For a built-up roof system in California's seismic design category D, what is the maximum allowable aggregate surfacing weight per square foot according to CBC provisions?
Correct Answer
C) 300 pounds per square foot
CBC Section 1511.1.1 limits aggregate surfacing on built-up roofs to 300 pounds per square foot in seismic design categories D, E, and F. This restriction prevents excessive dead loads that could create seismic hazards and ensures structural integrity during earthquake events common in California.
Why This Is the Correct Answer
CBC Section 1511.1.1 caps aggregate surfacing on built-up roofs at 300 pounds per square (per 100 sq ft) in Seismic Design Categories D, E, and F. Aggregate adds significant dead load, and in high-seismic zones this mass increases lateral forces on the building's structural system during an earthquake, so the code limits it to manage seismic risk.
Why the Other Options Are Wrong
Option A: No aggregate surfacing permitted
Prohibiting aggregate surfacing entirely is too restrictive; the CBC allows it up to the 300 lb/square limit. Aggregate provides important UV protection and ballast for BUR membranes, so an outright ban would eliminate a widely used and code-compliant roofing option.
Option B: 400 pounds per square foot
400 pounds per square exceeds the CBC limit of 300 lb/square for SDC D. Exceeding this threshold would increase the seismic weight of the roof diaphragm beyond what the structural system may be designed to accommodate.
Option D: 500 pounds per square foot
500 pounds per square is significantly above the code limit and would impose dangerous additional seismic dead load. This value may be confused with older code provisions or non-seismic-zone limits, but it is not acceptable in SDC D under current CBC.
Memory Technique
300 = 3 × 100 = three layers of protection. Think of the number 3: Seismic Design Category D is the 4th letter but the rule uses 300, and you can remember '3 and D go together' since both appear in the same provision.
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