What is the minimum roof slope required for built-up roofing systems according to the California Building Code?
Correct Answer
D) 1/4 inch per foot
California Building Code Section 1507.10.2 requires built-up roofs to have a minimum slope of 1/4 inch per foot (2% grade) to ensure proper drainage. Adequate slope prevents water ponding which can lead to premature roof failure and structural damage.
Why This Is the Correct Answer
CBC Section 1507.10.2 mandates a minimum slope of 1/4 inch per foot (approximately 2% grade) for built-up roofing. This minimal slope is enough to prevent prolonged water ponding — a primary cause of membrane deterioration — while still allowing built-up systems to be used on near-flat commercial roofs.
Why the Other Options Are Wrong
Option A: 1/2 inch per foot
1/2 inch per foot is twice the minimum required slope. While a steeper slope improves drainage, specifying it as the minimum is incorrect and would unnecessarily restrict built-up roof applications on low-slope commercial buildings.
Option B: 1/8 inch per foot
1/8 inch per foot is below the CBC minimum and does not provide adequate drainage for built-up systems. Ponding water at this slope would accelerate membrane failure and potentially overload the structure.
Option C: 1 inch per foot
1 inch per foot is the minimum slope for self-sealing strip shingles, not for built-up roofing. Applying this value to BUR would be a category confusion between steep-slope and low-slope roofing systems.
Memory Technique
Quarter-inch, quarter-dollar: just like a quarter is the smallest common coin that 'counts,' 1/4 inch per foot is the smallest slope that 'counts' for built-up roofing drainage. Both are the minimum meaningful unit.
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