A building permit application in Hawaii County includes a structure with a living roof system. What additional structural load must be considered beyond standard roof loads?
Correct Answer
D) Saturated soil weight plus 20 psf live load
Living roofs in Hawaii must account for saturated soil weight (typically 80-120 psf) plus 20 psf additional live load due to high rainfall conditions.
Why This Is the Correct Answer
Living (green) roofs retain water in their growing medium. In Hawaii's high-rainfall environment, the soil/growth medium becomes saturated, typically adding 80–120 psf of dead load. On top of this, an additional 20 psf live load is required to account for maintenance workers, equipment, and plant growth variations. Ignoring saturated weight is the most dangerous design error on living roofs.
Why the Other Options Are Wrong
Option A: 20 psf additional live load
A flat 20 psf additional live load accounts only for occupancy/maintenance but completely ignores the dominant load contribution — the weight of saturated growing medium. In Hawaii's climate, rainfall saturation of the growing medium far exceeds this single figure.
Option B: 40 psf additional live load
40 psf additional live load is still insufficient because it omits the dead load from saturated soil. The growing medium itself, when wet, can weigh 80–120 psf, which is separate from and in addition to any live load allowance.
Option C: Standard roof loads are sufficient
Standard roof loads are never sufficient for living roofs. Even a thin sedum mat system adds significant dead and live load that standard residential roof framing cannot accommodate without redesign.
Memory Technique
Remember 'Wet Dirt + People' — Saturated soil weight (dead load) PLUS 20 psf for people (live load). Hawaii's rain makes the dirt heavy; always add both components.
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