This is an old video but in case someone is doing research, I have used this design. Make it even better by ending the 2x4's 3' above the roof, attach your pullies at the top. put the shelf about 10" below the top of the sled, attach the rope or cable about 4" below the shelf. I made my sleds a little longer on the bottom side so the shelf was 3' high at the bottom for on loading. The shelf will go past the pully at the top and the shingles will be waist high for off loading from the roof side. There will be nothing in the way so no need to reach around. You'll always be away from the roofs edge.
Very good design, as basic as it gets. Works very well. Nice work you did there! I made one using 1/2 of an extension ladder, it works very well. I'll post a video if it in few days.
