Which of the following materials is commonly used in BUR surfacing?

Gravel is commonly used as a surfacing material in Built-Up Roofing (BUR) systems. In BUR applications, the gravel serves multiple purposes. Primarily, it acts as a protective layer that shields the underlying bitumen layers from ultraviolet (UV) rays, which can degrade the material over time. Additionally, the gravel provides weight to hold down the roofing system, preventing uplift from wind or other environmental forces. It also enhances the aesthetic appeal of the roof and helps in managing drainage by providing a textured surface that encourages water runoff.

The other materials mentioned, such as modified bitumen, EPDM rubber, and PVC sheeting, are typically associated with different types of roofing systems. Modified bitumen, for example, is more often used as a type of membrane roofing or as a layer within BUR, but it doesn't represent the finished surfacing material like gravel does in this context. EPDM and PVC are single-ply membrane systems and are distinct from the traditional BUR structure, which is characterized by its multiple layers that are built up with bitumen and capped with gravel or a similar surface treatment.