How many layers of No.15 Asphalt-saturated underlayment should be used for roofs with slopes of 18 degrees or more?

For roofs with slopes of 18 degrees or more, using a single layer of No.15 Asphalt-saturated underlayment is appropriate because it provides adequate protection against moisture intrusion while adhering to standard building codes and practices. The purpose of underlayment is to serve as a secondary water barrier, especially in instances where roofing materials may fail or in areas that experience heavy rain or snow.

When the slope is steep, such as 18 degrees or more, the likelihood of water pooling is significantly reduced compared to flatter roofs, which typically require additional layers of underlayment for extra protection. Therefore, one layer is sufficient to repel water and perform effectively under such conditions. This approach balances performance with cost and installation efficiency, making it the preferred option in roofing practices for steeper slopes.