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Asphalt shingles may only be used on roof slopes of 2 units vertical in 12 units horizontal (2:12) or greater.
For roof slopes from 2:12 to 4:12, double underlayment is required. Underlayment must conform to ASTM D 226 Type I, ASTM D 4869 Type I, or ASTM 6757.
For roof slopes of less than 4:12, two layers of underlayment are required and must be applied in the following manner.
For slopes of 4:12 and greater, underlayment must be applied shingle fashion. Laps must be a minimum of 2-inches. End laps must be offset by at least 6 feet.
For normal application, strip shingles must be fastened with a minimum of four nails. For interlocking shingles, two nails are required.