I grew up in a family of carpenters and have been on hundreds of roofs..not saying that I like it at all, but it sure as hell beats crawling around under the house! I do alot of independent roof jobs, I've never had to turn one down because I didn't want to get up there...although I have turned around to go get my rope lol.
If it's too steep or wet or pollen covered to believe that I can stay where I want to, I will anchor a 1/2" manilla bull rope to something on the opposite side I want to work on and toss it over. It's cheap, plenty strong to hold you and you don't have to worry about nails or sharp wood or edge metal ruining your climbing line. I go to the edge holding onto my line, and tie a figure 8 around my waist, wrapping it around my belt a few times so that it doesn't move up or down, and with the leftover tail put a prusik to the line. I don't use my saddle or fall harness because that stuff gets in the way of tool belts. I can get to where I need to be and snug my prusik up and still lean against the rope to access that entire side of the roof, even to hang over the edge. We use the same method when we have to have a guy on the roof when we're taking limbs down over houses. Not super comfortable but it will keep you up there, especially if having to do heavy lifting or something that could cause you to loose your balance