For roof slopes under 4 units in 12, how many layers of asphalt underlayment are required?

For roof slopes that are less than 4 units in 12, the requirement for asphalt underlayment is based on the increased risk of water infiltration typically associated with low-slope roofs. In these scenarios, the building codes, such as the International Residential Code (IRC), mandate the use of two layers of asphalt underlayment to provide additional protection against leaks.

The lower slope means that water has a greater chance of pooling, which can lead to moisture issues if not properly managed. By requiring two layers, the code aims to enhance the waterproofing capability of the roofing system, ensuring better resistance to water penetration. This doubling of the underlayment creates a secondary barrier, which is crucial for maintaining the integrity of the roof structure over time.

In contrast, roofs with a slope of 4 units in 12 or greater generally require only one layer of asphalt underlayment, as they are better equipped to shed water quickly, minimizing the risk of leaks. Therefore, the requirement for two layers specifically for slopes under that threshold reflects an understanding of how drainage characteristics change with roof steepness.