In message , TheScullster
writes

You really must get some proper advice before embarking on this project.

I was hoping to size the beam myself and run the ideas past the building
control structural engineer.
I have a HNC in civil/structural engineering, but as my questions make
clear, there is no practical experience to support this.

Ignoring the support from the end walls you are creating a structure
rather like two parallel hard back books stood on edge with a third,
open book, forming the roof ridge balanced on top.

There may be a way of carrying the *tie* over the top of the joists
leaving clearance for the top flange or additional *collars* may be a
way out but putting the question to an expert should not cost money;
only if you invite him to do the sums:-)

I have to go harvesting.

regards
--
Tim Lamb