The invention provides a roof assembly (2) comprising an outer roof component (4, 104, 204) that is arranged, in use, to be mutually interlocked with an inner roof component (6, 106, 206). The outer roof component (4, 104,204) comprises a depending key element (14) (14) that is a snap-fit in an upwardly facing dovetail slot (36) provided in the inner roof component (6, 106, 206). Embodiments of the invention enable roofs of various pitches to be assembled, the components (204, 206) having mutually inter-engaging bulbous configurations for this purpose. The key element (14) and the inner roof component (6, 106, 206) are moulded from foamed polystyrene. The invention also provides a method of forming a roof using the roof assembly (2).