Roof Slope for Shingles

Terminology

• A designation “four in 12,” “four to 12,” 4:12 and 4/12 in roofing and architectural literature all mean a ratio of 4 inches of rise to 12 horizontal inches of roof or 4 feet of rise to 12 horizontal feet of roof. A flat roof is 2:12. A low slope is between 2:12 and 4:24. A conventional or normal roof is 4:12 to 9:12. A steep slope is 9:12 or higher.

Wood, Cement, Slate

• Lay wood shingles on a minimum 3:12 slope. Lay wooden shakes, asbestos cement and slate shingles on a 4:12 slope.

Asphalt Shingles

• Asphalt shingles are the most commonly used and least expensive shingle in Canada and the U.S. Lay them only on a roof slope 2:12 or greater, but to avoid premature leaks, lay them on a minimum 4:12 slope. If you lay asphalt shingles on a 4:12 or steeper slope, apply them on a single layer of 15-pound felt saturated with asphalt.

Asphalt Low Slope

• Ice collecting on low slope roofs can cause water to back up under asphalt shingles. Many building codes in colder climates prohibit laying asphalt shingles on slopes of 2:12 or 3:12 without special precautions. For example, the city of Bloomington, MN, requires that asphalt shingles on a 2:12 roof be laid on two overlapping layers of felt 19 and 36 inches wide and an interior water underlayment.