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.
Step 1: Build Platform
Cut 2ft long 1x3 for Cleats
Place the 3 cleats at the bottom, 33 inches and 66 inches glue and screw them from underneath make sure the heads do not protrude.
Take the 2x4 by 4ft cleat and attach it to the top (you can use strapping but let it protrude from the edges I will tell you later why) again screw from underneath and glue liberally.
Now cut three pieces to fit vertically between the cleats place them so that the cleat is about 8 inches from the edge (see picture). This places the feet of ladder straddle the center of the plywood this is what you are shooting for.
Step 2: Carpet the Bottom
Spread the carpet cement/glue on the plywood (clean plywood and back of carpet well before you place glue).
Apply the carpet as per glue instructions.
Let stand until dry.
Step 3: Affix Ladder Leveler to Ladder.
As I stated the square ones are best for stabilization.
Even if you don't have a garage but your property is not level buy and install these you will love the added stability especially +20ft up.
Step 4: Use With Caution and Be Safe
Just don't overreach (I do now and then when I get lazy and my wife does not like it and she is correct).
In painting the entire area I need only move it 3 times (not counting initial placement), during which time you hang the ladder over the ridge line and let the upper stabilizer hold it in place.
So you may be wondering; why the 2x4 at the top?
As you can imagine bringing this thing up the ladder can be cumbersome pulling it up one step at a time as is taking it down when the job is over (even harder). Well solution is a rope loop loosely around the 2x4 and bring the end up with you and use the rope to hoist it up because it is loose (bowline knot) it simply comes off when not taught.
20 years and still going, I loan it out to neighbors when needed, it can also be used on any roof to cut down on roof wear for chimney work or any other type of repair.
These pictures were taken last Summer, I have another Instructable for window masking (not up yet) that is wicked (Rhode Island speak for extremely) easy and really helps when you are staining.