For assistance, if you take a typical construction required R value for a pitched roof of 5.803W/m²K, (I can post the calculation, but it's a bit long winded), then you can calculate any thickness requirement for any material, whether it be rockwool, polyiso, fibreglass, etc. as long as you have the material U value.
Just do the following calculation:
Thickness = R x U of material being used
=> Thickness = 5.803 x say 0.023 for polyiso
=> Thickness = 0.133
=> Thickness = 133mm
Pitched roof U value requirement = 0.16W/m²K
150mm thick Polyiso will exceed requirement to 0.14W/m²K
Assumptions:
Existing Roof R Value = 0.447 (without any insulation)
Required R Value = 6.250
=> Required Insulation R Value = 5.803