What is an important characteristic of the Base Sheet?

The Base Sheet is indeed important as it acts as a vapor retarder. This characteristic is crucial in roofing systems because it helps prevent moisture from penetrating into the roofing layers, which can lead to serious issues like mold growth or material degradation. By implementing a vapor retarder, the roofing system can maintain its integrity and longevity.

While other layers may contribute to the overall roofing structure, the Base Sheet's role in controlling moisture is essential in ensuring the performance of the roof system. This function is particularly significant in areas with high humidity or in structures where temperature fluctuations can cause condensation.

The other characteristics mentioned in the options do not accurately represent the function of the Base Sheet. It is not the final layer of roofing nor is it primarily decorative or made of lightweight materials. Instead, it serves an essential technical purpose within the roofing system.