For a built-up roof system in California seismic Design Category D, what additional requirement must be considered for the roof membrane attachment to prevent seismic damage?
Correct Answer
D) Enhanced adhesive bond strength and mechanical attachment at perimeters and penetrations
In high seismic areas like Design Category D, CBC requires enhanced roof membrane attachment including stronger adhesive bonds and mechanical fastening at critical areas like perimeters and penetrations. This prevents membrane failure during seismic movement, which is crucial for California's earthquake-prone regions per CBC Section 1507.10.
Why This Is the Correct Answer
In seismic Design Category D — California's high-seismic zone — the CBC (Section 1507.10) requires enhanced roof membrane attachment that goes beyond standard methods. This means both stronger adhesive bond strength and mechanical fastening at critical vulnerable areas: perimeters (where uplift and lateral forces concentrate) and penetrations (where the membrane is interrupted and most susceptible to tearing). A dual-method approach addresses the dynamic forces of seismic movement.
Why the Other Options Are Wrong
Option A: Flexible membrane joints at building expansion points only
Flexible membrane joints at expansion points only is insufficient for seismic Design Category D. While expansion joint flexibility is important, seismic forces affect the entire membrane system — especially perimeters and penetrations — not just expansion points. This partial measure would leave most of the roof vulnerable.
Option B: Mechanical fastening at 6-inch centers throughout
Mechanical fastening at 6-inch centers throughout would be excessive and is not the CBC requirement. The code takes a targeted approach, requiring enhanced attachment at the highest-risk zones (perimeters and penetrations) rather than uniform fastening across the entire field of the membrane.
Option C: Standard adhesive application with increased thickness
Standard adhesive with increased thickness does not meet the enhanced attachment requirement for seismic Design Category D. Simply using more adhesive in the field does not address the mechanical forces at perimeters and penetrations where membrane failure is most likely during seismic events.
Memory Technique
Think 'PERIL at the perimeter': in earthquake country, the Perimeter and penetrations are the high-risk zones requiring Enhanced attachment with both adhesive and mechanical fasteners. The middle of the roof is safer; the edges and holes are the danger zones.
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