For asphalt shingles, which slope indicates the need for a two-ply application?

A slope of 4:12 indicates the need for a two-ply application of asphalt shingles due to the potential for water infiltration. When the slope is between 2:12 and 4:12, the roof is considered low slope, which makes it more susceptible to water pooling and penetration. In areas where moisture can accumulate, a two-ply installation enhances the roof's durability and helps prevent leaks. The additional layer acts as a safeguard against water ingress, which is crucial in ensuring the longevity and effectiveness of the roofing system.

On slopes steeper than 4:12, such as 5:12 and above, a single layer of shingles is generally regarded as sufficient because the slope allows water to shed more effectively, reducing the risk of water damage.