Roofing material is the outermost layer on the
roof of a building, sometimes self-supporting, but generally supported by an underlying structure. A building's roofing material provides shelter from the natural elements. The outer layer of a roof shows great variation dependent upon availability of material, and the nature of the supporting structure. Those types of roofing material which are commercially available range from natural products such as
thatch and
slate to commercially produced products such as
tiles and
polycarbonate sheeting. Roofing materials may be placed on top of a secondary water-resistant material called underlayment.