A roof area measures 2,400 square feet. If asphalt shingles are installed with 10% waste factor and cost $95 per square (100 sq ft), what is the material cost?
Correct Answer
B) $2,508
Roof area with waste: 2,400 × 1.10 = 2,640 sq ft. Number of squares: 2,640 ÷ 100 = 26.4 squares. Cost: 26.4 × $95 = $2,508. Always account for waste and round up to the next whole square when ordering materials.
Why This Is the Correct Answer
Option B correctly applies the 10% waste factor to the roof area (2,400 × 1.10 = 2,640 sq ft), then converts to squares by dividing by 100 (2,640 ÷ 100 = 26.4 squares), and finally multiplies by the cost per square (26.4 × $95 = $2,508). This follows the proper sequence of accounting for waste before calculating material quantities and costs.
Why the Other Options Are Wrong
Option C: $2,640
This answer ($2,850) appears to round up to 30 squares instead of using the precise 26.4 squares calculation (30 × $95 = $2,850), which overestimates the material cost.
Option D: $2,850
This answer ($2,640) incorrectly treats the waste-adjusted square footage as the final cost, failing to convert to squares and apply the $95 per square pricing.
Memory Technique
Remember 'WAC': Waste factor first, Area conversion to squares, then Cost calculation. Think 'We Always Calculate waste before ordering materials.'
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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