The roof spars and blocking provides stability and lays the foundation to mount the window and fan as well as the foundation to glue the outside skin. The roof blocking was created in a similar way to the hatch ribs. I used the wall template to cut out the section that would follow the mortise in the walls.
Here is a finished blocking section resting on the inside wall template.
You can see how the blocking fits nicely up against the front of the deck.
For the roof spars I used 4/4 poplar boards. I ran them through a jointer and plainer and then ripped them down to 1×2 size (3/4″ x 1.5″) and cut them to 5′.