What is not a function of stone used on BUR?

The choice indicating that decoration of the roof is not a function of stone used on built-up roofing (BUR) is correct because the primary purposes of stone in BUR systems revolve around performance and functionality rather than aesthetic appeal.

In BUR applications, stone serves several essential roles. It provides ballast, which helps to secure the roofing material against wind uplift, ensuring structural integrity and longevity. Additionally, stone can aid in temperature reduction by reflecting sunlight, thus helping to maintain a cooler roof surface and improve energy efficiency. Lastly, the fire resistance improvement offered by stone contributes to the overall safety of the building, as it can slow the spread of fire and enhance the roof's fire rating.

While stone may have visual appeal, its use in BUR is fundamentally driven by practical functions related to the roof's stability, thermal performance, and safety. Thus, decoration is not a functional role of stone in this context.