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.