When estimating roofing materials, a contractor calculates 2,400 square feet of roof area. Using a 10% waste factor, how many squares of shingles are needed?
Correct Answer
D) 27 squares
Roof area with waste = 2,400 × 1.10 = 2,640 sq ft. Since one square = 100 sq ft, 2,640 ÷ 100 = 26.4 squares. Rounding up to the next whole square for ordering purposes gives 27 squares.
Why This Is the Correct Answer
Option D is correct because when calculating roofing materials, you must first add the waste factor (2,400 × 1.10 = 2,640 sq ft), then convert to squares by dividing by 100 (2,640 ÷ 100 = 26.4 squares). Since you cannot order partial squares of shingles, you must round up to the next whole number, which is 27 squares. This ensures adequate material coverage for the project.
Why the Other Options Are Wrong
Option A: 24 squares
Option A fails to include the 10% waste factor and simply converts the base roof area to squares (2,400 ÷ 100 = 24), which would result in insufficient materials for the job.
Option B: 26 squares
Option B appears to add the waste factor but rounds down to 26 squares instead of rounding up, which would leave the contractor short of materials since 26.4 squares are actually needed.
Option C: 26.4 squares
Option C provides the mathematically correct calculation (26.4 squares) but fails to account for the practical reality that shingles are sold in whole squares only, requiring rounding up for ordering purposes.
Memory Technique
Remember 'WAR': Waste factor first, Area calculation second, Round UP third - because you can't buy partial squares and running short is worse than having extra.
Reference Hint
Look up roofing material calculations and waste factors in the estimating chapter of your contractor reference manual, typically found under 'Material Takeoffs' or 'Roofing Systems'.
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