Would a standing seam metal roof (Looking at ssr2 by catnic) be viable ontop of three layers of 100mm xps (Staggered horizontally and vertically) with a vapour control layer beneath the insulation. Ontop of a concrete deck, at about 14* 

How would I ensure the standing seam roofing stays put, I dont really understand how the clips work can I just nail them into the xps or do i need some kind of OBS board underneath?

Best regards,
Jon fitz.