In message , TheScullster
writes
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".

I'm a farmer so run my comments past the structural experts before
cutting anything:-)

The 8"x2" beams are purlins.

Unless the outside walls are buttressed, you must have something
preventing the roof thrust pushing them flat. This can be done with ties
above ceiling level but usually the ceiling joist does both jobs.

I assume you are trying to avoid having the beam projecting down into
the room.

Put the beam under the rafters and make the fire proofing a feature. You
can Hilti nail wood to the bottom flange and secure the side pieces to
noggins bolted in the web. If you are careful to match the grain, you
can make a realistic timber beam. Somebody will know how thick the wood
has to be to meet the regulations (if needed).

Do you actually need a beam, if there is no load on the joists?

regards

--
Tim Lamb