Square edge OSB for warm flat roof

[Olf]

In normal times I'd just go for stock OSB/3 T&G boards, but with current problems with sourcing anything I'm considering all possible options.

I put to sleep idea of using P5 chipboards (for now...), but trying to find what is wrong with square edge OSB: with joists at 400 mm centres, 1200 mm wide boards of the bottom layer should always meet over the joist (75 wide, so at least 34 mm support each) and any buckling that could still try to happen will never be visible, and should be reduced once the sandwich is sqeezed with the fixings. With top it should be even easier, with rigid insulation boards providing flat surface, OSB boards will only be providing extra lateral stifness (and load distribution of anything pressing from above), so lack of interconnection should not matter much. Should it?

Additional question: going square edge, what gap between the boards is sensible to allow for any expansion? EDPM on top, so quite forgiving.

[DragsterDriver]

Fine for a warm roof, joists, firring pieces,thin osb, membrane, pir board, 18mm osb fixed with long screws.

[tonyshouse]

Leave gaps, yes. Ring shank gun nails for me and separating membrane. 
 

I hate flat roofs, destined to leak , all of them 

[Oz07]

I gave 3mm on last one I did.

[TonyT]

Use a nail as a spacer