The joint at the roof shall be made watertight using which materials?

The correct choice emphasizes the importance of using approved materials, specifically copper, lead, or other approved flashing, for making roof joints watertight. Flashing serves a critical function in roofing by directing water away from joints and seams, thereby preventing leaks and water damage. These materials are highly durable and resistant to the elements, ensuring a long-lasting seal that can withstand various weather conditions. Copper and lead have been long trusted in roofing applications because of their corrosion resistance and longevity.

Other materials, while they may have some water-resistant properties, are often not suitable for this specific application due to factors such as their durability, maintenance needs, or compatibility with roofing systems. Tar and gravel, for instance, are more commonly used in built-up roofing systems rather than as flashing at joints. Fiberglass is typically used in different applications, such as insulation or as part of composite materials. PVC sheets might offer waterproofing but lack the necessary flexibility and longevity needed for effective joint sealing on roofs when compared to approved flashing materials.