I don't know if that's the kind of info greener wanted ...
If the tenons on both ends of the purlins are cut as per the drawings in the "Tools" link they can't be flown in from above with a crane. We do what we call a "step cut"; the housing is cut as detailed but imagine the tenon sliding up so that it's upper face is lying in the plane of the roof. (Actually, if it was up to me I would prefer cutting the
tenons as shown in those drawings and there is a way of getting everything together but it's time consuming).
When I get back online tomorrow I can post some simple graphics (if requested) of chamfering or square cutting; the principle behind the housing and tenon cuts.
