A low-slope roof takeoff measures 18,400 sf of membrane field area. The estimator adds 7 percent for laps, perimeter waste, and detailing. What order area should be carried?
Correct Answer
C) 19,688 sf
Adjusted order area is 18,400 x 1.07 = 19,688 sf. Other choices add only 7 sf, subtract the allowance, or use a 10 percent allowance instead of the stated 7 percent.
Why This Is the Correct Answer
Adjusted order area is 18,400 x 1.07 = 19,688 sf. Other choices add only 7 sf, subtract the allowance, or use a 10 percent allowance instead of the stated 7 percent.
Why the Other Options Are Wrong
Option A: 18,407 sf
roof waste allowance, choice b: 18,407 sf is produced by 18,400. The error is using that partial/alternate setup.
Option B: 17,112 sf
roof waste allowance, choice c: 18,400 x 0.93 gives 17,112 sf. That calculation follows multiplies those two listed numbers.
Option D: 20,240 sf
roof waste allowance, choice d: 20,240 sf adds an extra unstated add-on/subtraction; the numeric gap is 552.
Memory Technique
Say the roof waste allowance rule aloud as a chain; the chain ends at 19,688 sf, not an intermediate stop.
Reference Hint
Open Roofing Construction and Estimating, Atcheson, 1995/2022 reprint; use the index entry or section for "Membrane roof material allowance" to confirm that Adjusted order area is 18,400 x 1.07 = 19,688 sf.
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