One of the tools we haven't talked much about that fits into the ladder and scaffold family is the use of extension ladders with ladder jacks and a plank. For residential construction, ladder jacks can be used up to 20 feet of elevation off the ground.
The trick is that once you pass the 6-foot mark, you have to have fall protection in place. This means railings or safety harnesses. Most of my experience with ladder jacks is they are used above 8 feet, which would normally require some type of fall protection. The higher you go, the more critical this becomes as the risk of serious injury increases. When you reach 8 feet or above you may want to consider scaffolding which is a much safer alternative, easier to access, more stable and more room to maneuver.
Sometimes ladder jacks on extension ladders work out better because of the situation. Just remember the need for fall protection.
Also keep in mind that you must use ladders that are Type 1A and a plank that meets the deflection requirement which for all practical terms is very minimal (deflection that is no greater than 1/60 of the length). The old idea of using a 2x10 or 2x12 with ladder jacks 14 feet or more apart will give too much spring and cause a dangerous situation, besides being too narrow. A 14-inch aluminum plank or specifically engineered wood scaffolding plank is required.