Clay and concrete roof tile underlayment for slopes under 2-1/2 units vertical in 12 horizontal should comply with what requirement?

For clay and concrete roof tile underlayment on slopes under 2-1/2 units vertical in 12 horizontal, the requirement for a single Type I underlayment is correct because a single layer of Type I underlayment is typically adequate for low-sloped roofs. Type I underlayment is designed to provide effective water resistance and is suitable for use under tile roofing materials, offering the necessary protection against leaks.

In contrast, other options such as double underlayment applications or specific types of modified bitumen sheets are generally reserved for different conditions or higher sloped roofs. The single Type I underlayment meets the minimum requirements for this application, ensuring both compliance with building codes and sufficient protection against moisture intrusion.