Saturday, June 12, 2010 While operating at a structure fire in a 3 story single family dwelling, we initiated an interior attack on what was, at the time, a room and contents fire on the C-D corner of the 1st floor. After battling the blaze for several minutes, the IC noted that due to balloon construction, the fire had spread to the 2nd and 3rd floors, as well as the attic, and the roof was in danger of collapse. The IC ordered an evacuation of the structure. Search teams on the 2nd and 3rd floor evacuated immediately, but due to radio problems, the attack team on the 1st floor didn't hear the call. After approximately two minutes, the IC ordered the attack engine to sound three blasts on the air horn, our local back-up evac command. It was then realized that the air horn on the attack engine was inoperable. Sirens were then activated, and the interior attack team evacuated safely. The fire was knocked down from an exterior attack, and mopped up successfully without collapse. LESSONS LEARNED: Full and frequent testing of all equipment, including radios and audible warning devices is vital to firefighter safety. As demonstrated here, if it were not for the driver/operators quick thinking to activate the siren, there very well could have been injury/loss of life from a collapse, had the firefighters not been able to evacuate.