Insulation, Radiant and Vapor Barriers - air flow

We are gutting a cement block room 17x30 13' high.
The room is attached to a 3 story builing ( used for a commercial store front) It has a flat roof. Maybe a 3' pitch. there are 2x12 holding up the roof. these 2x12 were blocked out about 3" before the top of the cement blocks, allowing no air flow between insulation and roof. The old insulation was put up between the 2x12's. We have to replace the old insulation. We are putting in a dropped ceiling about 10' high. We do not want to just lay the new insulation ontop of the ceiling panels. We want to put it back up between the 2x12's, but understand you need ventilation between roof and insulation. How do we get air flow? Thank you.

I have the solution for you, but first need to know what the ceiling or roof is made of. It is a metal roof, or wood asphalt.

decking is wood with EPDM or polyurethane rubber over it.

Hello Jack. Would like to make correction on pitch of roof would be more like six or eight inches. Thanks for your responce.