When installing asphalt shingles over one ply of fifteen-pound felt, what is the minimum allowable roof slope?

The correct answer is 4:12 because this is the minimum roof slope recommended for installing asphalt shingles over a single layer of fifteen-pound felt. A roof slope of 4:12 means that for every 12 horizontal units, the roof rises 4 vertical units. At this slope, asphalt shingles can perform effectively, allowing for proper water drainage and reducing the risk of leaks or shingle damage. This pitch helps ensure that water runs off the roof efficiently, preventing pooling or infiltration beneath the shingles.

Lower slopes, such as 2:12 or 3:12, while they can be used for some roofing materials, typically require additional underlayment or different installation practices to mitigate the risks associated with water penetration. The 4:12 slope strikes a balance, providing adequate drainage while adhering to building codes and standards regarding asphalt shingle installation.