According to IBC, slate roof systems installed over ASTM D226 require what kind of underlayment?

The correct underlayment for slate roof systems installed over ASTM D226, as per the International Building Code (IBC), is Type II underlayment. Type II underlayment is specifically designed to provide superior performance in terms of moisture resistance and durability, which is essential for handling the significant weight and potential water infiltration risks associated with slate roofing materials.

Slate roofs are heavy and can create a unique set of challenges, particularly around water intrusion. Type II underlayment meets the requirements set forth in the IBC for such applications, ensuring adequate protection against leaks and roof system failures.

The other types of underlayment mentioned are not appropriate for this particular application. For example, Type I underlayment may not offer the same level of durability and moisture resistance required for slate roofs, while polyethylene underlayment is typically used in different roofing contexts, not specifically for slate. Fiberglass mesh underlayment serves specialized purposes, such as reinforcement in low-slope applications, rather than as a primary underlayment for slate. Thus, Type II underlayment is the recommended and appropriate choice to ensure the integrity and longevity of the slate roofing system.