If you ever see an abandoned building with a spraypainted square (with a slash through it, as you can see below), this building has been identified by the FDNY as dangerous for its firefighters because of a missing or weakened roof:

The FDNY has developed a unique communication system for colleagues – a secret language spraypainted on abandoned building walls.

The most common, an "X" or slash within a square (and the letters “RO), signifies a "roof out" – a potentially unstable rooftop that could collapse during firefighting operations. Other symbols, like "F.O." (floor out) or "H2O" (no water source), warn of specific hazards within the building, guiding firefighters and protecting lives.

These markings forewarn other FDNY mermbers/teams of possible structural danger. The spray-painted code system, implemented in the 1970s, has demonstrably reduced firefighter injuries and fatalities.

But the FDNY's code goes beyond just symbols. Different colors signify varying degrees of danger. Red, the most alarming, highlights major hazards like roof instability or structural damage. Orange indicates lesser concerns, while yellow might mark potential access points or utilities.

Looking Up 5th to Marcus Garvey Park

A view from NYU all the way up 5th Avenue.