What is the minimum roof slope for asphalt shingles?

Asphalt shingles require a slope of at least 2:12, and double underlayment is required for slopes from 2:12 up to 4:12 (IRC R905.2.2).

The IRC permits asphalt shingles only on roof slopes of two units vertical in 12 units horizontal (2:12) or steeper. Below that slope, asphalt shingles are not an approved covering.

For low slopes from 2:12 up to but not including 4:12, a double layer of underlayment must be applied because water drains more slowly and wind-driven rain is more likely to get under the shingles.

At 4:12 and steeper, a single underlayment layer is generally permitted, subject to the underlayment provisions for the covering.