In message , Roger
writes
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.

I was writing in jest:-)

My construction is such that there are no rafters. The building has
three trusses at roughly 3m intervals. There is no ridge board but a
purlin close to the ridge and a second midway (it may not be proper to
call these *purlins* as they are load carrying members). I can secure
the ridge such that a rigid triangle is formed taking all the load to
the wall plates. Assuming snowloading, windloading and thrust on walls
are not issues (the building has withstood these for the last century) I
am concerned that there will be some sagging midway between the trusses
with the additional sheeting weight.

The agricultural solution is to try it and see: beefing up the structure
if necessary.

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

Yes. Probably:-)

regards


--
Tim Lamb