For a building described as unknown era, with commercial or commercial-residential use, Type III, and medium size, what is the described fire spread pattern?

• A. Vertical spread
• B. Horizontal spread
• C. Diagonal spread
• D. External spread only

Answer: (A)

In Type III ordinary construction, the interior can be quite combustible even though the exterior walls are noncombustible. In a medium-sized commercial or mixed-use building, vertical paths such as stairwells, elevator shafts, and service chases create direct conduits for fire to move from one floor to another. Because these vertical openings connect multiple levels, flames and heat can rise quickly, driving the fire upward (and continuing to spread to lower levels if heat and smoke travel downward through openings as rooms burn). This makes vertical spread the most characteristic and rapid pattern in this scenario. Horizontal spread across a single floor can occur as rooms and corridors burn, but the defining and most likely progression in a building of this type is vertical movement through those vertical voids. External spread requires the fire to move to the exterior, which isn’t the focus here, and diagonal spread isn’t the typical pattern without a specific structural layout.