Flat roofs can be insulated in the same manner as walls and are filled from the inside up into the ceiling, or from above but only when the roof is being replaced. Most flat roofs are either empty or have little insulation in them. Heat from the sun, in the summer, can make living under a flat roof very hot and uncomfortable, not to mention resulting in significant air-conditioning energy costs.
In the winter, heat penetrates quickly, leaving the house cold, and resulting in the furnace being overworked while trying to compensate for the heat loss. Cathedral ceilings are insulated in much the same way as flat roofs unless there is more than 12 inches of cavity available – venting is required in this specific situation which then allows the ceiling to "breathe".