The interior of your new room could look something like this with skimmed plaster ceiling, lighting of your choice and possibly Fakro ceiling roof windows.

 

The Guardian Roof system in compliance with LABC/LABSS is a ventilated roof. Slots in the ring and ridge beams allow air to flow freely through the roof allowing any moisture to exist.

With many unproven roof conversions, moisture can form inside the roof causing staining and mould growth to the plasterboard.  The Guardian ventilated system prevents this happening.