This will form a rafter for the barrel roof.
How to frame a barrell roof.
The arches must be at least 6 inches wide then glue and screws two arcs together.
Make rafters for the complete arch out of two layers of 3 4 in.
First frame your barrel roof dormer up to the top plates in the conventional way but leave the roof sheathing and framing on the main house intact above the dormer top plates.
Plywood glued and screwed together.
Cut a notch at the height of the finished bow.