struggling with Bay window flat roof adjoining ventilated cavity.

[SuperJohnG]

Hi See below picture, I'm struggling with a detail for a flat roof that adjoins the main house as I have a ventilated cavity and the main house is render on renderboard. 

 

I would like to finish the edges tidy and not leave a big gap. Its an area I haven't paid attention to but now need to do it before render board goes on in 2-3 weeks.

 

It's a SIP panel roof (partly) hence, can I just add batons for the ventilation gap, then OSB then membrane and whatever covering I decide? (GRP, Rubber? lead?). 

 

Would you just build up the left and right edges higher, then lay the finish, then add the gutter to the front face?

 

I've 100 questions but don't know where to start. I'm likley overthinking it, but a bit like a ribbit in the headlights