In a 2-story residential building described as small, what fire spread mechanism is noted?

• A. Horizontal spread through floors
• B. Vertical spread through open truss and HVAC poke-throughs
• C. Spiral spread through ducts
• D. Limited spread due to heavy fire doors

Answer: (B)

In a small two-story residence, fire often moves between floors through vertical channels created by the structure itself. Open truss floor systems leave voids that flames and hot gases can travel through upward, and any poke-throughs for HVAC ducts and pipes provide additional vertical paths. These features together act like chimneys, allowing the fire to spread from the lower level to the upper level quickly. That’s why vertical spread through open truss and HVAC penetrations is the best description here. Horizontal spread within a single floor would describe movement across the same level, which isn’t what’s being noted, and spiral spread through ducts isn’t the typical pattern, while heavy fire doors influence containment but don’t define the primary vertical path.