According to code, what type of underlayment is required for asphalt shingles on roofs with a slope of 4:12 or greater?

The required type of underlayment for asphalt shingles on roofs with a slope of 4:12 or greater is one layer of #15 non-perforated felt. This type of underlayment is specifically designed to provide adequate protection against moisture infiltration and to act as a secondary barrier in case the shingles themselves fail. The code mandates this underlayment because a 4:12 slope provides a sufficient gradient for water runoff but still requires some level of protection against potential leaks and moisture damage.

Asphalt shingles are sensitive to moisture, and the non-perforated nature of this felt prevents water from penetrating the underlayment while allowing any condensation that might form underneath the shingles to escape. This balance is critical for the longevity and performance of the roofing system. Moreover, using #15 felt is standard due to its greater weight and durability compared to lighter materials, ensuring it can withstand both the elements and the weight of the roofing system above it.

In contrast, the other options do not meet the code requirements for this specific application due to either their insufficient protective qualities or compatibility with certain roofing conditions.