What term is used to describe the layer of material applied to a roof surface to provide an even base or support?

The term that describes the layer of material applied to a roof surface to provide an even base or support is "decking." Decking refers specifically to the structural layer that lies beneath the roof covering, often made from plywood or oriented strand board (OSB). It creates a stable foundation for roofing materials and helps to transfer the weight of the roofing system to the building's structural frame.

While sheathing is sometimes confused with decking, it generally refers to the outer protective layer that gives the roof surface its shape and integrity, often containing additional moisture barriers. Underlayment serves a different purpose, acting as a protective barrier against water and other elements between the roof deck and the roofing material itself. Insulation, on the other hand, is used for thermal resistance within the building but does not contribute to the structural support or integrity of the roof surface.