By Vincent Dunn

There are three ways in which a masonry exterior building wall may collapse. The wall may fall straight out in a monolithic piece at a 90-degree angle, in a manner similar to a falling tree; the wall may crumble straight down in a so-called "curtain" fall collapse; or the wall may collapse in an inward / outward fashion, with the top falling inward and the bottom outward.

At fires where the officer in command suspects an explosion or other factor to cause a wall to collapse out into the street for a distance greater than the
height of the wall, a chief or fire officer should keep his men away from the front wall of the fire building altogether, positioning heavy-caliber streams in a
flanking position: that is, on either side of the front wall beyond the outer perimeter of the building's width.

If it is absolutely necessary to operate a master stream inside a collapse zone, and the wall appears unsafe, the portable deluge nozzle or aerial stream should
be secured to direct the stream effectively and safely, and then left unattended.

• Vincent Dunn is a retired FDNY Deputy Chief. For more tactical advice and tips, go to Vincentdunn.com. For lecture information, call 1-800-231-3388.