Thanks nick, the loft has 3 purlins, 1 on each of the external walls, semi detached. It’s not half lapped from what I can see.
I’m sorry I’m not very technical, I did a 2 year carpentry course with my employer as a secondary apprenticeship but as you could imagine it’s not very thorough and through lockdown didn’t get to college much and didn’t end up passing.
so you mean use the 9x2s straddle the opening like a lintel?