Yes, you will be able to insulate a flat roof in a similar ways as an apex roof. Please see this page for advice on insulating a log cabin.
For a flat roof you will however need to source additional OSB board or similar, the insulation will go on as normal and then the OSB board to create a hard surface for your final roof covering.
80mm screws will also been needed and you may also require batten material equal to the thickness of your insulation to create a frame to slot the insulation into.
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