A concrete slab in Hawaii requires a vapor barrier due to moisture conditions. The slab area is 1,200 sq ft and vapor barrier costs $0.85 per sq ft including 10% waste factor. What is the total material cost?
Correct Answer
B) $1,122
Area with waste factor: 1,200 × 1.10 = 1,320 sq ft. Total cost: 1,320 × $0.85 = $1,122.
Why This Is the Correct Answer
The calculation proceeds in two steps: (1) Apply the 10% waste factor to the net area: 1,200 × 1.10 = 1,320 sq ft of material needed. (2) Multiply by unit cost: 1,320 × $0.85 = $1,122. This is the standard method for material takeoffs — always apply waste before multiplying by unit cost, not after.
Why the Other Options Are Wrong
Option A: $1,224
$1,224 results from an incorrect waste calculation. This figure would come from computing 1,200 × $0.85 = $1,020, then adding 20% waste instead of 10% (or making a similar arithmetic error). It does not match any valid application of a 10% waste factor.
Option C: $1,326
$1,326 results from over-applying the waste factor. This would come from using a 30% waste factor (1,200 × 1.30 = 1,560 × $0.85 = $1,326) or a different miscalculation. Vapor barriers typically use a 10% waste factor, not 30%.
Option D: $1,020
$1,020 is the cost with zero waste factor: 1,200 × $0.85 = $1,020. This is the most common error — forgetting to apply the waste factor entirely. Always include waste in material estimates.
Memory Technique
Waste Factor Rule: 'Area first, then price.' Multiply the raw area by (1 + waste%) to get the material area, then multiply by unit cost. Think: 1,200 → expand by 10% → 1,320 → price at $0.85 → $1,122.
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