What happens to the asphalt in built-up roofing systems over time?

Asphalt in built-up roofing systems undergoes significant changes over time due to environmental exposure and the aging process. The correct answer highlights that asphalt crystallizes and becomes brittle as it ages.

This crystallization occurs due to the oxidation of the asphalt, which is a process accelerated by UV exposure, heat, and other environmental factors. As the asphalt oxidizes, the molecular structure changes, leading to a loss of flexibility and increased brittleness. This brittleness can result in cracks and splits in the roofing material, compromising its effectiveness and potentially leading to leaks and other forms of damage.

In contrast, options suggesting that asphalt becomes more elastic, maintains flexibility, or evaporates completely do not accurately reflect the nature of asphalt aging. As time progresses, rather than becoming more elastic or retaining its flexibility, the material tends to lose these qualities, resulting in a less durable and more vulnerable roof system. Similarly, while some components in roofing systems can evaporate, asphalt itself does not evaporate completely; instead, it deteriorates and hardens due to crystallization.