Tim Said

Umm.. nothing to do with pad stones but, you did mention a conventional
roof. Is there any chance that the ceiling joists are tieing the feet of
the rafters? Or have I misunderstood the scheme:-)

Good call Tim

This is one aspect that I am not clear on.
Is it usual for traditional roofs to rely on the joists as ties, or are the
rafters nailed to the wall plates?
The roof:
comprises 9 frames at 450 centres
two of these frames have ties (I think these are called collars) 6" x 2"
about 1/3 of the way up the rafters there are 8" x 2" beams edge on to the
rafters and built in to the wall at each end, but I suppose these will take
the weight of the roof rather than prevent it "splaying".

Thanks

Phil