Which of the following is NOT a characteristic of a built-up roof?

A built-up roof, commonly used in commercial construction, is characterized by several key features. It is constructed using multiple layers of roofing felt and an adhesive material such as asphalt or tar, which are alternately applied and allow for water resistance and durability.

One important aspect is the requirement for a vapor barrier in certain climates, as it helps to prevent moisture accumulation beneath the roof, which can lead to structural damage and decreased insulation effectiveness. The layering system of built-up roofs contributes significantly to their overall strength and longevity, and they are indeed notable for having layers of tar and felt, which provide additional waterproofing and protection against the elements.

However, while built-up roofs are durable and can withstand various weather conditions, they are not typically classified as lightweight. In fact, their multiple layers make them relatively heavy compared to other roofing systems, like single-ply membranes or shingles. Therefore, the correct answer highlights that built-up roofs are generally not lightweight, as their composition and construction method often results in a substantial weight that requires appropriate structural support.