a roof unit having an outer skin , an inward facing ceiling , whereby the outer skin and the ceiling are combined to form a unit ready for installation , a base structure having longitudinal frames . the ceiling and the outer skin along with the basic structure have joining configurations that mate the ceiling and the outer skin to the base structure at mutual places for joining . the ceiling has a sandwich structure including a middle layer of foam , at least one side of the middle layer being provided with one of a fiber mat and a glass fiber fleece and at least one of a plastic film , a cover film , a decorative overlay and a functional overlay .