For a built-up roof system in California seismic Design Category D, what is the maximum allowable aggregate surfacing weight per square foot when the roof diaphragm is required to transfer seismic forces?
Correct Answer
A) 16 pounds per square foot
California Building Code Section 1511.3.3, referencing seismic provisions, limits aggregate surfacing to 16 pounds per square foot in high seismic areas when the roof serves as a diaphragm. This limitation prevents excessive dead loads that could compromise seismic performance and structural integrity during earthquake events.
Why This Is the Correct Answer
California Building Code Section 1511.3.3 limits aggregate surfacing on built-up roofs to 16 pounds per square foot in Seismic Design Category D, where roofs function as diaphragms to transfer lateral seismic forces. Excessive aggregate dead load increases seismic mass, amplifies earthquake forces, and can compromise the roof diaphragm's structural performance.
Why the Other Options Are Wrong
Option B: 12 pounds per square foot
12 pounds per square foot is below the 16 psf limit and is not the codified maximum for SDC D. It may represent a stricter project-specific specification, but it is not the CBC standard.
Option C: 8 pounds per square foot
8 pounds per square foot is too restrictive and does not appear in the California Building Code as a maximum for aggregate surfacing on built-up roofs. It would unnecessarily limit common roofing applications.
Option D: 20 pounds per square foot
20 pounds per square foot exceeds the CBC limit for high-seismic zones. In Seismic Design Category D, the code restricts aggregate to 16 psf to reduce seismic mass on the roof diaphragm.
Memory Technique
SDC D roofs = 16 psf max. Think '16 = sweet sixteen — the CBC's upper limit for aggregate in the highest seismic zones.' Above 16 psf and the roof becomes a seismic liability.
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