Rigid foam panels are available in thickness.
Flat roof insulation techniques.
The insulation material is placed between the supporting joists leaving a gap of 50 60mm between the roof and insulation.
Warm roof is the best way to insulate your flat roof as there are no condensation issues to contend with.
Underneath the basics flat roofs are usually constructed with a ceiling nailed to joists which support a deck usually wood in residential construction a vapor barrier insulation and a roof board.
Flat roof insulation is unfaced polustyrene and is suitable for use with loosely laid single ply membranes.
Use rigid foam insulation panels on your flat roof before applying the final roof covering or inside the house before adding sheetrock or paneling.
Foam control plus flat roof insulation.
This will enable the airflow to circulate freely.
From there the flat roof system you choose will determine the materials you apply.
If insulation is being added to an older building then it is common practice for the ceiling itself to be taken down.
Finally depending on the make up of your property and the access to the flat roof be prepared to factor in the cost of scaffolding.