The roof is on and covered in felt. I really wanted to get to this stage a week ago just in case the weather turned wet. I have been lucky so far but it is a milestone to have it done. The roof construction depends on two oak beams down the outside of each wall and eight poplar purlins on nine-inch centers inside. This keeps the roof structure light but relatively stiff. The bent plywood further stiffens the entire structure and makes it feel rock-solid. More photos here.
For the roof covering… I have long debated with myself over a traditional canvas roof or using metal. Both have advantages and cost is similar for each. I will decide in the next few days.