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from The Natural Philosopher contains these words:

I am not at home atm so haven't the time to even take a look at John's
hymn sheet much less expand on what I have written above.

It may actually turn out that existing rafters and joists are adequate -
the purlin function being replaced by spreaders at ceiling height to
stop the rafters bowing inwards. And existing joists enough top prevent
outward loads on the upper walls at the rafter eave level.

In your dreams.

I don't think so. Why not do the calculations?

Time for one thing but I am so rusty on the nitty-gritty that I would
not trust any figures I came up with.

FWIW I think wind is a bigger factor than you make out but like so many
aspects of a roof it depends very much on other factors which also have
a considerable outcome on the calculations. Angle of roof makes a
tremendous difference both to the compressive load on the rafters and to
the wind force on the surface of the roof. Wind is also a complex force
that probably can't be properly catered for without knowing how a roof
behaves in a wind tunnel. Then there is the weight of the roof varying
from almost bugger all for corrugated sheets to roofs* like mine with an
average of perhaps 3" thickness of stone.

*BTW roofs might be pronounced rooves but AFAICT spelling it that way is
not standard English.

--
Roger Chapman