When I was little (too small to help lift heavy sheets on a ladder ) Dad made a small jib boom from some pipe. Slid it down through the roof and secured inside.

It could pivot 360 degrees. I would drill small 1/4" holes in a sheet of plywood and insert a lifting eye in it. Hook the block and tackle through the eye then use the lawnmower to pull the rope on the block and tackle to get the sheet up to him.

Once up he could grab the sheet and pivot it over onto the roof. He would put several nails in to hold it and mark where the rafters were then I would go up and help him screw them in place.

This would have been about 1977 long before battery tool and deck screws. We drilled and countersunk every hole and used an electric drill with a straight screw driver bit to put in tapered wood screws. Dad did not want nails he wanted screws. 

Only picture I have sorry poor quality taken with 110 camera and scanned into computer.

Edit: This was the old school "it builds character" method before we had loader or skidloader and no real other way to get sheets up there. How high is your roof edge ? do you have access to loader, forklift, stacked flatbed wagons to get sheets up to edge of roof ? Does lumber yard have boom truck they could set them on the roof for you? just don't set all the sheets in one place though. Nail some cleats down to keep sheets from sliding off or around.

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