When I ladder such areas, I use a five gallon bucket with a paint grid in it suspended off the side of the extension ladder with a pot hook. I use a 7inch roller frame with a 1/2 inch knap roller cover. The "hook" on the handle of the roller handle allows it to hang on the top of the grid while you are cutting in with the brush.

When you are working off a ladder, you are in effect a one handed painter. One hand is holding the ladder, the other is holding either the roller OR the brush. What do you do with the brush while you are holding the roller? I drive a small finishing nail into the brush handle just above the metal ferrule. I then take a pliers and bend the nail downward to form a hook. The brush can now be hung on the side of the bucket while you are rolling.

A 7 inch roller will just fit the reveal of the lap on HardiePlank. The area under the lap will have to be brushed in. I back brush everything.

Normally, I would use two ladders with jacks and a Stinson plank between them. However, when working up high, I did not feel comfortabel on a narrow plank and would ladder the gable area down to a height at which I felt comfortable. ( yes, I know this is not kosher in Canada!) You can bet your life that I always knew if I was working over plain grass or a picket fence!