Purlins
are the small 2" x 3" trellis members. They don't block
as much sunlight as the rafters, but they do provide support
for vines and ivy. The purlins are notched on their bottoms
to fit over the rafters.
Most
people choose to run their rafters in the direction that
is perpendicular to the suns rays. If you want shade during
the midday you should run your rafters from east to west.
