According to California Building Code Chapter 15, what is the minimum roof slope required for clay and concrete tile roofing?
Correct Answer
A) 2.5:12 (21% slope)
CBC Section 1507.3.3 requires clay and concrete tile roofing to have a minimum slope of 2.5:12 (21%). This is specific to California's adoption of the International Building Code with state amendments for tile roofing common in California's climate.
Why This Is the Correct Answer
CBC Section 1507.3.3 requires a minimum slope of 2.5:12 (approximately 21%) for clay and concrete tile roofing. Below this slope, water can back up under tiles and into the underlayment. California's code specifically adopts this threshold as a state amendment to the IBC given the prevalence of tile roofing in California climates.
Why the Other Options Are Wrong
Option B: 6:12 (50% slope)
6:12 is a much steeper threshold, sometimes associated with certain slate or wood shake requirements. Requiring 6:12 as a minimum for tile would eliminate most residential tile applications in California, which are commonly installed in the 3:12 to 5:12 range — well below 6:12.
Option C: 3:12 (25% slope)
3:12 is a plausible but incorrect answer. While tile can be installed at 3:12 with proper underlayment, the code-specified minimum is actually 2.5:12 — a slightly lower threshold. Selecting 3:12 would cause a contractor to decline installations that the code actually permits.
Option D: 4:12 (33% slope)
4:12 is above the minimum requirement and represents a more conservative slope than the code mandates. Installing tile at 2.5:12 is code-compliant; telling a client their 2.5:12 or 3:12 roof needs to be 4:12 before tile can be installed would be incorrect.
Memory Technique
Two-and-a-half bricks stacked = 2.5:12 slope. Visualize two and a half bricks leaning against a wall — that slight angle is just enough for water to run off tile before pooling. The '2.5' is the key number to lock in.
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