What are the layers that make up a built-up roof commonly called?

In roofing terminology, the layers that make up a built-up roof are referred to as "plys." This term specifically denotes the individual layers of felt or membranes that are combined to create a watertight surface. A built-up roof, typically used in commercial buildings, consists of multiple plys that are adhered together with asphalt or a similar adhesive.

Using specific terms like "plys" helps professionals communicate more clearly about roofing construction and maintenance. In contrast, the terms "tiers," "layers," or "sheets" do not provide the precise nomenclature used in the roofing industry for this type of roofing system. While those terms may describe concepts related to construction or other types of roofing systems, they lack the specialized meaning that "plys" carries. Understanding this terminology is essential for anyone working in roofing, as it relates to both the installation process and the assessment of roof performance.