1507.2 Asphalt shingles. The installation of asphalt shingles shall comply with the provisions of this section.
1507.2.1 Deck requirements. Asphalt shingles shall be fastened to solidly sheathed decks.
1507.2.2 Slope. Asphalt shingles shall only be used on roof slopes of two units vertical in 12 units horizontal (17-percent slope) or greater. For roof slopes from two units vertical in 12 units horizontal (17-percent slope) up to four units vertical in 12 units horizontal (33-percent slope), double underlayment application is required in accordance with Section 1507.2.8.
This code implies that asphalt shingles shall only be applied on steep-slope roof assemblies. By code, steep-slope roofs are those that are “two units vertical in 12 units horizontal (17 percent slope) or greater.”
The code also states that on roof slopes between 2:12 to 4:12 a double layer of underlayment must be applied over the structural deck. This requirement is designed to eliminate moisture entry into the structure for all potential ponding water. Typically, the materials used in steep-slope applications serve to shed water and they are prone to moisture entry if exposed to standing water. The underlayment application must be in compliance with Section 1507.2.8