There are various ways that a roof can be insulated. The type of roof you are insulating will influence the best choice. (1)

Flat roofs should be insulated from above. It is possible to insulate from below, but not advisable because this typically results in condensation problems. A layer of rigid insulation board can be added above either the weather-proof layer or the timber roof surface.

On a pitched roof, the choice of insulation will be dictated by the available roof space and how or if the space is used.

Unused roof spaces, perhaps due to inaccessibility, can have wool fibre, treated cellulose or polyurethane foam insulation blown into the void.

If the roof space is used for storage, consider insulating between and over the joists and then laying boards on top. To install enough insulation, the level of the floor might need raising by either inserting timber battens across the joists or inserting purpose-built legs. It is imperative to leave an air gap for ventilation to prevent damp problems. If the joists are regular, rolls of mineral wool insulation can be used. Insulating between and over the joists means the roof space is colder. Pipes and water tanks are therefore more likely to freeze, so they too need insulating. Insulating roof space access hatches with strips of draught-excluding material around the edges will also improve efficiency.

To avoid raising the floor, or to make the roof space warmer, perhaps to prevent water tanks and pipes freezing, insulate between and over the rafters.  Either rigid insulation boards or sprayed insulation foam, can be used, but it is a more expensive alternative.

(1) Energy Saving Trust, Reducing Home Heat Loss, Roof and Loft Insulation. Available from: https://energysavingtrust.org.uk/advice/roof-and-loft-insulation/ [Accessed 25th January 2021].