Gravel surfacing on roofs is typically set in what material?

Gravel surfacing on roofs is primarily set in hot asphalt because hot asphalt serves as an effective adhesive that can bond the gravel securely to the roofing material. When the asphalt is heated, it becomes more fluid, allowing it to effectively penetrate the gravel and provide a strong hold while also creating a durable barrier against weather elements. This method is especially popular in built-up roofing (BUR) systems where a layer of gravel helps provide protection from UV rays and physical damage, extending the lifespan of the roof.

Other materials listed, such as cold asphalt, epoxy, and concrete, do not provide the same effectiveness or desirable characteristics in this specific application. Cold asphalt typically lacks the fluidity needed for proper adhesion and may not properly secure the gravel. Epoxy is not commonly used for roofing applications like gravel surfacing due to its different properties and curing process. Concrete, while strong, does not serve as an adhesive for gravel in roofing systems and is generally used in entirely different contexts within construction.