I live near the coast and salt spray makes your shingles take a beating, therefore I stain one side of my house each year. So every 4 years I have to paint over my attached garage. It extends about 10ft above the garage and I came up with this: Plywood with vertical and horizontal cleats, indoor carpet on the bottom and ladder stabilizers attached to a 16ft extension ladder.
The plywood distributes the weight and the carpet protects and grips onto the garage roof. This unit has never shifted ever, I have a matching one for the other side but don't seem to need it. I originality had a 4x4 flat scaffold that easily assembled in place stradling the roof peak, I haven't used it in years but I have loaned it out.
2x8 1/2 inch plywood sheet
8ft of cheap indoor outdoor carpet (HD sells them in 3ft runner width)
Outdoor carpet glue
2, 1x3 x 8ft stapping
2x4 x 4ft
1 1/4 sheet rock wood screws (all the rest)
2 in sheet rock screws (just a few)
Liquid nails glue (1 tube)
Set of ladder levelers (try to get the ones with the square bottom they lock in better).
No they are not cheap, but long ago I determined my life was worth more than a shingle and a brick, both of my ladders have these my house is a walkout in the back and they were needed for the slope on the sides the swivel ball feet are necessary for safety.
Here is one from Amazon http://www.amazon.com/Louisville-Ladder-LP-2300-00-Leveler/dp/B001BIVSI8/ref=sr_1_1?s=hi&ie=UTF8&qid=1359483843&sr=1-1&keywords=ladder+leveler
I won't go into their installation they are all different just follow the directions.
Drill (driver bit)
Caulk gun (for Liquid Nails)
Build / use at your own risk, I make no guarantees.