High-rise and commercial buildings are marvels of modern engineering, but their design also introduces subtle paths for fire and smoke to travel. One of the most influential is the slab edge, the junction where the floor meets the curtain wall. Without a robust Curtainwall firestop and perimeter Barrier system like Firoblock, fire and smoke can move with surprising speed through hidden voids, turning a local incident into a multi-floor emergency.

The perimeter gap at the slab edge between the floor and the curtain wall is often out of sight, yet it can become a fast escape route for heat, flame, and smoke when it is left unprotected. This is the story the headlines rarely tell, until a fire breaks out.

1. The Perimeter Gap Problem

A perimeter gap is a deliberate void left at the junction of the floor slab and the curtain wall to allow for structural movement, thermal expansion, and shifts during seismic events. But every gap is a ready-made path.

When this void is left unprotected, fire does what it always does—it looks for
weakness. In minutes, flames and choking smoke can bypass solid slab divisions, traveling internally via the Chimney Effect or externally through Leapfrogging, defeating years of careful planning.

Most building codes now demand perimeter Barriers and certified Curtainwall firestop products like Firoblock because real-world tragedies showed how disastrous things get when those lines are ignored.

2. Case Study – The First Interstate Bank Fire (Los Angeles, 1988)

On May 4, 1988, a fire broke out on the 12th floor of the 62-storey First Interstate Bank building in Los Angeles. NIST subsequently examined the fire, including exterior fire propagation, smoke movement, detector response and sprinkler operation.

The investigation provides a documented case for studying fire spread in a high-rise building. Read NIST's investigation overview and engineering report for the event chronology and technical analysis.

For further context, explore our guide to curtain-wall perimeter gaps and curtain-wall firestop system.