Omit the bird’s-mouth from four rafters and use these on the ends. Cut the 2x4s for the ridge and subfascia to length and mark the rafter positions on them. Line up the rafters with the marks and nail through the ridge and subfascia with 16d nails to secure them. When the roof frame is complete, line up the subfascia with the chalk line on the platform and tack it in three or four places with toe screws to hold the frame straight while you install the soffit.
Working from ladders is more dangerous than working from scaffolding. Plus, having to constantly move ladders around is time consuming. When you get to the roof construction, consider renting a set of scaffolding with wheels. You’ll be amazed at how much easier it is to have a stable platform to work from and to set your tools and materials on. You can rent a 5-ft.-tall section of scaffold with three planks and wheels for about $110 per week.