If the flat roof is vented then you can treat it as a pitched roof so you would place the insulation between the joists/rafters and leave a 50mm air gap above and make sure all ventilation pathways are unobstructed.
If the roof is unvented then vapour control is critical. The space can be full filled with insulation and a VSD membrane installed below ensuring all joins are taped and gaps sealed.
Please contact us on 017684 86285 or email enquiries@naturalinsulations.co.uk if you would like to speak to our technical team for further information