said before, I am the worry wort, so I stayed to watch to make sure he didn't fall and crack his head on the concrete. He can't be dialing 911 if he is out of it.
But it went fine. The roof has a bit of a texture to it. It isn't slippery even with the sloping angle of it. Will not make cleaning it any fun though.