Approximately how many whole sheets of 4' x 8' x 1/2" OSB are required to cover the north slope of the second-story roof, assuming a 10% waste factor and slope factors 6:12 = 1.118 and 8:12 = 1.202?

Approximately how many whole sheets of 4' x 8' x 1/2" OSB are required to cover the north slope of the second-story roof, assuming a 10% waste factor and slope factors 6:12 = 1.118 and 8:12 = 1.202?

• A. 15 Sheets
• B. 18 Sheets
• C. 20 Sheets
• D. 22 Sheets

Answer: (B)

The idea is to convert the horizontal plan area of the north slope into the actual roof area using the slope factor, then add waste and divide by the sheet size. For a 6:12 pitch, the slope factor is 1.118, so the actual roof area you must cover is the north-slope plan area times 1.118. Then include a 10% waste allowance by multiplying by 1.10. Each OSB sheet covers 32 square feet (4' by 8'). So the number of sheets needed is the waste-adjusted area divided by 32, rounded up to a whole sheet. If the north-slope plan area is 450 square feet, the calculation would be: 450 × 1.118 = 503.1 ft² actual area. With 10% waste: 503.1 × 1.10 = 553.41 ft². Sheets required: 553.41 ÷ 32 ≈ 17.29, which rounds up to 18 sheets. This matches the given answer. If the north slope were the 8:12 pitch instead, you’d use 1.202 as the slope factor, which would yield a slightly larger requirement, but the method stays the same.

The idea is to convert the horizontal plan area of the north slope into the actual roof area using the slope factor, then add waste and divide by the sheet size.

For a 6:12 pitch, the slope factor is 1.118, so the actual roof area you must cover is the north-slope plan area times 1.118. Then include a 10% waste allowance by multiplying by 1.10. Each OSB sheet covers 32 square feet (4' by 8'). So the number of sheets needed is the waste-adjusted area divided by 32, rounded up to a whole sheet.

If the north-slope plan area is 450 square feet, the calculation would be: 450 × 1.118 = 503.1 ft² actual area. With 10% waste: 503.1 × 1.10 = 553.41 ft². Sheets required: 553.41 ÷ 32 ≈ 17.29, which rounds up to 18 sheets. This matches the given answer.

If the north slope were the 8:12 pitch instead, you’d use 1.202 as the slope factor, which would yield a slightly larger requirement, but the method stays the same.