Which characteristic defines the structure of BUR systems?

The defining characteristic of Built-Up Roofing (BUR) systems is that they consist of multiple layers of materials. This multilayer structure includes alternating layers of felt and bitumen, providing a durable and waterproof roofing solution. Each layer adds to the system's strength, insulation capabilities, and resistance to punctures and other forms of damage. The layering also contributes to the longevity of BUR systems, making them a popular choice for both commercial and residential roofing applications.

The other options do not accurately represent the essential characteristics of BUR systems. For example, while some roofing materials may be lightweight and flexible, BUR systems are typically heavier and rigid due to their multiple layers. Single-layer installations describe systems like single-ply membranes, which contrast sharply with the layered approach of BUR. Lastly, the notion that BUR systems should only be used in warm climates is misleading, as they can function effectively in a variety of climates, providing proper insulation and protection regardless of temperature conditions.