When converting areas to liveable space then it is likely that any existing roof needs to be checked for adequacy in terms of weather resistance and thermal insulation.
If the underside of the roof (the ceiling) is to be lined with plasterboard then the resulting void may also need to be ventilated.
The roof will need to be ventilated. Typically, a 50mm gap should be maintained between any insulation and the underside of the roof. Through ventilation is then achieved by incorporating eaves venting.
If the roof is to be re-covered then the insulation can be installed on top of the joists making a ‘warm deck’ roof (check with manufacturers for details). This avoids the need for ventilation and the roof covering can be re-applied over the top.
Insulation can be placed between the ceiling joists. Again, the thickness will vary depending on the material you choose to use.
If the roof has no ceiling then the insulation can be placed between the rafters and ventilation maintained as described above - in which case the ridge should also have vent tiles installed to allow for through ventilation.