I use this Werner folding ladder. Configure as an 8ft step for general purpose, and unfolded to 16ft straight for roof access.
For roof work I place it as described by Lv2Roam and use the slide top as the top step both up and down. Very comfortable setup. For traveling, the ladder folds down to 4ft long and hangs on the attached rear ladder.
I've looked at that installed ladder numerous times, counted the attach points, counted the individual mounting screws. I've calculated and know in my head that the ladder should hold me without difficulty, but I cannot bring myself to trust it. I have also (in the service) had some experience with vertical ladders. Didn't much care for them-- and that was when I was young and crazy!
John & Diane, fulltiming since '12 02 DS40, FL, Cat, '04 Element NHSO RVM103