Which material is commonly used as a waterproofing agent in low slope BUR membranes?

Coal-tar pitch is commonly used as a waterproofing agent in low slope Built-Up Roofing (BUR) membranes because of its superior water-resistant properties and durability. It has been a traditional choice in roofing applications due to its ability to form a strong, protective layer when heated and applied in multiple plies. This material provides excellent adhesion and flexibility, which is crucial for low slope roofs that may be prone to ponding water.

Additionally, coal-tar pitch possesses a high softening point, making it effective in protecting the roofing system from extreme temperatures and wear from environmental elements. Its resistance to UV radiation and its ability to withstand the harsh conditions found in some climates also contribute to its enduring performance in BUR systems.

Other materials mentioned, such as asphalt, rubber, and polyurethane, serve different functions and may be beneficial in specific scenarios, but coal-tar pitch remains the more established and traditionally preferred option for waterproofing in low slope BUR membranes.