For lead caulked joints in cast-iron hub and spigot soil pipe, what material must be used to pack the joints?

The correct material to pack lead caulked joints in cast-iron hub and spigot soil pipe is oakum. Oakum is a fiber used in caulking where it provides both a sealing function and a support structure for lead, which is poured over it to create a watertight seal. This fiber material is traditionally made from jute or hemp, making it highly suitable for use in plumbing applications due to its absorbent properties and ability to adhere to lead.

While asphalt, cotton, and bitumen may serve various purposes in other applications, they do not provide the same structural support or sealing capabilities as oakum when used with lead. Asphalt and bitumen are typically used in different contexts within construction, such as waterproofing or roofing materials, and cotton does not have the necessary durability or structural characteristics for this specific joint application in plumbing. Thus, the use of oakum is essential for creating a reliable and effective joint in cast-iron soil piping systems.