Roof timer query


I have to get retrospective Building Control approval for a roof which has had its slates replaced by heavier tiles. I understand I need to fit an inverted 'A' frame which I want to do my self (to save 350+).
Whilst I understand how to make and fit the diagonals of the A frame, to the purlins, I don't have any info on the 'collar', the horizontal beam/s of the A frame (which will link the two purlins) e.g.
- what diameter bolts? - how many - is it two beams or just one 4 * 2 - it is connected to the rafter?
Does anyone have a spec or drawing or could describe it or has photo?
Thanks
Si
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It's amazing what a difference a 'b' makes to a request ;-)
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Leave it to the experts because building control will want to see structural engineers design/calculations,
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