What is a common characteristic of materials used for coping parapet walls?

Coping parapet walls serve a vital role in protecting the roofing system and the wall itself from water infiltration and weathering. Therefore, the materials selected for this application must exhibit weatherproof characteristics to ensure durability and longevity. This means they should effectively resist moisture penetration, provide protection against UV rays, and withstand various weather conditions without degrading.

Weatherproof materials such as metal, concrete, or masonry are commonly used for coping because they can effectively channel water away from the wall, preventing deterioration that can lead to structural issues over time. Ensuring that coping is weatherproof also helps maintain the integrity of the entire building's envelope, reducing the likelihood of mold growth, water damage, and other complications associated with moisture intrusion.

The other options, while they may describe certain materials in other contexts, do not capture the essential requirement for coping materials as effectively as the need for them to be weatherproof. For example, lightweight materials might not necessarily provide the robustness needed for parapet walls, flexibility can lead to issues with maintaining the structure over time, and combustibility is a critical concern; therefore, it is generally unacceptable for materials in this application.