The message
from Tim Lamb contains these words:

OK again. So provided the rafters/sheets are rigid and the wall plate
prevented from spreading by the truss tie, the purlin and ridge are
mainly decorative?

That is not what I intended to impart. Looking at one side of the roof
in isolation the wall plate and the ridge will take 25% of the total
weight of the roof and the purlin 50%. As the ridge is also supporting
the other side of the roof its actual loading is the same as the purlin.

You didn't quote any actual dimensions and I was assuming the rafters
were bound to be strong enough if the purlin was. I have 3" or more of
stone on my barn roof with 2 purlins each side but the rafters are only
4" by 2" at 18" centres. If you want to check a rafter consider the
rafter in 2 parts with a uniformly distributed load. Initially as a
simply supported beam but if you want to model it more closely as one
end built in and one end simply supported.

I have no knowledge of Superbeam but I assume it will allow that sort of
calculation.

--
Roger Chapman